NEET Biology Plant Kingdom Notes

NEET Biology Chapter 3 Plant Kingdom notes covering the classification systems of plants, Thallophyta and algae (Chlorophyceae, Phaeophyceae, Rhodophyceae), Bryophytes (liverworts, mosses such as Funaria), Pteridophytes, Gymnosperms, Angiosperms with dicot and monocot differences, and plant life cycles including haplontic, diplontic and haplo-diplontic patterns.

At a Glance

While going through the chapter pay special attention to the following.

Terms

1. Phenetics
2. Cladistics
3. Algin
4. Carrageen
5. Synergids
6. Antipodal cells
7. Algology
8. Floridean starch
9. Laminarin
10. Pyrenoids
11. Fucoxanthin
12. Frond

Differences

(i) Isogamous, anisogamous and oogamous fusion.
(ii) Chlorophyceae, Phaeophyceae and Rhodophyceae.
(iii) Monocotyledons and dicotyledons.
(iv) Haplontic, diplontic, and haplo-diplontic life cycles.

Plant Kingdom

The plant kingdom includes eukaryotic, autotrophic or photosynthetic and non-motile organisms. They have a well defined cellulosic cell wall and plastids are present.

The plant kingdom includes five major plant groups, i.e., Algae, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms, as proposed by R.H. Whittaker in 1969.

In earlier classifications the kingdom Plantae also included fungi and members of Protista and Monera having cell walls.

Types of Classification

There are various types of classification in plants.

1. Artificial system of classification: All taxonomists, from Aristotle to Linnaeus, classified organisms on the basis of external observable (morphological) characters such as the structure of floral parts (number of stamens), root modification, leaf venation etc. No weightage was given to natural and phylogenetic relationships. Such a system is based on one or a few superficial similarities and is an arbitrary system of classification. Linnaeus also used this system. It may also be based upon the habit and habitat of the organisms.

2. Natural system of classification: It uses more number of characters and is based upon natural affinities using homology and comparative study. Bentham and Hooker used this system of classification for angiosperms.

3. Numerical taxonomy (Phenetics) or quantitative taxonomy: This system uses numerical methods for evaluating the similarities and differences between species. It uses the maximum number of characters without giving extra emphasis on any one. Sophisticated calculating machines and computers are used for statistical analysis. This system gives equal importance and weightage to all the characters used for taxonomy. As this system was first attempted by Adanson (1763), numerical taxonomy is also known as the Adansonian system. Phenetics is considered to be a better classification as it uses a huge number of comparative features.

4. Phylogenetic classification (Cladistics): This system is based upon evolutionary relationships and uses morphological characters, origin and evolution of the different organisms. The family tree in this system is called a cladogram. This system has led to the emergence of new systematics and bio-systematics. The phylogenetic system of classification was proposed by Hutchinson. Adolf Engler and K. Prantl, both German botanists, published a phylogenetic system of classification in plants and considered monocots to be more primitive than dicot angiosperms. They published twenty volumes of classification (from algae to angiosperms) in Die Natuerlichen Pflanzenfamilien.

5. Karyotaxonomy: This system of classification uses information like chromosome number, structure of chromosomes, size and shape of chromosomes and the behaviour of chromosomes during meiosis.

6. Chemotaxonomy: This system is based on chemical products, particularly secondary metabolites. Various families of plants have been identified on the basis of raphides (crystals of calcium oxalate).

7. Experimental taxonomy: Here the relationship is determined on the basis of genetics, breeding experiments and the effect of captivity.

8. Bio-chemical taxonomy: It is based on the biochemistry of various chemicals such as hormones and pheromones.

Classification by Eichler and Engler

Eichler (1883) divided the plant kingdom into two subkingdoms mainly on the basis of the presence or absence of seeds.

Cryptogamae: Lower plants in which sex organs are hidden and seeds and flowers are absent. It includes Thallophytes, Bryophytes and Pteridophytes.

Phanerogamae: Higher plants in which sex organs are evident and seeds are present. It includes Gymnosperms and Angiosperms.

Engler (1886) divided plants into Thallophyta (plant body thallus-like and no embryo formation) and Embryophyta (zygote develops into a multicellular embryo).

Classification of Plants

• Plant Kingdom
  • Cryptogams (plants without seeds)
    • Thallophyta
      • Algae
    • Bryophyta
    • Pteridophyta
  • Phanerogams (plants bearing seeds)
    • Gymnosperm
    • Angiosperm
      • Monocots
      • Dicots

Thallophyta

It includes Algae, Fungi, Bacteria and Lichens. In the modern system of classification such as Whittaker (1969), Fungi, Lichens and Bacteria are excluded from this group and are placed in separate kingdoms.

Thallophytes are the simplest seedless, flowerless, non-vascular plants. The plant body is a thallus which is haploid (n) and gametophytic and does not show any differentiation into roots, stem or leaves. The vascular system is completely absent. The sex organs are unicellular and called gametangia. These are not jacketed by sterile cells.

Algae

Connecting Concepts: Morris is the father of algae. M.O.P. Iyengar is the father of Indian phycology. Linnaeus (1753) coined the term algae.

The branch of botany which deals with the study of algae is termed algology or phycology.

In Whittaker's classification, algae are grouped in three kingdoms: Monera (blue green algae), Protista (dinoflagellates, diatoms, euglenoids) and Plantae (green algae, red algae and brown algae).

Algae are haploid, gametophytic, eukaryotic, autotrophic, chlorophyllous, aquatic plants. Algae consist of a thallus-like plant body, i.e., there is no differentiation into roots, stem and leaves. Algae are plants because they have chlorophyll a, a cellulosic cell wall, starch as reserve food and absorption of food in liquid form. Chlorophyll a, carotenes and xanthophylls occur in all algae. Every algal group has some additional photosynthetic pigments, such as fucoxanthin and chlorophyll c in brown algae, phycobilins in red algae, and chlorophyll b and beta-carotene in green algae.

Connecting Concepts: Algae are the primary producers of food and occupy three-fourths of the surface of earth. They account for 90% of total photosynthesis. Algae absorb food in liquid form.

Every algal group has a specific reserve food, e.g., floridean starch (red algae), laminarin (brown algae) and starch (green algae).

Asexual reproduction occurs by unicellular mitospores. Sex organs are unicellular, non-jacketed and called gametangia. In multicellular reproductive organs (in some brown algae), all the cells are fertile and take part in reproduction. There is no jacket of sterile cells surrounding the reproductive organs.

Fertilization requires water. The zygote sheds and develops into a perennating, thick walled zygospore. The zygote never develops into an embryo. True alternation of generations is absent (exception: Ectocarpus, which shows isomorphic alternation of generations). The life cycle is mostly of the haplontic type.

Algae do not require mechanical tissue because buoyancy holds them upright.

The size ranges from microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox, and to filamentous forms like Ulothrix and Spirogyra.

Reproduction in Algae

Vegetative reproduction: by fragmentation, where each fragment develops into a thallus.

Asexual reproduction: by spores such as zoospores, aplanospores, autospores, akinetes and endospores. The most common is the zoospore, which is flagellated.

Sexual reproduction: by fusion of gametes, which may be isogamous, anisogamous or oogamous.

Isogamous fusion: gametes can be flagellated or non-flagellated but are similar in size. E.g., Ulothrix, Chlamydomonas, Chlamydomonas debaryana.

Anisogamous fusion: gametes are dissimilar in size. E.g., Chlamydomonas bravni (sic; correctly braunii).

Oogamous fusion: a fusion between one large, non-motile female gamete and one small but motile male gamete. E.g., Volvox, Fucus, Chlamydomonas coccilera (sic; correctly coccifera).

Economic Importance of Algae

At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.

Algae are primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.

Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae), which are used commercially.

Agar, one of the commercial products obtained from Gelidium and Gracilaria, is used to grow microbes and in the preparation of ice creams and jellies.

Chlorella and Spirulina are unicellular algae rich in proteins and are used as food supplements by space travellers.

Classes of Algae

The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.

Chlorophyceae (Green Algae)

Green algae are mostly freshwater (Spirogyra, Ulothrix, Chara, Zygnema, Volvox, Chlamydomonas) and hardly 10% are marine (e.g., Acetabularia, Ulva, Caulerpa etc.).

The plant body may be unicellular (e.g., Chlamydomonas), colonial (e.g., Volvox) or filamentous (Spirogyra, Ulothrix).

They are green due to the dominance of the pigments chlorophyll a and b. The pigments are localised in chloroplasts. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.

Starch is the reserve food, stored as sheets in pyrenoids present in the chloroplast.

They have a rigid cell wall made of cellulose and pectose. Vegetative reproduction usually takes place by fragmentation. Asexual reproduction takes place by zoospores produced in zoosporangia.

Sexual reproduction involves the formation of haploid gametes, their fusion to form a diploid zygote, followed by meiosis to form meiospores which on germination produce a new haploid gametophytic plant. Sexual reproduction shows considerable variation in the type and formation of sex cells and may be isogamous, anisogamous or oogamous.

Connecting Concepts: Green algae are regarded as the ancestors of land plants because both green algae and land plants possess chlorophyll a and b, beta carotenoids, cellulosic cell walls and starch as reserve food.

Common examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.

Connecting Concepts: The Sargasso Sea (called the oceanic desert) in the North Atlantic is named so due to the abundant occurrence of free floating Sargassum fluitans, which is a menace to shipping. Algae Chlamydomonas snowiae and C. nivalis inhabit ice or snow and make it appear red. The Red Sea is part of the Mediterranean Sea, where a blue green alga Trichodesmium erythraeum grows on the surface and imparts a red colour.

Fig. 3.1 Green algae (labelled parts given as text):

• (a) Volvox: Daughter colony, Parent colony
• (b) Chara: Branches, Axis
• (c) Chlamydomonas: Flagella, Apical papilla, Contractile vacuoles, Basal granules, Eye spot, Cell wall, Nucleus, Volutin grains, Chloroplast, Pyrenoid, Starch

Phaeophyceae (Brown Algae)

These are brown coloured marine eukaryotic algae. They show great variation in size and form. They range from simple branched filamentous forms (Ectocarpus) to profusely branched forms such as the kelps, which may reach a height of 100 metres (the largest marine plants).

They possess chlorophyll a and c, carotenoids and xanthophylls, and a characteristic brown pigment fucoxanthin. Chlorophyll b is absent.

They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment fucoxanthin present in them.

Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol.

The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. This prevents drying of the plant during low tide, freezing of the plant in water in winter, and protects cells when waves beat the plant against rocks.

The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus. Thylakoids are found in groups of three to five in the cytoplasm (not in chloroplast) as chromatophores. Pyrenoids are naked.

The plant body is usually attached to the substratum by a holdfast and has a stalk, the stipe, and a leaf-like photosynthetic structure, the frond.

Fig. 3.2 Brown algae (labelled parts given as text):

• (a) Laminaria: Frond, Stipe, Holdfast
• (b) Fucus: Air bladder, Frond, Midrib, Holdfast
• (c) Dictyota: Frond, Stipe

Vegetative reproduction takes place by fragmentation.

Asexual reproduction in most brown algae is by biflagellate zoospores that are pear shaped and have two unequal, laterally attached flagella.

Sexual reproduction may be isogamous, anisogamous or oogamous. Union of gametes may take place in water or within the oogonium (oogamous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella.

Common brown algae are Ectocarpus, Dictyota, Laminaria, Fucus and Sargassum.

Rhodophyceae (Red Algae)

They are commonly called red algae because of the predominance of the red pigment r-phycoerythrin in the body.

The majority of red algae are marine, with greater concentrations found in warmer areas. They occur both in well-lighted regions close to the surface of water and at great depths in oceans where relatively little light penetrates.

When growing deep, red algae appear red due to the excess formation of r-phycoerythrin, which captures short wavelength (blue green part of the visible spectrum) light that is able to reach the maximum depth in water. These algae therefore reach the maximum depth (30 to 90 m) in the sea where no other photosynthetic organism grows. They appear greenish, bluish green or purple when growing near the surface of the sea due to little production of phycoerythrin. This property of colour change is called chromatic adaptation.

The thalli of most red algae are multicellular. Some have complex body organisation.

Batrachospermum is the only freshwater red alga, found growing in well aerated water, but it is not red.

The food is stored as floridean starch, which is very similar to amylopectin and glycogen in structure.

Red algae reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Flagellated cells are totally absent in the life cycle.

Sexual reproduction is oogamous and is accompanied by complex post fertilisation developments.

Common members are Polysiphonia, Porphyra, Gracilaria and Gelidium.

Fig. 3.3 Red algae (labelled parts given as text):

• (a) Porphyra: Frond
• (b) Polysiphonia: Main axis, Branches

Divisions of Algae and their Main Characteristics

Check Point

Fill in the blanks:
(i) Red colour of red algae is due to pigment _________.
(ii) The largest unicellular and uninucleate algae is _________.
(iii) In brown algae, the plant has three parts _________, _________ and _________.
(iv) Agar is commercially obtained from _________.
(v) In brown algae, food is stored as _________.

Bryophytes

Connecting Concepts: Bryophytes do not grow tall. The average size is hardly 5 to 6 cm. Greater heights are rare because bryophytes (i) do not have roots which are required to absorb water from the soil and provide proper fixation in the soil; (ii) lack an efficient internal transport system or vascular system; and (iii) lack mechanical tissues.

Bryology is the study of bryophytes. Father of Bryology: Hedwig. Father of Indian Bryology: S.R. Kashyap. Connecting Concepts: Gold mines of liverworts are in the Western Himalayas.

Bryophytes originated in the Ordovician period about 420 to 500 million years ago. The age of bryophytes is the Silurian period of the Palaeozoic era.

They usually occur in damp, humid and shaded localities.

The plant body of bryophytes is more differentiated than that of algae. It is thallus-like, prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stems or leaves. They may possess root-like, leaf-like or stem-like structures.

Bryophytes are adapted to grow under land conditions by having rhizoids for fixation; a waxy covering on the epidermis to reduce loss of water and protect against desiccation; stomata or air pores for gaseous exchange; multicellular jacketed sex organs to protect gametes against the drying effects of air; and retention of the zygote inside the archegonium to provide nourishment to the developing embryo.

Bryophytes are fundamentally terrestrial plants but they require water for dehiscence of antheridia, liberation and swimming of antherozoids, fertilization of the egg, opening of the archegonial neck and entry of sperms into the archegonium. Because of this peculiar habitat they are appropriately called the amphibians of the plant kingdom.

Bryophytes completely lack a vascular tissue system and they absorb water by the general body surface.

The main plant body of the bryophyte is haploid. It produces gametes, hence it is called the gametophyte. The gametophyte is dominant over the sporophyte. The gametophyte is green, branched, haploid, long lived and independent, whereas the sporophyte is diploid, short lived and dependent upon the gametophyte.

The sex organs in bryophytes are multicellular. The male sex organ is called the antheridium and produces biflagellate motile antherozoids. The female sex organ, called the archegonium, is flask-shaped and produces a single non-motile egg.

The antherozoids are released into water where they come in contact with the archegonium. An antherozoid fuses with the egg to produce the zygote. Zygotes do not undergo reduction division immediately. They produce a multicellular body called the sporophyte.

The sporophyte is not free-living but is attached to the photosynthetic gametophyte and derives nourishment from it. Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores in the sporogonium. The sporogonium is short lived and is differentiated into either foot, seta and capsule, or capsule only.

All spores are alike (homosporous). These spores germinate to produce the gametophyte.

Asexual reproduction is absent.

Vegetative reproduction takes place by fragmentation of the thallus, tubers (Riccia, Anthoceros), gemmae (Funaria, Marchantia), protonema (mosses), adventitious branches (Pellia), progressive death and decay of the posterior older region of the thallus, and persistent growing apices (liverworts).

Sexual reproduction is oogamous. The male gamete is motile whereas the female gamete is non-motile. Archegonia attract sperms by secreting mucilage rich in potassium salts, proteins and sucrose.

Fig. 3.4 Life cycle of Bryophyte (labelled parts given as text): Young gametophyte, Protonema, Spores, Capsule, Operculum, Calyptra; Antheridial head with Antheridium and Sperm; Archegonial head with Egg cell and Venter; Zygote; Early embryo; Developing calyptra; Sporophyte. The cycle runs: spores germinate into protonema and young gametophyte, which bears antheridial and archegonial heads; sperm fertilises the egg to form the zygote, which develops through the early embryo into the sporophyte whose capsule releases spores.

Economic Importance of Bryophytes

Some mosses provide food for herbivorous mammals, birds and other animals.

Species of Sphagnum provide peat, which has long been used as fuel. Because of their capacity to hold water, they are used as packing material for the trans-shipment of living material.

Mosses along with lichens are the first organisms to colonise rocks and are therefore of great ecological importance. They decompose rocks, making the substrate suitable for the growth of higher plants. In other words, they initiate the conversion of rocks into soil.

Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

The Bryophytes are divided into the following classes: Liverworts, Mosses.

Liverworts (Marchantia)

Liverworts usually grow in moist, shady habitats such as banks of streams, marshy ground, damp soil and the bark of trees.

The plant body is haploid (n), gametophytic, small, dorsoventrally flattened and thallose, and is dichotomously branched. It is fixed by unicellular and unbranched rhizoids.

On the dorsal side, gemma cups are found. In gemma cups, asexual buds called gemmae are formed. On the ventral side, scales are found for retaining moisture and protecting the growing apex.

Water is absorbed by the general surface of the thallus. Internally, the thallus is differentiated into an upper photosynthetic region and a lower storage region. The chlorophyllous cells of the thallus are without pyrenoids.

Asexual reproduction in liverworts takes place by fragmentation of thalli, or by the detachment of gemmae from the parent body and their germination to form a new individual.

Check Point

Name any one function of these structures:
(i) Gemmae
(ii) Rhizoids
(iii) Sporogonium
(iv) Seta
(v) Capsule

Fig. 3.5 Bryophytes Marchantia (a) Female thallus (b) Male thallus (c) Funaria gametophyte and sporophyte (d) Sphagnum. Labelled parts:

• (a) Archegoniophore, Gemma cup, Rhizoids
• (b) Antheridiophore, Gemma cup, Rhizoids
• (c) Sporophyte (Capsule, Seta); Gametophyte (Leaves, Main axis, Rhizoids)
• (d) Antheridial branch, Branches, Archegonial branch

During sexual reproduction, male and female sex organs are produced either on the same or on different thalli. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule. These spores germinate to form free-living gametophytes.

Moss (Funaria)

Mosses are either monoecious or dioecious. The common Indian species Funaria hygrometrica is monoecious. Mosses are found in moist, shady places, in patches on damp soil, moist rocks, burnt humid soil, damp walls or on tree trunks.

Gametophyte

The main body of Funaria is the gametophyte, which has two forms.

Juvenile form (creeping protonema): Protonema is a green, filamentous, branched structure. From the protonema, some branches grow below the substratum. These branches have no chloroplasts and are known as rhizoidal branches. They fix the protonema in the soil and help in the absorption of water and minerals. Green branches develop buds that grow into leafy gametophores.

Adult form (leafy gametophore): This is the main plant body of the moss, consisting of a small erect axis (stem) surrounded by spirally arranged leaves and numerous rhizoids at the base. The leafy gametophore develops from buds produced on the protonema.

Reproduction

Vegetative reproduction: The gametophyte reproduces vegetatively by fragmentation of the primary protonema, secondary protonema, gemmae and bulbils.

Sexual reproduction: Funaria is monoecious and autoicous, i.e., antheridia and archegonia occur on different branches of the same plant. The male branch is the main branch and develops first. The antheridia mature first, i.e., it is protandrous. The sex organs are borne on the apices of fertile branches called gametophores or receptacles. The receptacle bearing antheridia is called the antheridiophore and the one bearing archegonia is called the archegoniophore.

Sporophyte

The diploid zygote develops into the spore producing plant called the sporophyte. The mature sporophyte is differentiated into foot, seta and capsule.

Foot: The foot remains embedded in the apical tissue of the archegonial head and obtains nourishment from it. It functions as an anchorage and absorbing organ.

Seta: It is a long, slender, stalk-like structure bearing the capsule at the top.

Capsule: The capsule is pear shaped, green, yellow or orange, and erect or pendent. Its function is the production and dispersal of spores. It consists of three parts: apophysis, theca and operculum.

Apophysis: It is the basal sterile region.
Theca: It is the central fertile region. It consists of a spore sac bearing numerous diploid spore mother cells that divide to give a large number of haploid spores.
Operculum: It is the upper region of the capsule enclosing the peristome.

Just below the operculum lies the peristome, consisting of two rings of long conical teeth, one within the other. Each ring of the peristome possesses sixteen teeth.

Dehiscence of capsule: The spores are dispersed due to the hygroscopic movements of the outer ring (exostome) of peristomial teeth. The inner ring (endostome) of peristomial teeth does not show hygroscopic movements.

Germination of spores: On getting a suitable moist substratum, the spore germinates to produce a young filamentous stage of the gametophyte called protonema.

Common examples: Funaria, Polytrichum and Sphagnum.

Fig. 3.6 Funaria: Life cycle (labelled parts given as text):

• Gametophyte (n): Spore tetrad, Spore, Germination of spore (Germ tube), Primary protonema with bud, Bud, Gametophyte (leafy gametophore) with Leaves; Archegonium (Lid cell, Neck canal cells, Venter canal cell, Egg, Stalk); Antheridium (Opercular cells, Antherozoids, Jacket, Stalk); Egg inside mature archegonium; Antherozoid with Cilia.
• Sporophyte (2n): Zygote or oospore; 2-celled zygote (Epibasal, Hypobasal); apical cell (capsule + upper part of seta) and basal cell (foot + lower part of seta); Differentiation of apical cells; Embryo; Sporophyte attached to gametophyte (Calyptra, Capsule, Seta, Leaves, Rhizoids, Axis); L.S. capsule (Calyptra, Peristome, Spore mother cell, Columella, Lacuna, Seta); Spore mother cell, which gives the Spore tetrad.

Pteridophyta

Pteridophytes are found in cool, damp, shady places, though some may flourish well in sandy soil conditions.

Evolutionarily, they are the first terrestrial plants to possess vascular tissues, xylem and phloem. They are therefore known as vascular cryptogams. They are flowerless and seedless plants.

The main plant body is a sporophyte differentiated into true root, stem and leaves. These organs possess well differentiated vascular tissues. A cambium is altogether absent. In xylem, trachea are absent, and in phloem, companion cells are absent.

The primary root is short lived and is replaced by adventitious roots.

The leaves in pteridophytes are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.

The stem is usually an underground rhizome or an erect trunk as in tree ferns. Leaves are large (megaphyllous) and variously shaped.

The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases sporophylls may form distinct compact structures called strobili or cones (Selaginella, Equisetum).

The sporangia produce spores by meiosis in spore mother cells. Sporangia are borne on the abaxial side of fertile leaves. They are borne either singly or in groups called sori (singular: sorus). The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothalli.

Connecting Concepts

Theophrastus is the father of pteridophytes, botany and ecology.
Selaginella bryopteris is Hasanpadi of Ayurved or Sanjivani Booti.
Selaginella rupestris is the bird's nest moss.
Lycopodium is famous as Christmas green.
Sorrow of Kashmir: the aquatic weed Salvinia.
Ophioglossum reticulatum, commonly called Adder's Tongue fern, has the maximum number of chromosomes in Plantae, i.e., 2n = 1262.
Azolla-Anabaena show symbiosis and Azolla is used as a biofertilizer in rice fields.
Smallest pteridophyte: water fern Azolla. Tallest pteridophyte: Alsophila. Rootless pteridophyte: Salvinia (root-like structures are modified leaves).
Pteridology is the study of pteridophytes.
Xylem vessels are present in the stem of Equisetum.
Isoetes is a pteridophyte with secondary growth, commonly called quillwort.
Pteridophytes originated in the Silurian period of the Palaeozoic era. The golden age of pteridophytes is the Devonian period.

Fig. 3.7 Pteridophytes (labelled parts given as text):

• (a) Selaginella: Leaves, Stem, Roots
• (b) Equisetum: Strobilus, Node, Internode, Branch, Rhizome

Pteridophytes show the origin and evolution of the stele (i.e., vascular tissue, pericycle and pith).

These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited to narrow geographical regions.

The gametophytes bear male and female sex organs called antheridia and archegonia, respectively.

Antheridia (male sex organs) produce motile sperms (biflagellated in Selaginella and multiflagellated in ferns).

Archegonia (female sex organs) are sessile and flask shaped with a neck and swollen venter. The neck is made up of four vertical rows of cells, each row with 2 to 6 cells. The venter is embedded and has no wall of its own, and encloses a non-motile ventral canal cell and an egg cell (oosphere).

Sexual reproduction is oogamous. Water is required for the transfer of antherozoids, the male gametes released from the antheridia, to the mouth of the archegonium. Fusion of the male gamete with the egg present in the archegonium results in the formation of the zygote, which represents the first cell of the sporophyte.

The zygote thereafter produces a multicellular, well differentiated sporophyte, which is the dominant phase of pteridophytes.

The life cycle is diplohaplontic and the alternation of generations is of the heteromorphic type.

Genera like Selaginella and Salvinia produce two kinds of spores, macro (large) and micro (small) spores, and are known as heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively.

The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos takes place within the female gametophytes. This event is a precursor to the seed habit and is considered an important step in evolution.

Connecting Concepts

Apospory: Formation of a gametophyte directly from cells of a sporophyte without the formation of spores. It is induced by excess salts, injury and dim light. The aposporous gametophyte is diploid. It produces diploid gametes which can introduce polyploidy in the race. Apospory is also a means of shortening the life cycle by producing diploid sporophytes parthenogenetically from diploid oospheres.

Apogamy: Formation of a sporophyte directly from a gametophyte without the formation and fusion of gametes. It is induced by partial desiccation, ageing, strong light, high temperature, the presence of sugars and ethylene. Haploid sporophytes do not survive, therefore apospory is successful only if the sporophyte develops from a diploid gametophyte formed either through apospory or from diploid spores produced due to failure of some division prior to sporogenesis.

Classification of Pteridophytes

Pteridophytes have been divided into the following four subphyla.

Subphylum I: Psilophyta. These plants represent rootless sporophytes which mainly constitute fossils. The example of a living form is Psilotum. Vascular tissue is present only in the stem, and rhizoids are present for absorption of water and minerals.

Subphylum II: Lycophyta. These are club mosses, differentiated into root, stems and leaves. Vascular tissue is found in all organs of the plant.

Subphylum III: Arthrophyta. The horsetails or scouring rushes belong to this group. Silica particles are deposited in their stems, e.g., Equisetum (horsetail).

Subphylum IV: Filicophyta. Usually fan shaped leaves. Stems usually rhizome. Homosporous, e.g., Dryopteris, Pteris, or heterosporous, e.g., Marsilea.

Gymnosperms

Theophrastus in 300 BC introduced the term gymnosperm. Gymnosperms are plants which bear naked seeds, i.e., the ovules and the seeds that develop from these ovules after fertilisation are not enclosed in a fruit wall.

They act as a bridge between pteridophyta and angiosperms.

Gymnosperms include medium-sized trees, tall trees and shrubs. Gymnosperms are represented by only about 900 species, of which 500 species are conifers.

Tallest gymnosperm: Sequoia sempervirens (111.6 m), commonly called Red Douglas Fir or Red Wood Tree or Californian Fir. Father of forest: Sequoia sempervirens (redwood tree).

Smallest gymnosperm and smallest cycad: Zamia pygmaea (26 cm). Largest cycad: Macrozamia (10 m).

Most plants are perennial and woody. Plants possess well developed tap root systems. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in others (Cycas) small specialised roots called coralloid roots are present. These are associated with N2-fixing cyanobacteria.

The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound. In Cycas the pinnate leaves persist for a few years. The leaves in gymnosperms are well adapted to withstand extremes of temperature, humidity and wind. In conifers, the needle-like leaves reduce the surface area, and their thick cuticle and sunken stomata also help to reduce water loss.

Vascular tissues are arranged in vascular bundles. In most gymnosperms, secondary growth occurs and annual rings are distinct. The xylem is without vessels (except in Gnetales) and the phloem lacks companion cells.

The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia borne on sporophylls. These are arranged spirally along an axis to form compact strobili or cones in order to protect their ovules.

Male strobili: The strobili bearing microsporophylls and microsporangia are called male strobili. The microspores develop into a male gametophytic generation which is highly reduced and confined to a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia.

Female strobili: The cones bearing megasporophylls with ovules or megasporangia are called female strobili. The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form female cones. The megaspores enclosed within the megasporangium (nucellus) develop into a multicellular female gametophyte known as endosperm. Thus endosperm is a pre-fertilisation tissue in gymnosperms. It bears two or more archegonia (female sex organs). The multicellular female gametophyte is also retained within the megasporangium.

In gymnosperms, male and female gametophytes do not have an independent free-living existence. They remain within the sporangia. The pollen grain is released from the microsporangium, is carried in air currents and comes in contact with the opening of the ovules borne on megasporophylls.

Following fertilisation, the zygote develops into an embryo and the ovules develop into seeds. These seeds are not covered.

Usually polyembryony is found in seeds, although ultimately only one embryo survives. The embryo bears two cotyledons, as in Cycas, or more (10 to 11 in Pinus).

Cycas as a living fossil: Cycas is a living representative of a once large group of gymnosperms which has become largely extinct. It is unable to compete with more evolved angiosperms except in a few selected arid localities where other plants do not flourish. Its primitive characters include: (i) slow growth; (ii) shedding of seed when the embryo is still immature; (iii) little secondary growth; (iv) leaf-like megasporophylls; (v) flagellate sperms even when a pollen tube is present; (vi) persistent leaf bases; (vii) circinate ptyxis; (viii) arrangement of microsporangia in ill-defined sori.

Other notes: Ephedra has the largest pollen chamber. Cycas, Ginkgo and Metasequoia are living fossils. Largest ovule: Cycas. Largest egg cell: Cycas. Largest antherozoid: Cycas. Ciliated sperms: Cycas and Ginkgo. Double fertilisation is absent in gymnosperms but present in Ephedra. There is no female cone in Cycas. The male or female cones (strobili) may be borne on the same tree (Pinus) or on different trees (Cycas). Archegonia lack neck canal cells. None of the gymnosperms is annual or herbaceous. Gnetum and Welwitschia have no archegonium. Ephedra is a connecting link between angiosperms and gymnosperms.

Fig. 3.8 Cycas circinalis: (A) Female plant, (B) Male plant. Labelled parts: Foliage leaves, Stem, Old leaf bases (A); Male cone, Stem (B).

Connecting Concepts: Classification of Gymnosperms

Gifford and Foster in 1989 divided gymnosperms into seven major taxa:
Progymnospermatophyta (extinct).
Pteridospermatophyta (extinct).
Cycadophyta (plants are called cycads).
Cycadeoidophyta (extinct).
Ginkgophyta (extinct, except the maidenhair tree, Ginkgo biloba).
Coniferophyta (extinct and living plants are commonly called conifers).
Gnetophyta (living plants close to angiosperms, having Gnetalian vessels and a flower-like arrangement of sporophylls).

Check Point

State True or False:
1. In gymnosperms ovules change into seeds after fertilization.
2. Endosperm is the female gametophyte in Cycas.
3. Two or more archegonia act as female sex organs in gymnosperms.
4. Gymnosperms are mostly shrubs and ephemerals (short lived).
5. Archegonia lack neck canal cells in Cycas.

Angiosperms (Flowering Plants)

Angiosperms appeared about 130 million years ago in the Jurassic (dicot plants) and Cretaceous (monocot plants) periods of the Mesozoic era. The Tertiary period of the Cenozoic era is called the age of angiosperms.

Angiosperms are found in every habitat and constitute more than 50% of the total plants on earth. Connecting Concepts: Marine angiosperms are Zostera marina and Thalassia. The National Tree of India is Ficus religiosa, i.e., Peepal, also called the Enlightenment Tree or Bodhi Tree. The Flame of the Forest is Butea monosperma, due to its reddish flowers.

Angiosperms are seed bearing, flowering vascular plants in which seeds are enclosed in fruits. The term angiosperm means 'enclosed seeds', as seeds (ovules) are found enclosed in the ovary wall.

Instead of cones, angiosperms bear flowers. The flower is the most characteristic structure of angiosperms. A flower is defined as a modified shoot meant essentially for the reproduction of the plant. The flower usually develops as a branch from a bud, growing in the axil of a small leaf-like structure known as a bract. A stalk called the pedicel supports the flower in the axil of the bract. The upper swollen end of the pedicel, called the receptacle or thalamus, bears all the floral parts.

A typical angiospermic flower consists of four whorls: calyx, corolla, androecium and gynoecium. Both calyx and corolla are known as accessory or helping whorls. The androecium and gynoecium are called reproductive or essential whorls.

The androecium (stamen) is the microsporophyll and the gynoecium (carpel) is the megasporophyll. The megasporophyll is differentiated into ovary, style and stigma.

Angiosperms lack both archegonia and antheridia. The archegonium is replaced by the gynoecium (pistil) and the antheridium by the pollen tube.

The outermost whorl of the flower is the calyx, represented by the sepals. The corolla is the second whorl, present inner to the calyx, and is composed of petals. The bright colour of the petals combined with the scent of essential oils present in some flowers make the flower highly attractive to insects, which act as agents for pollination. Flowers are adapted for various modes of pollination by birds, air, water, insects, etc.

Angiosperms are an exceptionally large group of plants occurring in a wide range of habitats. They range in size from the tiny, almost microscopic Wolffia to tall trees of Eucalyptus (over 100 metres). They provide food, fodder, fuel, medicines and several other commercially important products.

The male sex organ of a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The anthers, after meiosis, produce pollen grains.

The female sex organ of a flower is the pistil or carpel. The pistil consists of an ovary enclosing one to many ovules. Within the ovules are present highly reduced female gametophytes termed embryo sacs. Embryo sac formation is preceded by meiosis, hence each of the cells of an embryo sac is haploid. Each embryo sac has a three-celled egg apparatus (one egg cell and two synergids), three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

Pollen grains are shed at the 2 to 3 celled stage after dispersal from the anthers, and are carried by wind or other agencies to the stigma of a pistil. This is termed pollination.

The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of the stigma and style and reach the ovule. The pollen tubes enter the embryo sac where two male gametes are discharged.

One of the male gametes fuses with the egg cell to form a zygote (syngamy). The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). This is known as triple fusion. Because of the involvement of two fusions, this event is termed double fertilisation, an event unique to angiosperms.

The zygote develops into an embryo (with one or two cotyledons) and the PEN develops into endosperm, which provides nourishment to the developing embryo.

Connecting Concepts: The tallest angiosperm is Eucalyptus regnans, at around 114 m.

Angiosperms are the most dominant and most evolved plants on earth.

The synergids and antipodals degenerate after fertilisation. During these events the ovules develop into seeds and the ovaries develop into fruits. A fruit is a ripened ovary. The ovary wall enlarges due to cell division and cell enlargement and is termed the pericarp. In fleshy fruits, the pericarp is often differentiated into epicarp, mesocarp and endocarp.

A true fruit is one which develops directly from an ovary. Sometimes other parts of the flower, such as the thalamus or sepals, become part of the fruit. Such fruits are termed false, spurious or pseudocarps, as in cashew nut, apple and strawberry.

Angiosperms are divided into two classes: the dicotyledons and the monocotyledons. Dicotyledons have two cotyledons in their seeds, while monocotyledons have only one.

Characteristic features of dicots and monocots

Dicotyledons are characterised by seeds having two cotyledons, reticulate venation in leaves, tetramerous or pentamerous flowers (having four or five members in each floral whorl), concentric arrangement of tissues in the stem, open, conjoint and collateral vascular bundles in the stem, secondary growth present, and fewer than 8 radial vascular bundles in the root. Exceptions to reticulate venation are Calophyllum and Corymbium. Examples of dicots are Eucalyptus, mustard, sunflower, rose, cotton and apple. The number of dicot species is about 200,000.

Monocotyledons are characterised by single cotyledon seeds, parallel venation in leaves, trimerous flowers (three members in each floral whorl), scattered, closed, collateral and conjoint vascular bundles in the stem, vascular bundles with a bundle sheath, and more than 8 radial vascular bundles in the root. Exceptions to parallel venation are Colocasia, Alocasia and Smilax. Examples of monocots are grasses, cereals, orchids, lilies, palms, banana, sugarcane and bamboos. There are some 50,000 species.

Differences between Dicotyledons and Monocotyledons

Check Point

Fill in the blanks:
1. A flower has sepals, petals _______ and _______.
2. Trimerous condition of floral whorl is characteristic of _______.
3. One of the male gametes fuses with the secondary nucleus to form _______. This is known as _______.
4. In dicotyledonous root, vascular bundles are _______ than eight.
5. Fruit is ripened _______.
Fruit is ripened _______.

Fig. 3.9 Life cycle of an Angiospermic plant (labelled parts given as text): Sporophyte diploid (flower); Meiosis in certain stamen and carpel cells. Anther gives Microspores haploid (pollen), which become Microgametophytes haploid (pollen) and Microgametes (sperm cells) haploid. Carpel gives Megaspore haploid, then Nucellus (2n) with Antipodal cells haploid, Megagametophyte, Polar nuclei (n + n), Megagamete (egg) haploid; then Ovule with Male gametes in pollen tube. Syngamy + Triple fusion = Double fertilization. Then Former egg is now zygote diploid, Primary endosperm nuclear (3n). Growth by mitosis and cell division, then Endosperm, Seed, Embryo; Seed germination, growth and development; Seed; back to the Sporophyte.

[Figure: Fig. 3.10 Diagrammatic representation of life history of an Angiospermic plant]

Fig. 3.10 (flow chart as text):

• Mature flowering plant (diploid sporophyte, 2n) bears Flowers.
  • Stamens (microsporophylls) give Pollen sacs (microsporangial group of cells), then a Pollen mother cell, then by Meiosis Pollen grains (haploid microspores); the vegetative nucleus (male gametophyte) plus generative cell gives Two male gametes (n).
  • Ovaries (megasporophylls) give Ovules (integumented megasporangia), then a Megaspore mother cell, then by Meiosis the Embryo sac (haploid) as the GAMETOPHYTE (n), containing the Egg cell (n), the female gamete, and Polar nuclei (n + n).
• Double fertilisation (this ensures that the endosperm tissue possesses stored food for the developing embryo): the Ist male gamete fuses with the egg to form the oospore; the IInd male gamete fuses with the polar nuclei (secondary nucleus) to form the endosperm nucleus.
• Embryo with protective testa (seed coat) forms the seed, then Dispersal, then Germination, giving the SPOROPHYTE (2n) again.

Plant Life Cycle and Alternation of Generation

In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies, haploid and diploid.

The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte. Following fertilization, the zygote also divides by mitosis to produce a diploid sporophytic plant body.

Haploid spores are produced by this sporophytic plant body by meiosis. These in turn divide by mitosis to form a haploid plant body once again.

Thus, the life cycle of any sexually reproducing plant consists of two morphological phases, i.e., the haploid gametophyte and the diploid sporophyte, which regularly alternate with each other. This phenomenon is termed alternation of generation.

However, life cycles in different plant groups differ in the following manner.

Haplontic life cycle

There is a single dominant vegetative phase, i.e., the haploid gametophyte, which is a free living photosynthetic phase. It produces gametes that fuse to form a zygote.

Meiosis in the zygote at the time of its formation results in haploid spores called meiospores. These meiospores divide mitotically to form the gametophyte.

Thus, in the haplontic life cycle there is no true alternation of generation, as the sporophytic generation is represented only by the one celled zygote. There is no free living sporophyte.

This life cycle is found in algae like Chlamydomonas, Volvox, Ulothrix and Spirogyra.

Diplontic life cycle

In the diplontic life cycle, the diploid sporophyte is the dominant vegetative phase. The sporophyte is the independent photosynthetic phase of the plant.

It bears sex organs that produce gametes through meiosis. These gametes fuse to form a zygote which again gives rise to the sporophytic plant through mitosis.

The gametophytic phase is represented by haploid gametes only. Thus, in the diplontic life cycle also, there is no true alternation of generation.

This type of life cycle is found in all seed bearing plants, i.e., gymnosperms and angiosperms. The alga Fucus shows a diplontic life cycle.

Haplo-diplontic life cycle

This is an intermediate condition where both phases are multicellular and often free living.

The haploid gametophyte produces gametes in sex organs. The gametes fuse to form a zygote which on germination gives rise to the diploid sporophyte.

The diploid sporophyte produces spores after meiosis. These spores on germination give rise to the gametophytic phase.

Thus it represents a true alternation of generation. This type of life cycle is found in bryophytes, pteridophytes and in some algae like Ectocarpus and Polysiphonia.

[Figure: Fig. 3.11 Life cycle patterns: (a) Haplontic (b) Diplontic (c) Haplo-diplontic]

• (a) Haplontic: Gametophyte (n) → Gametogenesis → Syngamy → Zygote (2n) → Meiosis → Spores (n) → Gametophyte (n). Only the zygote is diploid.
• (b) Diplontic: Sporophyte (2n) → Meiosis → Gametogenesis (gametes, n) → Syngamy → Zygote (2n) → Sporophyte (2n). Only the gametes are haploid.
• (c) Haplo-diplontic: Gametophyte (n) → Gametogenesis → Syngamy → Zygote (2n) → Sporophyte (2n) → Meiosis → Spores (n) → Gametophyte (n).