1.  PLANT CLASSIFICATION

System
♦   Artificial

• Classification based on one or few morphological characters.
• Gave equal weightage to vegetative and sexual characteristics, this is not acceptable as vegetative characters are more easily affected by environment.
• This system is not followed because it results in separation of closely related families.
• Most famous system is given by Linnaeus, which is based upon number of stamen, due to this it is also known as sexual system of classification.
  (a) Theophrastus (370-285 B.C.), a Greek botanist, classified plants into four groups, herbs, undershrubs, shrubs and trees on the basis of their habit.
  (b) Pliny the Elder (23-79 A.D.) distinguished animals into flight and nonflight ones. Flight animals included bats, birds and insects. He divided plants into herbs, shrubs, undershrubs, trees, vines, succulents, aquatic and terrestrial.
  (c)  Carolus Linnaeus (1707-1778), classified plants into 24 classes on the basis of sexual characters. He took only the number, length and union of stamens and carpels into consideration for 23 classes. For example, he proposed classes Monandria (1. stamen), Diandria (2. stamens), Tri ­and Polyandria (3 and more stamens). 24th class was Cryptogamia, for flowerless plants. Hence, Linnaeus’s system is also known sexual system of classificationor numerical system of classification.
• Artificial system is simpler and easier to practice in the field, however it has several drawbacks in this system of classification that are follows:
• It lacks the natural relationship amongst the organism.
• Organisms do not show a clear-cut evolutionary line.
• It leads to heterogeneous assemblage of unrelated organisms.

The traits used for artificial system are liable to change.
♦   Natural System

• Based on natural affinities among the organisms.
• Along with morphology it also includes ultra-structure, anatomy, embryology and phyto-chemistry.
• Most famous natural system is Bentham and Hooker system. This system is followed in India.
• Bentham and Hooker published their classification in 3-volumes of Genera Plantarum, they studied 97,205 species of plants.

♦   Some of the natural systems are briefly discussed below:

• L. de Jussieu (1686-1758) attempted a natural classification in his Genera Plantarum (1789). He adopted the views of John Ray as to the primary divisions and applies them to the system of Tournefort which was in common use in France. He particularly took into consideration the position of stamens with respect to the ovary and number of cotyledons. He considered 15 classes of plants. These classes were further divided into 100 natural orders (equivalent to our present day families).
• P. de Candolle in 1819 published a system of classification in his book Theorie Elementaire de la Botanique. He was first to use the characteristic of vascular tissues in the classification of plants and recognized two groups of plants, Vasculares(Vascular plants with cotyledons), Cellulares(Plants without vascular bundles or cotyledons). The former includes Pteridophyta, Gymnosperms and Angiosperm, and the latter includes Thallophyta and Bryophyta.
• George Bentham (1800-1884) and Joseph Dalton Hooker (1817-1911) worked together at Royal Botanical Gardens, Kew, England and proposed the most important and the last of the natural system for classification of seed plants. The system presented by them was published in Latin language in their treaties Genera Plantarum (1883) which appeared in three volumes. In these volumes, they have described some 97,205 seed plants according to their classification. An important feature of their classification is that they have given Gymnosperms a rank equal to Dicotyledons and Monocotyledons. Bentham and Hooker’s system of classification is used by most of the well-known herbaria of the world.

♦  Natural system of classification not only brings out natural relationships but also studies the evolutionary tendencies and phylogeny with the help of all the available data including fossils. This system of classification is certainly better than any artificial system of classification because:

• It establishes natural relationships amongst organisms.
• It places only related organisms in a group.
• It prevents grouping of unrelated organisms.
• It brings out phylogenetic relationships and indicates the possible origin of different taxa.

♦  Phylogenetic System

• The term phylogeny is coined by Lamarck and the concept of phylogeny was given by Haeckel.
• First phylogenetic system was given by Eichler.
• In this system all the evidences along with Fossil evidence is taken into account.
• This system assumes that organisms belonging to same taxa have a common ancestor.

♦  Classification becomes difficult when there is no supporting fossil evidence, then information from other sources is collected.

Phylogenetic system

♦  The evolutionary history of a group of organisms is called phylogeny. The system of classification reflecting the evolutionary sequence as well as the genetic interrelationships of organisms is called phylogenetic system. Adolf Engler (1884-1930) and his associate Karl Prantl (1849-1893) published a phylogenetic system in the monograph Die Naturlichen Pflanzenfamilien. They placed families and orders of the flowering plants in ascending series based on the complexity of floral morphology. The characters like one whorl of perianth or no perianth, unisexual flowers and pollination by wind were considered primitive as compared to perianth with two whorls, bisexual flowers and pollination by insects. The plant Kingdom according to their classification is further divided into divisions, sub-divisions, classes, orders and families. Asteraceae (Compositae) among dicots and Orchidaceae (Orchid family) among monocots are considered highly advanced. According to them monocots are more primitive than dicots.

♦  This system considered evolution of angiosperms from a single source and the sequence of order and families show parallel evolution.

♦  Ideally, a classification must reflect possible evolutionary relationships. Organisms belonging to same taxa are believed to have a common ancestor and may be represented in the form of a family tree (cladogram). The systems of classification proposed after Darwin’s theory of natural selection, are mostly claimed to be phylogenetic.

♦  Modern attempts at developing a phylogenetic classification of flowering plants are those of Cronquist, Hutchinson (1959), Takhtajan (1967), etc. However, a phylogenetic system is not static but highly dynamic. Its major source is fossil record. As newer and better fossils are discovered, our knowledge about past organisms and their linkage with the present forms are liable to undergo change. Therefore, phylogenetic classification is under refinement ever since the concept was born.

♦  Numerical Taxonomy / Adansonian taxonomy – In this all the characters were given equal importance. Number and codes are assigned to all the characters at the same time hundreds of characters are analysed using computers.

♦  Karyotaxonomy / Cytotaxonomy – It is based on cytological information like chromosome number, structure, behavior.

♦  Chemotaxonomy – Chemical constituent of plant is used to resolve taxonomical problem.

ALGAE

2.  GENERAL STUDY OF ALGAE

♦  Photosynthetic, simple, thalloid, mainly aquatic organisms, but also present on moist stones, soils and wood.

♦  They occur in association with fungi (Lichen) and animal (on Sloth bear).

♦  Form and size is highly variable, unicellular Chlamydomonas, colonial Volvox, filamentous Ulothrix and Spirogyra.

♦  Reproduction by vegetative, asexual and sexual means.

Vegetative	Asexual			Sexual
Fragmentation	Spores
	Zoospores	Aplanospores	Hypnospores	Isogamous	Anisogamous	Oogamous

• Algae usually occur in a variety of habitats such as water, land as well as on the other plants and even animals. Some grow in marine water called seaweeds.
• Vascular tissues are absent.
• A mechanical tissue is absent.
• A variety of pigments in algae provide different colours.
• Sexual reproduction involves isogamy, anisogamy and oogamy. Sex organs are unicellular and non-jacketed. An embryo stage is absent.
• Life cycle is various-haplontic, diplontic or diplohaplontic.

Classification of Algae
♦  Algae are usually differentiated on the basis of their pigments and storage products. Algae included under kingdom Plantae by Whittaker (1969) are of three types: red algae, brown algae and green algae.
Chlorophyceae: Green Algae
♦  About 90% of the total species grow in fresh water habitats and 10% are marine. They are cosmopolitan in distribution. They may be

• Terrestrial– growing on moist soil, walls and rocks, e.g., Fritschiella
• Epiphytes– growing on other plants, e.g., Trenteopohlia, Protococcus,
• Endophytes– growing inside the other plants, e.g., Coleochaete nutellainside the thallus of
• Epizoic– growing on the surface of animals, e.g., Cladophora and Charaeilum on mollusc shells and crustaceans, respectively.
• Endozoic– living inside body of animals, e.g., Zoochlorella inside sponges, Chlorella in the body of
• Cryophytes– growing in the polar region on ice and snow, e.g., Chlamydomonas nivalis, (causing red snow ball).
• Thermophilic– growing in hot springs, e.g., Chlorella
• Parasitic– growing as pathogens and causing diseases, e.g., Cephaleuros (causing red rust disease of tea and coffee).
• Symbionts – as components of certain lichens.

♦  Thallus is of various types: unicellular flagellate (e.g. Chlamydomonas), unicellular non flagellate (e.g., Chlorella, Characium, Acetabularia) or flagellate colonies, (e.g., Volvox), non-flagellate colonies (e.g., Hydrodictyon), coenocytic and siphonaceous (e.g., Vaucheria, Caulerpa), unbranched filament (e.g., Ulothrix), simple branched (e.g., Cladophora), heterotrichous with prostrate and vertical branches, (e.g., Draparnaldia), and parenchymatous (e.g., Ulva).
♦  Cell wall contains cellulose with a few exceptions.
♦  Photosynthetic pigments are similar to those of higher plants: chlorophyll a, chlorophyll b, carotenes and xanthopylls.
♦  Food reserve is Starch.
♦  Vegetative reproduction occurs by fragmentation, stolons and tubers.
♦  Asexual reproduction takes place by mitospores. The common asexual spores are zoospores, aplanospores, hypnospores, akinetes, etc.
♦  Sexual reproduction is effected by isogamy, anisogamy and oogamy.
♦  Three types of life cycle occur in green algae: haplontic, diplontic and diplohaplontic.

• In haplontic life cycle the dominant phase is haploid. The diploid stage is present only in the form of zygote or zygospore. Meiosis occurs at the time of its germination of zygote (zygotic meiosis, e.g., Ulothrix, Spirogyra and Chlamydomonas).
• In diplontic life cycle, the dominant phase of the alga is diploid. It gives rise to haploid gametes through meiosis (gametic meiosis, e.g., Caulerpa).The gametes fuse and the fusion product or zygote regenerates the diploid phase.
• The haplodiplontic life cycle possesses well developed multicellular haploid and diploid structures. The haploid gametophyte gives rise to haploid gametes. The fusion product of gametes or diploid zygote grows directly into diploid sporophyte. The sporophyte produces haploid spores through meiosis (sporic meiosis, e.g., Cladophora). Meiospores germinate into new gametophytes.
• Grass-green due to presence of chlorophyll ‘a’ and ‘b’ pigments.
• Chloroplast may be discoid, plate like, reticulate, cup-shaped, spiral or ribbon-shaped.
• Pyrenoid is present in chloroplast, it is a protein core surrounded by starch.
• Some green algae store food in the form of oil.
• Zoospore is produced in zoosporangia.
• In Spirogyra, sexual reproduction is isogamous and isogametes are non-motile.
  E.g., Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

CHLAMYDOMONAS: LIFE CYCLE
HABITAT
Chlamydomonas is widely distributed fresh water unicellular alga, commonly occurring in standing or stagnant rainwater, ponds, pools, ditches and on moist soils. It grows, in abundance in water rich in ammonium compounds.
Plant Body

It is a simple, unicellular motile, green algae. The individuals are spherical or ellipsoidal. In many species a papilla like out growth is visible in the anterior region.

• The protoplast is surrounded by a definite layer of glucoprotein wall and motile cells of some species have a gelatinous pectic sheath outside the cellulose layer.
• Most of the species have a single large cup shaped chloroplast and occupies most of protoplast. Chloroplasts of most of species have a single pyrenoid, which is a protein body and is the site of starch storage. Some species like C. reticulata do not have pyrenoids.
• At the pointed anterior end of the cell, arise two flagella emerging through the same or separate canals. The flagella are acronematic(whiplash) and are of equal length. Each flagellum has a granule (blepharoplast)at the points of its origin. They are connected together by a transverse fibre called paradesmos,which is again connected with the intranuclear centrosome of the nucleus through a cytoplasmic strand, rhizoplast.The entire setup is well coordinated to perform sensory as well as locomotory functions and is known as neuromotor apparatus.
• Each cell typically possesses two contractile vacuoles located at the base of flagella in a plane at right angles to them.
• A tiny spot of an orange or reddish colour, known as stigma or eyespot,lies at the anterior end. It is a photoreceptive organ concerned with the direction of the movement.
• Each individual has a single nucleus lying in the colourless cytoplasm filling the cup. The nucleus is typically of eukaryotic type.

Reproduction

Chlamydomonas reproduces both asexually and sexually.
Asexual Reproduction

• Zoospores: The protoplasm of each vegetative cell undergoes repeated longitudinal divisions, either into 2, 4, 8 or 16 daughter protoplasts. The parent cell normally loses its flagella before the onset of division. After the last series of divisions, each daughter protoplast secretes a cell wall and neuromotor apparatus that develops two flagella, eyespot and contractile vacuoles. The daughter cells (zoospores) are liberated by gelatinization or by the rupture of the cell wall. Thus, each daughter cell resembles the parent cell in all respects, except that it is smaller in size. In nature the zoospore formation is very common when conditions are favourable.

• Palmella spores: In adverse condition of drought, when pond or pool dries up or when plant grows on moist soil or on agar medium in laboratory, the daughter protoplast, formed as a result of divisions, do not develop neuromotor apparatus and become motile. But the parent cell wall gelatinizes and forms a matrix around the daughter protoplast. Divisions and redivisions of these daughter protoplasts ultimately may produce an amorphous colony with hundreds or thousands cells. All cells of such palmella stages (named after the genus Palmella) develop flagella, become motile and escape from the gelatinous matrix when flooded with water.
• Aplanospores: During unfavourable conditions, sometimes undischarged zoospores develop into aplanospores. The aplanospores are thin walled, uninucleate, unicellular structure. Often they develop singly in the cell and may germinate in situ (i.e., before liberation). On germination, each develops into a new filament.
• Hypnospores: In some species (e.g., C. nivalis), the protoplast withdraws from the cell wall, rounds up and develops a thick wall under unfavourable conditions. These resting spores are called hypnospores. Hypnospores usually develop red colour due to the formation of haematochrome.
• Akinetes: Akinetes are formed during extreme conditions. They are formed in certain cells that accumulate food and secrete a thick and resistant wall. During favourable condition each germinates to produce a new cell.

 Sexual Reproduction

Some species of Chlamydomonas are homothallic, while ot
hers are heterothallic. Gametic union may be isogamous, anisogamous, or oogamous.
Isogam

• Each Chlamydomonas cell may produce 8, 16, 32 or 64 biflagellate gametes that are (+)ve or (-)ve in character. In C. longistigma, the gametes are naked (gymnogametes)whereas in C. media, the gametes become covered with a wall, just before their emergence from the cell (calyptogamete).

HOLOGAMY

In most of the isogamous species any vegetative cell may function as gamete and their walls fuse prior to the gametic union e.g., C. debaryana.
PHYSIOLOGICAL ANISOGAMY

In species where the two uniting gametes, though they are morphologically similar, behave differently such as the cell contents of one gamete may pass into another gamete, the process is called as physiological anisogamy.
ANISOGAMY

In anisogamous species like C. braunii, 2-4 large female gametes are formed in one cell and 8-16 small male gametes are formed in another cell. Both the gametes are provided with a wall. The male and female gametes join by their anterior ends. At the point of contact, their membranes dissolve and contents of the male gamete pass into the female gamete with the result of formation of zygote. The gametes do not shed their walls at the time of gametic union.
OOGAMY

In oogamous species, like C. coccifera the male cell divides to form 8, 16 or 32 small biflagellate antherozoids.The large female cell loses its flagella and becomes an egg cell or oogonium. Fusion takes place between a male gamete and an egg. Both the gametes are covered with a cell wall and form a zygote.

ZYGOTE AND ITS GERMINATION

• Disappearance of flagella from quadriflagellate zygote of isogamous or anisogamous species is followed by the formation of a wall around it. The two nuclei fuse and it becomes a spherical structure, which undergoes a period of rest. It secretes a thick wall, which may be smooth and spiny. The resting zygote enlarges to 2-5 times of its original size, owing to the accumulation of reserve food material during photosynthesis. However, there are few species, in which there is no increase in size of the resting zygote that develops a red pigment, called hematochrome.In C. eugametos, however, zygotes remain green.
• When the resting period is over and the conditions are favourable, the zygote germinates. The diploid nucleus undergoes reduction division and forms four nuclei and the cytoplasm gets accumulated around each nucleus. The daughter protoplasts are liberated to the outside by the breaking up of the zygote wall. Thus, the new cells formed are usually four in number, from a single zygote. But in some species, there may be eight (c. reinhardi) or 16 to 32 (c. intermedia) biflagellate zoospores.

3.  ULOTHRIX: LIFE CYCLE
Occurrence

Ulothrix with its about 30 species, largely grown in fresh water ponds, pools, tanks and running streams. Some species like U. zonata, are distinctly cold water forms.

Structure of The Thallus

The plant body is a thallus consisting of an extremely fine unbranched filament. The filament comprises of single row of cells placed end to end, and firmly united. The filaments appear slender, thread like and may be up to 0.04 mm in diameter. Except for basal one, all the cells of the filament are similar in structure and behaviour. The basal cell (holdfast) is slightly elongated, hyaline or achlorophyllous and through it, the filament remains attached to the substratum. The remaining cells of a filament are barrel shaped and are often wider than long. However, the apical cell is down-shaped. The growth is apical.

Structure Of A Cell

• The cells are usually cylindrical, sometimes slightly swollen in middle, and often broader than long as in U. zonata and most other species. Each cell consists of a cell wall enclosing the protoplast. The cell wall consists of two layers:
  (a)  the inner layer consisting of cellulose, and
  (b)  the outer layer consisting of propectin which is insoluble in water.
• The protoplast is differentiated into a cell membrane, cytoplasm, a single nucleus, chloroplast with one or more pyrenoids and a central vacuole. Cytoplasm forms the lining layer or primordial utricle and is closely invested by the cell membrane. The central portion of the cell is occupied by a large vacuole containing a cell sap. The cells are always uninucleate and possess a single girdle, collar or ring shaped chloroplast with one (e.g., U. variabilis) or more (e.g., U. zonata) pyrenoids. The number of pyrenoids may increase during cell division. The chloroplast may be closed (e.g., U zonata) or open at one end.
• The cell are uninucleate. The nucleus in resting condition possesses a prominent nucleolus.

Reproduction

Ulothrixreproduces by the following means:

VEGETATIVE REPRODUCTIONS

Accidental breaking or the death of intermediate cell causes breaking of the filament into fragments, the process being known as fragmentation. Each fragmented part then grows into a new plant.

ASEXUAL REPRODUCTION

• Following modes of asexual reproduction are known to occur in Ulothrix:
• Zoospores: The zoospores are formed during favourable conditions. All cells are capable to form zoospores except the holdfast.
• During the formation of zoospores, the protoplast divides mitotically into number of daughter protoplasts by repeated longitudinal divisions. Each develops into a zoospore. Following are the two types of zoospores:
  ♦  Macro zoospores: They are quadriflagellate, uninucleate and pyriform (pear shaped) with a pointed anterior end and are often fewer in number. Each macrozoospore consists of a pair of contractile vacuole, a single chloroplast with a pyrenoid and almost anteriorly placed stigma. The zoospores resemble morphologically with Chlamydomonas and are liberated from the parent cell through a pore in the lateral wall. They are first liberated in a thin vesicle that soon disappears making zoospores free in the water.
  •  After swimming for a short period the zoospores attach by the anterior pointed end on some solid object. They discard their flagella, secrete a cell wall and divide by a transverse division to produce a lower cell and an upper cell. The lower cell develops into a holdfast while the upper cell by repeated transverse divisions forms the filament.
  ♦   Microzoospores: Filaments of U. zonata produce microzoospores that are formed in large number (4 to 32) similar to that of macro zoospores. They are smaller in size, uninucleate, narrowly ovoid with a round posterior end and they may be quadri – or biflagellate. They swim for a longer period for about 2 to 6 days.
  •   After swimming phase, they attach to some solid object by their anterior end. The process of their development into a filament is similar to that of macro zoospores.

Aplanospores: During unfavourable conditions, sometimes undischarged zoospores develop into aplanospores. The aplanospores are thin walled, uninucleate, unicellular structure. Often they develop singly in the cell and may germinate in situ (i.e..before liberation). On germination, each develops into a new filament.

Hypnospores: During drought, the entire content of the cell rounds off and secretes a thick wall and is called hypnospore.On the onset of favourable conditions, the hypnospore germinates to produce a new plant.

Akinetes: Akinetes are formed during extreme conditions in U. idiospora. They are formed in certain cells that accumulate food and secrete a thick and resistant wall. During favourable conditions each germinates to produce a new filament.

Palmella spores: Occasionally, the wall of the parent cell producing aplanospores gelatinizes. Simultaneously, the aplanospore wall also gelatinizes resulting into number of rounded bodies embedded in common mucilaginous mass. This is called palmella spore that serves to protect against desiccation. With the return of favourable conditions, each rounded body liberates as a zoospore. Palmella stage is commonly formed when the plant reaches damp banks of the pools and ponds.

SEXUALREPRODUCTION

♦  Isogamous type of sexual reproduction is found in Ulothrix and in majority, the plants are heterothallic, The gametes are formed in large number i.e., 32 to 64 in number in each gametangium. Each gamete looks quite similar to biflagellate microzoospore. However, gametes are smaller in size. These are formed and liberated in a way similar to zoospores. Each gamete is biflagellate, pyriform and has prominent stigma and a chloroplast. They look like Chlamydomonas. but are naked. Since, male and female gametes are indistinguishable, they are denoted as (+) or (–) strain gametes.

♦  During fusion the two gametes fuse from anterior to lateral side. As a result, a quadriflagellate zygospore is formed, which possesses a pair of nuclei, chloroplasts and eyespots. Plasmogamy (fusion of protoplasm) takes place at this stage that is followed by karyogamy (nuclear fusion). After swimming for a short while,­ it secrets a thick lamellated wall and undergoes or resting period.

GERMINATION OF ZYGOSPORES

With the return of favourable conditions, the diploid nucleus of zygospore divides meiotically and generally produces four motile (zoospores) or non-motile spores (aplanospores) of which’ two develop into male filament (+ type) and other two into female ones (- type). Each spore develops into a new plant. Sometimes, after meiosis, a few mitotic divisions may also occur in zygote each followed by protoplast cleavage. In such cases, 4 to 6 spores are formed. They may be motile and quadriflagellate and called zoomeiospores. The zoomeiospores are liberated into the surrounding water and each develops into a new filament.

4.  SPIROGYRA: LIFE CYCLE
OCCURRENCE

Spirogyra is a large genus consisting of about 300 species widely distributed throughout the world. It grows as free floating extensive masses and hence commonly called pond scum. It grows frequently in fresh water, stagnant reservoirs and in slow running streams and rivers. In natural conditions, Spirogyra looks like a mass of shining silky long filaments and hence it is popularly known as pond silk.

 STRUCTURE OF THE THALLUS

The plant body consists of slender, unbranched filament. The young filament of Spirogyra is found attached to some substratum by a modified basal cell, while adult plant is always free floating. The basal cell that helps in the attachment is called as hapteron.Each filament consists of a single row of cylindrical cells.

Structure of A Cell

♦  Each cell consists of a firm cell wall enclosing a mass of protoplast. Cell wall is commonly two layered, the inner composed of cellulose while outer of pectic substances. The pectic substances gelatinize in the presence of water and render the plants slimy touch.

♦  The protoplast consists of a single nucleus, mass of cytoplasm, variable number (1 to 2, sometime 24) flat, ribbon shaped chloroplasts and a large central vacuole. The nucleus is found centrally suspended by strands of cytoplasm or it may be parietal in position. The Cytoplasm is peripheral due to presence of large central vacuole but central vacuole is traversed by several cytoplasmic strands. The number of chloroplast is the characteristic feature of the alga. Their arrangement is also specific and spiral i.e., twisted to the right in the ascending order. The name of the alga is after the spiral arrangement of chloroplast. Each chloroplast remains at uniform intervals with linearly arranged pyrenoids.

REPRODUCTION

Spirogyra reproduces by vegetative and sexual methods. However, aplanospores formation has been reported in S. aplanosporum and akinetes in S. farlowii.

VEGETATIVE REPRODUCTION

Fragmentation is the common method of vegetative reproduction in Spirogyra. Accidental breaking or injury breaks the filaments into 2-3 celled pieces, each germinates to produce a new plant. However, in certain cases, cross walls also play a role in separating the two cells apart by the process of invagination.

SEXUAL REPRODUCTION

The sexual reproduction in Spirogyra is called conjugation, which involves fusion of two morphologically identical but physiologically dissimilar gametes. It is called as physiological anisogamy. The gametes are aflagellate (aplanogametes).For development of gametes some of the cells start to act like male and female gametangia. The cell contents taking part in development of gametangia become separated from the cell wall and shrink and are ultimately converted into gametes. The process of conjugation involves following methods:

SCALARIFORM CONJUGATION

Scalariform conjugation takes place mostly during night in recently divided cells. The process begins with two filaments getting intimately associated due to mucilage. Lateral outgrowths arise from the cells of these two filaments, placed opposite one another and are called papillae.The outgrowths enlarge because of the repulsion between the two conjugating filaments and result in the formation of conjugation tubes. Later the common walls of the conjugation tubes dissolve and a free passage is formed. Simultaneously, the protoplasts accumulate abundant starch. The male gamete moves in amoeboid manner through the conjugation tube into the female cell of another filament. Ultimately the nucleus of male gamete fuses with the nucleus of the female gamete involving plasmogamy followed by karyogamy and forms a diploid zygospore. At the completion of scalariform conjugation, the cells of the male filaments become empty while the cells of the female filament are filled with the zygospores. Sometimes, even three filaments are involved in the scalariform conjugation, of which the central filament acts as the female one in which male gametes from two other filaments move in and fuse to form zygospores. This type of conjugation has been reported in S. indica and S. elongata.

LATERAL CONJUGATION
In Spirogyra lateral conjugation takes place by one of the following three methods:

• Indirect lateral conjugation: In this process two adjacent cells of the filament take part. These cells develop tube like outgrowths close to the common cross walls. These outgrowths extend laterally and ultimately form conjugation tube like structure that connects the adjacent cells. The protoplast of conjugating cells contract and form gametes. The outgrowths of the adjacent cells fuse to form a passage between them. The contracted protoplast of one cell (so called male gametangium) moves through the conjugation passage into the adjacent cell (so called female gametangium). The fusion of both the gametic protoplasts results in the formation of a diploid zygote. The male cells or male gametangium becomes empty due to migration of its contents while zygospores occupies the female gametangium. This type of conjugation has been reported in S. gratiana.
• Terminal conjugation: A different process of conjugation has been described in S. colligatacalled terminal conjugation. In this method, conjugation tubes are produced on either side of the septum of the two conjugating cells. The male gamete enters the female gametangia by perforating the septum of the conjugation tube.
• Direct lateral conjugation: This type of lateral conjugation was reported in S. jogensis. The filament is attached to the substratum by its basal cell. Lateral conjugation takes place between the two cells placed immediately next to the basal cell. The protoplast of male cells pushes and pierces the septum between the two cells and the whole protoplast of the male cells moves into the female cell through the perforation. After fusion zygote is formed. It is believed that the secretion of an enzyme effects perforation.

ZYGOSPORES

The mature zygospores have a three layered wall; the outer smooth thin and cuticularized exosporium. The middle brown thick and ornamented mesosporium, and the inner thin endosporium. The mesosporium is made up of chitin while the remaining two are made up of cellulose. The mesosporium serves as a main distinguishing character for different species of Spirogyra.

GERMINATION OF ZYGOSPORE

Zygospore germinates after the onset of rains. During germination the diploid nucleus of the zygospore divides meiotically. As a result four haploid nuclei are formed. Of these, three degenerate and the remaining one enlarges. Meanwhile the two outer layers of the zygospore burst open and the endosporium comes out in the form of a cylindrical germ tube. Later, the germ tube divides transversely forming a two celled filament. The distal (upper) cell, by repeated transverse divisions, forms a long multicellular uniseriate filament. Thus in Spirogyra each zygospore on germination gives rise to a single filament.

PARTHENOGENESIS

Sometimes conjugation does not take place. The gametangia then are converted into thick walled bodies identical to zygospores. These bodies, formed parthenogenetically, are called azygospores or parthenospores. They germinate like zygospores to form new filaments but without the meiotic division.

5.  PHEOPHYCEAE

• Primarily marine in habitat, from simple branched filamentous Ectocarpus to very large Kelps (Upto 100 meters).
• They possess chlorophyll ‘a’ and ‘c’, carotenoid and xanthopylls, they are olive green to various shades of brown depending upon concentration of xanthopylls, fucoxanthin.
• Stored food is laminarin or mannitol.
• Inner wall is cellulosic and outer wall is gelatinous algin.
• The protoplast contains plastid, centrally located vacuole and nucleus.
• Plant body is divided into holdfast (helps in attachment), Stipe (Stalk) and frond (photosynthetic part).
• Vegetative reproduction by fragmentation, asexual reproduction by pear shaped biflagellated zoospores (flagella is unequal) which are laterally inserted.
• Sexual reproduction can be isogamous, anisogamous or Oogamous.
• Fusion of gametes can be   external or within the Oogonium (Oogamous species).
  E.g. Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

6.  RHODOPHYCEAE

• Red algae because of the predominance of the red pigment, r-phycoerythrin.
• Mostly marine and found in warmer areas.
• Mostly multicellular with complex body organization.
• Stored food is florideanstarch, which is similar to amylopectin and glycogen in structure.
• Reproduce vegetatively by fragmentation, asexually by non-motile spores, sexually by non-motile gametes.
• Sexual reproduction. Oogamous, with post fertilization development process.
• Polysiphonia, Porphyra, Gracilaria and Gelidium.

7.  ECONOMIC IMPORTANCE OF ALGAE

• 50% of CO₂ fixation on earth is by algae.
• They increase amount of dissolve O₂ in the surrounding environment.
• Being producer, aquatic food chain is based on it.
• Around 70 algae along with Porphyra, Laminaria, and Sargassum is used as food.
• Source of Hydrocolloids (Water holding substance) e.g., algin (pheophyceae) carrageen (red algae), Agar (Gelidium, Gracilaria) used as medium. Chlorella and Spirullina are unicellular algae rich in protein and used as food supplement.
• It also yields an antibiotic chlorellin. Chlorella can be used in prolonged space flights for food, oxygen, disposal of  and organic matter.
• Cephaleurosis parasitic on a number of plants. C. virescens causes red rust of tea whereas C. coffea is parasitic over coffee plants.

Divisions of Algae and their Main Characteristics

BRYOPHYTES

8.  GENERAL STUDY OF BRYOPHYTES

The term Bryophyta,(Gk. bryon: moss; phyton: plant), is used as a collective name to represent a group of plants that includes liverworts, hornworts and mosses growing predominantly in amphibious environment. The group, therefore, goes well with the name of amphibians of plant kingdomowing to the amphibious habitat of plants. The plants are characterized by the presence of conspicuous, green, well developed, nutritionally independent gametophytes to which are always attached physically and nutritionally dependent sporophytes. The gametophyte constitutes the dominant phase of life cycle that exhibits sharply defined alternation of generation.

• Lack true roots, stems or leaves.
• Unicellular or multicellular rhizoids are present.
• They are first embryophyta.
• Some cells of sporophyte undergo meiosis to produce haploid spore, which germinate to produce gametophyte.
• Diploid zygote is produced, which do not undergo immediate reduction division.
• Male gamete is biflagellate.
• Sex-Organ is multicelluar, male sex organ is antheridium and female sex organ is archegonium, which is flask shaped.
• Gametophyte is photosynthetic.
• Main plant body is gametophyte and sporophyte is dependent on it.

Gametophyte (The plant Body)

• The plants show two morphologically distinct heteromorphic generations, i.e., gametophytic and sporophytic generations.
• Gametophytic generation is the dominant phase of life cycle and in general the term plant body is used to represent this phase
• The gametophytes are well developed, green and autotrophic to which the sporophytes are not only attached but are also physically and physiologically (nutritionally) dependent.
• The plant body of primitive forms, e.g., Riccia and Marchantia is thalloid but in mosses. It is foliose and is differentiated into root like (rhizoids), stem like (axis) or cauloid and leaf like (phylloid) structures.
• The thalli of primitive forms are found attached to the substratum by unicellular unbranched rhizoids but in higher forms as in mosses these are attached by means of branched multicellular rhizoids. Rhizoids are absent in aquatic forms. In Marchantiales (e.g., Riccia, Marchantia etc.) often multicellular scales are present in addition to rhizoids. Scales/amphigastria take part in capillary action and protection.
• Bryophytes lack vascular tissues. The plant body consists of simple parenchymatous cells. Xylem, phloem and lignified cells are completely absent. The parenchymatous cells may be differentiated into several types e.g., chlorophyllous cells, storage cells, rhizoids, etc. to perform various functions, as in Riccia and Marchantia. Few thick walked cells called stereomare present in mosses. Mosses retain moisture like sponges.

Reproduction

Bryophytes reproduce by vegetative and sexual methods.

VEGETATIVE REPRODUCTION

Bryophytes largely multiply by means of vegetative reproduction which is accomplished by fragmentation, adventitious branches, tubers, persistent apices, buds, gemmae, rhizoids, primary protonema, secondary protonema, etc.

SEXUAL REPRODUCTION

• The sexual reproduction is invariably advanced oogamous type.
• Sex organs are multicellular and jacketed with sterile jacket. They may be embedded type, e.g., Riccia, Anthoceros or projected type, e.g., Marchantia, mosses, etc.
• The male reproductive organ is called antheridium.It consists of a central mass of androcytes enclosed by a single layer of sterile jacket. Each androcyte produces a single biflagellate spermatozoid or antherozoid.
• The female reproductive organ is called archegonium. Each archegonium is a multicellular and flask shaped structure. The basal swollen portion is called venter whereas slender and elongated upper portion is called neck.Wall of the neck is single layered made up of 6 rows of 6-8 cells each. Venter has 1-2 layered wall. The neck of archegonium is filled with 6-10 neck canal cells (4-6 in Riccia) and the venter has a large egg cell and small venter canal cell. The egg is large and non-motile.

FERTILIZATION

• Water is indispensable for fertilization (zooidiogamy).
• Many antherozoids swim to the neck to archegonium.
• All the neck canal cells and venter canal cell disorganise to form mucilage, carbohydrate, proteins, K+, etc. These chemicals not only provide the medium for swimming of antherozoids but also chemotract them. Many antherozoids enter into the venter but only one, the most active one, fuses with egg to form diploid zygote (oospore).
•  With the formation of diploid zygote, the gametophytic generation ends and the sporophytic generation starts.

The Sporophyte

• The zygote, immediately after fertilization, divides repeatedly without undergoing any resting period. The first division of zygote is transverse and the embryo proper develops from the outer cell. This type of embyrogeny is commonly known as exoscopic embryogeny.
• The embryo is not liberated but is retained within the archegonium where it develops into a sporophyte. The sporophyte in bryophytes is called sporogoniumbecause it is dependent and mainly meant for producing spores.
• The sporophyte consists of foot,seta and capsule. In a few cases only seta is absent as in Corsinia whereas in Riccia both foot and seta are absent. Wall of the venter proliferates to form a covering around the growing embryo. It is called calyptra.
• In a capsule, the spores are formed after meiosis. These spores (meiospores),that are all of one kind make the plants homosporous.

The Young Gametophyte

• The spore is the mother (first) cell of the gametophytic generation. .
• The spores are cutinized and non-motile. They are exclusively wind disseminated. The individual spores separate from the tetrad before they are discharged from the capsule.
• The spores germinate directly into the new gametophytic plants (e.g., Riccia and Marchantia), but in mosses they germinate into filamentous protonema from which are produced buds that give rise to a new plant. The protonema represents the juvenile stage of plant body.
• Thus, bryophytes have heteromorphic or heterologous alternation of generation.

CLASSIFICATION 

• Bryophytes are divided into three classes: Hepaticae (Liverworts), Anthocerotae (Hornworts), Musci (Mosses). However, ICBN recommended names of these classes as Hepaticopsida, Anthocerotopsida and Bryopsida.
• The recent classification of Bryophyta is a as follows:

9.  LIVERWORTS

• Present in moist and shady habitats, such as banks of streams, marshy ground, damp soil, bark of trees and deep in woods. E.g., Marchantia, Riccia.

• Liverworts are thalloid and thallus is dorsiventral which is closely appressed to the substratum.
• Some members like Pelia and Porella is leafy having tiny leaf like appendages in rows on the stem like structure.
• Asexual reproduction by fragmentation of thallus or by specialised structures called gemmae.
• Gemmae are green, multicellular, asexual buds, produced in small cup like receptacles called gemmacups, present on upper surface.
• When gemmae detaches from parent thallus, it germinates to form new individual.
• Sex organs are produced on same thallus or on different thallus.
• Sporophyte is differentiated into foot, seta and capsule.

10.  MOSSES
THE SPOROPHYTE of Funaria

• The mature sporophyte is a complex and highly elaborated structure differentiated into the foot, seta and capsule.
• Foot: It is a poorly developed, small, dagger like conical basal part of the capsule embedded in the apical tissue of the female shoot. It functions both as an absorbing and anchorage organ.
• Seta: It is a long, slender, tough and twisted reddish brown stalk like structure. It bears capsule at the top.
• Capsule: The capsule is highly organized, erect or pendent, pear shaped structure situated at the tip of long seta. It is chiefly concerned with the formation and dispersal of the spores. Externally it is differentiated into three well marked regions (i) sterile basal region the apophysis(ii) central fertile region, the theca and (iii) upper region the operculumenclosing peristome.

Apophysis: It appears as an expanded part of the seta. It constitutes the basal zone of the capsule that gradually narrows below. Its outermost boundary is surrounded by a single layered thick epidermis, interrupted at places by stomata. Each stomata has an annular guard cell with an opening in the middle that opens into a small air space. Within the epidermis is a broad spongy zone of sterile cells rich in chloroplast. The intercellular spaces occurring in between these chlorophyllous cells make it as an organ of active photosynthesis. The central tissue of this zone that consists of vertically elongated achlorophyllous cells and is collectively called as central strand.It is in continuation with the central strand of the seta below. Central strand is connected with columella by few bundles of filaments. The cells of which are thin walled, vertically elongated and fewer in number.

Theca: The central zone of the capsule situated in between the apophysis and the operculum is called theca.The wall of this zone is several cells thick and highly differentiated. A clearly defined row of cells form the outermost covering of the wall, the epidermis with few stomata. Internal to the epidermis are colourless compact parenchymatous cell arranged in 1 or 2 layers. It constitutes the hypodermis. The cells continuous with the inner side of the hypodermis constitute spongy layer. The spongy layer is also 1 or 2 cells thick. The cells of this zone are loosely arranged and contain chloroplasts. A large number of wide, cylindrical air spaces occurring within the capsule wall are formed in between transversely oriented strands of narrow, green elongated cellsthe trabeculaewhich connect the innermost wall of the capsule with the outer most wall of the spore sac. The spore sac on its outer side is surrounded by a single layer of cells (inner spores sac) representing the inner wall. Spore sac is a barrel shaped structure open at both the ends. It is situated in between the wall of the capsule and the columella and contains numerous spores. The central part of the theca, the columella looks like a solid cylinder composed of delicate, colourless compact parenchymatous cells. The columella is expanded above whereas its narrow basal part is connected with the central strand of the apophysis.

Operculum: The upper most part of the capsule is highly modified in relation to spore dispersal. It looks like an obliquely placed conical cap and is known as It consists of 4 or 5 layers of cells, of which the outer most layer with thick walled cells forms the epidermis. The thin parenchymatous cells of other layers occupy the major part of the operculum. The operculum is separated from the theca by a narrow circular construction below which occurs a rim or diaphragm of 2 or 3 layers of radially elongated cells. Just above this rim of theca lies the annulus that consists of 5 or 6 layers’ of’ cells, of which the upper most layer of elongated cells constitute the line along which the operculum separates from the theca.

Immediately underneath the operculum attached below the edge of the rim, lies the peristome consisting of two rings of long conical teeth one within the other. Each ring of peristome possesses 16 teeth. The teeth of inner ring of peristome are colourless and delicate with their bases covered by the outer peristomial teeth that are placed opposite to that of inner ones. The outer teeth are conspicuous, red in colour and possess thick transverse bands. The tapering distal ends of the outer peristome teeth and joined to a centrally placed disc of tissue. The opening of the spore sac is closed by these elaborately sculptured and highly hygroscopic teeth

Dehiscence of Capsule

In dry atmosphere, capsule begins to dry up loosing water from the thin walled cells. Thus a tension is developed which ruptures the spore sac. Thin walled delicate cells of the annulus break away, the operculum is thrown off and the peristome teeth are exposed. The outer peristome teeth, which by this time remain bent so as to cover the open spore sac, jerk and bend back dispersing the spores. The peristome teeth form a fringe around the mouth of the capsule. These by their hygroscopic movements assist in liberating the spores from the spore sac. The inner peristomial teeth usually do not exhibit hygroscopic movements but act as a further check over the spore sac.
The mechanical jerk exercised over the capsule by twisting and untwisting of the sets under the influence by hygroscopic movements further aids in the dispersal of spores. The presence of filaments of cells connecting theca and apophysis facilitate the wind effect on dispersal and at the same time provide mechanical support against wind. The whole capsule body bends easily.

The Young Gametophyte

• The spore: The spores are small 0.12–0.20 mm in diameter. Each spore is more or less spherical with a smooth surface. The spore wall is differentiated into an outer smooth, coloured exosporium and an inner colour less hyaline, smooth endosporium. Enclosed within the wall lies a single nucleus, chloroplasts and numerous oil globules.
• Germination of the spore: On reaching suitable moist substratum the spore germinates immediately. It absorbs water, swells and increases in size. The exosporium may rupture at one or both ends of the spores producing germ tube at one or both the ends, which are opposite to each other. Each of the germ tube develops a cross wall near the point of emergence and turns green. The germ tubes elongate, become septate and produce a filamentous protonema.
• The protonema branches freely and forms the green filaments. The filaments, which are erect or very close to the substratum, develop thick colourless cell walls and cross walls at right angles to the lateral walls and below the substratum and away from the light develop brown thin cell walls, oblique cross walls and a few inconspicuous chloroplasts or leucoplasts. These are known as rhizoidal filamentsand are primarily meant for fixing the protonema to the substratum. The rhizoidal branches when exposed to light behave as chloronemal filaments. The formation of these two type of filaments depends on the environmental conditions in response to which the protonema is extremely plastic. On account of variability in the morphological and genetic behaviour caused under the influence of environmental factors, they do not exhibit and stabilized pattern of growth. Soon after the formation of the buds near a cross wall towards the base of the chloronemal branches or even on the rhizoidal branches the growth of protonema stops. The buds after becoming 2-4 celled develop the tetrahedral apical cell with three cutting faces and cut segment to form the stem and three rows of leaves. The development of the rhizoids takes place from the base of the stem.
• The erect gametophores of the moss plants develop by the activity of the apical cell. The gametophores are usually very large in number even in the protonema formed by the germination of a single spore, become independent as soon as the protonema dies off. The germination of a large number of spores develop several protonema intermingled due to which a large number of gametophores are developed.
• Several gametophores are formed from each protonema of a spore and become independent shortly after its death. A dense growth of plants is seen because of this property.
• Gametophyte consists of two stage.

• Multicelluar, branched rhizoids are present.
• Vegetative reproduction by fragmentation and budding in secondary protonema.
• Sex-organs are produced on the apex of leafy-shoot.
• Sporophyte is divided into foot, seta and capsule, the sporophyte of mosses is more elaborate than that in liverworts.
• After meiosis spores are produced, they have elaborate mechanism of spore dispersal.

  

11.  ECONOMIC IMPORTANCE OF BRYOPHYTES

• Some of them serve as a food for herbaceous animals.
• Sphagnum is source of peat, which is used as  fuel.
• Used as packing material for trans-shipment of living material because of their capacity to hold water.
• Mosses along with Lichens are pioneer when succession is occurring on rock.
• As mosses forms dense mat on soil, they reduce the impact of falling rain drop and prevent soil erosion.

PTERIDOPHYTES

12.  GENERAL STUDY OF PTERIDOPHYTES

• First land vascular plant, also refered as vascular cryptogams.
• Found in cool, damp and shady places, some of them are also present in sandy area.
• Main plant body is a sporophyte which is differentiated into true root, stem and leaves.
• The leaves in pteridophyta are small (Microphylls) as in Selaginella or large (macrophylls) as in ferns.
• Sporangia is subtended by leaf like structure called sporophylls. In some cases sporophyll may form compact structure called strobill / cones (Selaginella, Equisetum).

• Gametophyte requires cool, damp and shady place to grow and the need of water for fertilization is responsible for limited spread of living pteridophytes.
• Spore germinates to form gametophyte which is inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid called prothallus.
• Male sex organ is antheridium and female is archegonium. Fertilization results in the production of diploid zygote which develops into sporophyte.
• Majority of pteridophyte is homosporous but some of them are heterosporous like Selaginella, Salvinia.

  

• Heterosporous pteridophyte has microspore and megaspore. Microspore germinates to produce male gametophyte and megaspore into female gametophyte.
• Female gametophyte is stored in parent sporophyte for variable period. The development of zygote into embryo takes place within the female gametophyte. It is a precursor of seed habit.

Stelar System in Pteridophytes

At one time the vascular bundle was considered the fundamental unit in the vascular system of the pteridophytes. Later a stelar theory was proposed by Van Tiegham and Douliot in 1886. According to this theory the primary structure of the stem and root are basically similar and the fundamental parts of each are the cortex and a central column, the stele, and that the two are separated from each other by the endodermis. According to the proponents of this theory, the endodermis constitutes the innermost layer of the cortex, and pericycle the outermost portion of the stele. The stele includes not only the primary vascular tissues but also the pericycle and pith (if present). The idea of the stelar theory was widely accepted by many scientists and proved to be useful in the comparative anatomical and phylogenetic studies of the vascular plants.

Classification of Peteridophyta

The Pteridophyta is divided into four classes, viz. Psilopsida, Lycopsida, Sphenopsida and Pteropsida, on the basis of organization of plant body including the nature of leaf, vascular system, and location of sporangia.

13.  DRYOPTERIS: LIFE CYCLE

• There are 150 species of Dryopteris; about 25 species have been reported in India. The most common species is Dryopteris fluxmas, which is commonly known as Beech fern or male shield fern or Hay scented fern. It is found in moist and shady places in tropical, sub-tropical and temperate regions.
• The plant body is a sporophyte, which is differentiated into root, stem and leaves. The roots are adventitious. The stem is dark brown underground rhizome growing obliquely. From upper surface of rhizome many leaves arise acropetally. The leaves are large, called fronds, and are bipinnately compound. The young leaves show circinate vernation. Persistent leaf bases of the dead leaves are present in the old part of rhizome. The younger part of rhizome and rachis are covered with multicellular brownish scales called ramenta.
  Reproduction takes place by vegetative methods and by spores.

VEGETATIVE REPRODUCTION

Vegetative reproduction takes place by adventitious budsthat develop on the rhizome. These buds give rise to new plants. Besides this, fragmentation of rhizome also helps in vegetative propagation.

SEXUAL REPRODUCTION
Spore Producing Organs

• Spores are formed within sporangia. They develop on the ventral (lower) surface of ordinary foliage leaves. The leaves bearing sporangia are known as sporophylls. Many sporangia arise irregularly from placental tissue developed at the tip of an ultimate vein. The sporangia of all developmental stages are aggregated in cluster called sori. These are arranged in two rows, one on each side of the main vein. Thus sori are sub marginal and discontinuous. Sorus is covered by a thin membranous shield like or kidney shaped outgrowth of the leaf, called indusium.

• Each sporangium develops from a single initial cell. The development is of leptosporangiatetype. Mature sporangium consists of a stalk and a body or capsule. The stalk is long, slender and multicellular. The capsule is lens shaped. The capsule wall is only one cell in thickness. A row of thin cells on one side of the capsule marks the line of dehiscence. It is called stomium. Besides this, a ring of modified cells, called annulus extends along the edge of the capsule. The cells of the annulus have thick radial and inner walls while the outer tangential walls are thin.
• Capsule encloses 8 or sometime 16 spore mother cells. Each of them divides meiotically and forms haploid spores. A mature sporangium so has 32 or 64 spores. The spores are the first cells of the new gametophytic generation. All the spores are similar in shape and structure, So, Dryopteris is a homosporous fern. The spores are asexual reproductive bodies.
• The spore wall is two layered: an outer thick ornamented exine and inner thin and smooth intine. Each spore has a single haploid nucleus.
• Dispersal of spores: Dispersal of spores takes place when the atmosphere is dry. In such conditions the indusium dries and shrivels and the sporangia are exposed to the dry atmosphere. The annulus and stomium help in the dehiscence of sporangium by a purely mechanical action. In dry atmosphere water evaporates from thin outer wall of the annulus which are pulled in and the thick radial walls contract. As a result, the annulus straightens out slowly. This causes the capsule to break open transversely at the stomium. In this process the upper half of the sporangial wall swings backward along with annulus in the form of a cup. At this stage almost all the spores are lodged in this cup. Later, due to continued drying, the annulus snaps back to its original position. This tosses the spores out into the air.

GAMETOPHYTE
The Prothallus

• The spore is the mother cell of gametophytic generation It germinates when temperature and moisture are suitable. As a result of moisture absorption, exine ruptures and intine comes out in the form of a germ tube. It develops chloroplasts and divides transversely to form a green filamentous structure resembling moss protonema. The germ tube attaches itself to the soil by rhizoids. At a very early stage in development, the uppemost cell of the filament divides and establishes an apical cell. It cuts off cells alternately on two sides till a heart shaped gametophyte is formed. Mature gametophyte is thin flat and green structure, approximately 1/4 inch in diameter. It is known as prothallus.The apical part of prothallus has an apical notch where the growing point is situated. The lobes of prothallus are only one cell in thickness whereas the central part lying immediately below the apical notch is several cell layers thick. Many one-celled rhizoids develop on the ventral surface of the thallus and serve as organs of attachment.

  

• All the cells of the prothallus, except rhizoids are green and so, the gametophyte is autotrophic.

 SEX ORGANS

The male and female reproductive structures are antheridiaand archegonia, respectively. The prothalli are strictly monoecious or homothallic and protandrous (antheridia maturing earlier than archegonia). Both the sex organs are produced on the ventral surface of the thallus.

Antheridia

The antheridia develop in the basal region of the prothallus, among the rhizoidal cushion. Mature antheridium is a dome shaped structure that projects beyond the surface of the prothallus. The wall of the antheridium is composed of only three cells. Two cells form a ring around antheridium and are known as first ring celland second ring cell. The third cell, which forms a cap of the antheridium, is known as cap cell.In each antheridium, there are usually 32 spirally coiled multiflagellate antherozoids. The antherozoids of ferns thus resemble with those of Cycas in their multiflagellate nature.

Archegonia

The archegonia are produced in the thickened portion of the prothallus,just behind the apical notch. Mature archegonium is a flask shaped structure. It has a basal enlarged venter that is deeply sunken in the tissue of the prothallus and a neck that project beyond the surface of the prothallus. The wall of the neck consists of four rows of neck cells. The venter has a large egg and a small venter canal cell. The neck canal of the young archegonium is occupied by an axial formation between the two neck canal cells and, therefore, almost invariably there is a single binucleate neck canal cell.

FERTILIZATION

Fertilization takes place when the antheridium absorbs water. This creates turgor force that pushes the cover cell of the antheridium and the antherozoids are set free. Almost at the same time the venter canal cell and the neck canal cell of the archegonium disintegrate to form a mucilaginous substance. It contains malic acid that attracts antherozoids towards the neck of the archegonium. This is a chemotactic movement. Many antherozoids swim into the neck of the archegonium, but only one of them fuses with the egg. The fusion results in the formation of a diploid zygote or oospore. The new sporophytic phase begins with the zygote.

Embryo and the Young Sporophyte

• Generally, many archegonia are fertilized but only one of them develops into embryo. So, only a single sporophyte is found attached to each prothallus. Soon after fertilization, the zygote secretes a thick wall and begins to grow. The first division of zygote is vertical, i.e., along the long axis of the archegonium. The resulting two hemispherical cells divide till eight cells are formed. These are arranged in two tiers of four cells each. It is called octant stage.The upper tier, known as epibasal tier, forms primary leaf (or first leaf) and primary root. The lower of the hypobasal tier forms foot and the embryonal stem. Subsequent divisions in the embryo are irregular. The foot, which is the first organ to develop, is a short, projecting Spherical mass of cells. It absorbs food, water and minerals. The young sporophyte is, totally dependent upon gametophyte in the beginning. The primary leaf, primary root and embryonal stem develop later.
• After the establishment of foot, the primary leaf and primary root, the embryo emerges from the archegonial wall. The embryonal stem forms rhizome that gives out a number of adventitious roots. The primary leaf is replaced by a bipinnately compound leaf. Thus, a new independent sporophyte is established and the prothallus dries up and disintegrates. The young sporophyte now manufactures its own carbohydrate food and absorbs water and minerals from the soil.

Alternation of Generation

• Dryopteris has a heteromorphic alternation of generation. There are two morphologically distinct phases in its life cycle: the sporophytic phase and the gametophytic phase, which alternate with each other. The sporophytic phase that forms spores is long lived and is differentiated into roots, stem and leaves. The gametophyte, on the other hand is relatively short lived and thalloid. It manufactures its organic food by itself. The sporophyte is dependent upon gametophyte when young but becomes independent as it attains maturity.
• The sporophyte is diploid. The diploid spore mother cells undergo meiosis and form haploid spores. The spore is the mother cell of the haploid gametophytic generation and forms gametophyte which produces haploid gametes. Haploid male and female gametes fuse to form diploid zygote, which is the mother cell of the sporophytic generation. The zygote grows into embryo and develops into sporophyte.

14.  SELAGINELLA: LIFE CYCLE

Selaginella, commonlyknown as spike moss or little club moss prefers moist, cool and shady places, though a few species (e.g., S. lepidophylla and S. rupestris) are xerophytes. The xerophytic species roll up to form ball-like structures (cespitose habit) during dry season but spread out and revive when sufficient moisture is available (e.g.,S. lepidophylla). Such plants are called resurrection plants. S. bryopteris is sold    in the market under the name “Sanjivani”.

External Features

• The main plant body represents the sporophyte (2n), and is an evergreen herb differentiated into root, stem and leaves.
• The stem is erect prostrate and dichotomously branched. Positively geotropic structures called rhizophores arise from the stem at the point of branching. Rhizophore bears adventitious roots at its tip. Rhizophores are non-green, leafless thread like structures. They lack root hair and root cap. The rhizophores develop exogenously from angle meristem (a special meristem) which lies at the point of branching. Since rhizophore resembles the stem in some characters, and the root in other characters, it was regarded as organ suigeneris (organ of independent origin) by Bower and Goebel.
• The leaves are small, simple, sessile called microphylls (being single-veined).
•  The leaves are ligulate (i.e., bear a small multicellular scale-like structure) at the base of the leaf on the adaxial side). The bulbous basal region of the ligule is made up of larger cells called glossopodium.

Internal Organisation

• Stem: Anatomically stem has an outer parenchymatous epidermis, a few layers of sclerenchymatous hypodermis, parenchymatous cortex and one or more steles. Thus the stem is polystelic with 1 to 16 steles depending upon the species (S. spinulosa – 1 stele; S. kraussiana – 2 steles; S. laevigata – polystelic). Each stele is a protostele and is surrounded by air space.
• The stele is connected to the cortex across the air space by radially elongated filament-like trabeculae. The trabeculae show casparian strips; hence, are believed to be endodermal cells.
• The xylem is diarch, exarch and made up of tracheids, though a few vessels are present in S. rupestris.

1. Root: Root has a layer of epiblema with occasional root hair. It is followed by cortex, endodermis, pericycle and a single monarch exarch xylem surrounded by C-shaped phloem.
2. Rhizophore: Rhizophore has an outer thick walled epidermis without root hair, sclerenchymatous hypodermis, thin walled cortex, endodermis, pericycle and phloem surrounding mesarch xylem on all sides.
3. Leaf: Leaf has an upper and lower epidermis with stomata. Mesophyll is not differentiated. Its cells contain one or more cup-shaped chloroplasts having granoids (having irregularly stacked thylakoids). One vascular strand with protostelic condition occurs in the mid-rib region. It is covered by bundle sheath.

Reproduction

Selaginella reproduces vegetatively and sexually.

VEGETATIVE REPRODUCTION

Vegetative reproduction occurs by fragmentation, bulbils (e.g. S. subdiaphana) and stem tubers (S. chrysocaulos).

SEXUAL REPRODUCTION

• It takes place by spores. Being heterosporous, Selaginella produce two types of spores microspores and megaspores produced-in microsporangia (producing numerous micro-spores) and megasporangia (producing usually only four megaspores).
• The microsporangia and megasporangia are borne on microsporophylls and megasporophylls, respectively, both of which aggregate to form cone (or strobilus) at the apices of branches.
• The sporangia are eusporangiate (developing from a group of initial cells).
• Each sporophyll is ligulate and bears only one sporangium at its base on the adaxial surface.
• Microsporangium consists of a multicellular stalk and a round/oval body covered by two-layered jacket. A single-layered tapetum present beneath the jacket, provides nourishment for developing spores. Many diploid microspore mother cells are formed in each sporangium which undergo meiosis to form numerous microspores.
• Megasporangium is large four-lobed structure (one megaspore per lobe) having two-layered jacket and a multicellular stalk. In S. monospora, only one megaspore survives. Megaspores are formed by meiosis in megaspore mother cell.

GAMETOPHYTIC GENERATION

• Microspore develops into male gametophyte which is highly reduced. At one stage it is 13-celled consisting of one prothallial cell plus 12 cells of the antheridium (8jacket cells + 4 primary androgonial cells.). The microspores are liberated at 13-celled stage. The primary androgonial cells divide (repeatedly to form 128 or 256 androcytes, each metamorphoses into an oval biflagellate antherozoid.
• The megaspore germinates in situ (even before dispersal) to form an autotrophic multicellular female gametophyte (female prothallus).
• After the release of megaspores at this multicellular stage, a few superficial cells act as archegonial initials each of which develops into an archegonium.
• Each archegonium has a short, projecting neck with venter embedded. The jacket of neck is one-cell thick and is made up of two tiers of four cells each. Cover cells/lid cells are absent.
•  Antherozoids swim through rainwater or dew towards archegonium to fuse with egg forming a diploid zygote that develops into embryo.
• The embryo is gradually pushed into the interior of female gametophyte (endoscopic embryo development).
• In some species of Selaginella (e.g., S.rupestris). Megaspore is not released from the megasporangium. Even the fertilization and embryo development occurs while the megaspore is still inside megasporangium (vivipary).

Gymnosperms

• Gymnos: naked, sperma: seed; this includes plants in which the ovules are not enclosed by any ovary wall and remain exposed before and after fertilization, i.e. seeds are naked.
• Smallest gymnosperm is Zamia and tallest is giant red-wood tree Sequoia.
• In Cycas coralloid roots are present, which shows association with N₂-fixing cyanobacteria Anabaena.
• In Pinus and other gymnosperms root shows association with fungal hyphae, this association is known as mycorrhiza.
• Stem is unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves are well adapted to withstand extremes of temperature, humidity and wind. In Conifers it is needle shaped to reduce surface area, thick cuticle and sunken stomata to reduce water loss.
• They are Hetrosporous.

• Male and female cone may be present on the same tree (Pinus) or on different trees as in Cycas.
• Male and female gametophyte do not have independent existence.

15.  GYMNOSPERMS
Economic Importance of Gymnosperms

Gymnosperms are of immense importance in nature and man’s economy. Their uses are discussed under the following headings:

RESIN

• Resins are plant exudates secreted in specialized ducts. These largely come from conifers as a result of tapping. They are insoluble in water and soluble in organic solvents and find extensive use in varnishes, paints, lacquers, medicines and in paper sizing. Various kinds of resins and their sources are given below.
• Rosin: Rosin (colophony) is obtained as residue after the distillation of pine oleoresin orturpentine. Indian turpentine is chiefly tapped from Pinus roxburghii, P wallichiana, P insularis and P merkusii. It is distilled to obtained rosin and turpentine oil. Rosin is used in paper sizing, varnish making enamels and in the preparation of plasters and ointments. Inferior grades of rosin are used in making yellow laundry soap, grease and oil, sealing wax, oilcloth, plastics, adhesive, insulator, insecticides, disinfectants, shoe polish, furniture and in several other commercial articles.
• Copal: Copal is tapped from Agathis australis as a green gum or candle gum. A. albaalso provides copal resin, the East Indian or Manila Copalwhich is used is spirit varnishes, in making linoleum, preparation of plastics, polishes and the articles for which rosins are used.
• Canada balsam: Canada balsam is obtained from Abies balsamea. It has a high refractive index as that of glass and is used as a mounting medium for microscopic objects and as a cementing agent for lenses in optical work.
• Sandarc: Sandarc is obtained from various Australian species of Callitris and from African Tetraclinis articulata. Resin of Australian types exude naturally from various species of Callitris and is used as metal varnish giving good lusture when applied in thin coats. It is also used as paper and leather varnish. In present day pharmaceuticals it is used as pill varnish. Cotton wool dipped in its alcoholic solution is used as temporary filling for teeth. In Arabian countries it is used as a remedy for diarrhoea. Resin of African type is obtained by tapping Tetraclinis articulata. It is pale yellow or orange and contains about 1 % volatile oil.
• Venice turpentine: Venice turpentine is obtained from Larix decidua. It is used for making special type of varnish and veterinary medicines. Jura turpentine is obtained from Picea abies and is used in making specific paints and varnishes.
• Essential oils: Various kinds of essential oils are obtained £rom different species of coniferous plants. Abies sachalinensis gives an essential oil ‘Japanese pine needle oil’that is used for making scented soaps. Similar oil obtained from Cedrus atlantica and C. deodara is also used in perfumery and in medicines to cure bronchitis, tuberculosis, skin diseases and gonorrhoea. Picea glauca yields a similar essential oil is used as a constituent of room sprays.

FATTY OILS

Fatty oils are extracted from various species of gymnosperm plants. Fleshy outer layer of the seed of Macrozamia yields oil that has properties similar to palm oil. Some species of Torreya (T. nucifera), Cephalotaxus(C. drupacea) and Pinus (P cembra) yield fatty oil from their seeds that are used as food and for paints. Oil obtained from the seeds of Gnetumulais used for illumination.

FIBRES

Stuffing fibres are obtained from ramantal hairs removed from the leaf bases of Macrozamia. Bark of Gnetumgnemonand G. latifoliumyields fibres of high tensile strength and are used for making ropes and fishing nets.

PAPER

Paper is made from wood pulp of some Indian species of coniferous plants (Picea smithiana, Pinus roxburghii, Abies pindrow, Cryptomeria japonica etc). Various species of Pinus provide newsprint almost all over the world. High grades of paper are made from the wood pulp of Abies balsamea and species of Picea and Tsuga. Pulp obtained from stem fibres of Gnetumgnemonis also used for making paper.

FOOD

The stem and the seeds of Cycas yield a starch sago or arrowroot. Kernels of Diooneduleare eaten after roasting and bread is made from the boiled seeds. The outer covering (sarcotesta) of the seeds of Dioonand Macrozamia is also eaten. Seeds of Pinus gerardiana(chilgoza) and other species of Pinus are edible and very nutritious. Seeds of Gnetumgnemon, G. latifoliumand G. ulaare also edible. Young leaves and strobili of G. gnemonare cooked as vegetable.

MEDICINES

Resin obtained from Cycas rumphiiis applied to ulcers. The juice of the tender leaves of Cycas is used for flatulence and vomiting and the seeds and bark in the form of a paste are used as poultice for sores and swellings. Cedrus deodara wood possesses diuretic and carminative properties and finds use 1n curing pulmonary disorders, piles and rheumatism. The oil obtained from Cupressussemipervirenspossesses vermifuge properties. Seeds of Pinus gerardianayield an oil that is applied as dressing to wounds and ulcers. The leaves of Taxusbaccataare used in asthma, bronchitis, cough, epilepsy and for indigestion and the seeds as sedative. Ephedra (E. intermedia, E. nebrodensisand E. gerardiana) contains good quantities of ‘ephedrine’that is used against asthma, hay fever and bronchial troubles. The fruit juice of Ephedra is applied to cure respiratory troubles.

ORNAMENTALS

Several species of Cycas are extensively grown as garden plants and for decorative purposes. Ginkgo. The maidenhair tree’ is grown as an ornamental plant in the temples in China and Japan and is worshipped. Thujaplicata, Biota orientalisand species of Juniperusare cultivated as ornamental trees throughout India especially in the planes. Species of Pinus and Aurocariaare also raised as ornamental plants in North India. Some species of Gnetumand Ephedra are also grown as ornamentals.

16.  ANGIOSPERMS

• Smallest angiosperm is Wolfiaand tallest is Eucalyptus.
• Source of food, fodder, fuel, medicines and several other economically important product.
• Divided into two classes: the dicotyledons and monocotyledons.
• Polar nuclei fuses to form diploid secondary nucleus.
• Pollen tube contains two male gametes, one fuses with egg to form zygote and other fuses with secondary nucleus to produce   triploid primary endosperm nucleus (PEN). This event is known as double fertilization.

•  Zygote develops into embryo and PEN into endosperm which provides nourishment to developing embryo.
• Synergids and antipodal cells degenerates after fertilization.
• Ovule develops into seed and ovary into fruit.

 17.  PLANT LIFE CYCLES AND ALTERNATION OF GENERATION

• In plants, both haploid and diploid cell can divide by mitosis.
• The haploid plant body produces gametes by mitosis, this plant represents gametophyte.
• After fertilization diploid zygote also divides by mitosis to produce sporophyte.
• As a result of meiosis in sporophyte haploid spores are produced.
• Syngamy and meiosis are two important factor which are responsible for alternation of generation.

Haplontic

• Sporophytic generation is represented by zygote (2n) only.
• No free living sporophyte.
• Meiosis is zygotic which results  in formation of spores.
• Spore undergoes repeated mitotic division to produce gametophyte, which is dominant photosynthetic phase in life-cycle.
  E.g., Volvox, Spirogyra, Chlamydomonas.

Diplontic

• Diploid sporophyte is dominant, photosynthetic and independent phase.
• Gametophyte is represented by single to few celled haploid gametophyte.
  E.g. All seed bearing plants, Fucus.

Haplo–Diplontic

• As in bryophytes and pteridophytes.
• In bryophytes, gametophyte is dominant and sporophyte is dependent on it.
• In pteridophyte sporophyte is dominant and gametophyte is dependent on it.
• Algae Ectocarpus, Polysiphonia and Kelps also shows this type of life-cycle.

18.  SOME IMPORTANT FACTS

• O.P. Iyengar is considered to be father of Indian phycology.
• Acetabularia, a green alga, is the largest unicellular plant.
• Ulva is known as sea lettuce.
• Spirogyra is commonly known as pond silk or pond scum or water silk.
• Volvox is called rolling algae.
• Chara is called as stone wort.
• Hydrodictyon is commonly called water-net.
• Sargassum is called Gulf weed.
• Chlorella a green alga, is more or less equivalent to soyabean and spinach having high proteins, lipids arid varieties of vitamins.
• Trumpet hyphae are found in brown algae and serve as conductive tissue like sieve tube. They conduct foodmaterials at the rate of 38-78 cm/hr.
• Is an antibiotic is obtained from Chlorella pyrenoidosa.
• Gongrosira stage of Vaucheria resembles palmella stage of
• Smallest gymnosperm is Zamia pygmea, which reaches a height of 25 cm.
• The tallest gymnosperm is Sequoia sempervirens with a height of 111 m.
• Cycas and Ginkgoaare regarded as living fossils
• Gnetumis considered as the most modern gymnosperms due its similarities with angiosperms.
• Transfusion tissue (hydrostereom) of Cycas leaflets is believed to represent vestigial internal lateral veins.
• Munkey puzzle is the common name of Araucaria imbricata.
• Cycas has largest male cone, antherozoids, ovule and egg.
• Male gametes of Cycas and Ginkgo are motile.The endosperm of Gnetumis polyploid.
• Medicinally most important gymnosperms are Taxusand
• In Gnetales, the ovules are bitegmie.
• Neck canal cells are absent in the-neck of archegonia of gymnosperms.
• Polyembryony is common in Cycas,Pinus also exhibits cleavage polyembryony.
• Welwitschiais peculiar in that it possesses only two large leaves with basal meristems.
• Gnetumis considered to be connecting link between gymnosperms and angiosperms.