Plant Kingdom

1. Chapter at a Glance

In the previous chapter we saw that Whittaker placed all eukaryotic, chlorophyll-containing, multicellular organisms in kingdom Plantae (fungi, blue-green algae and unicellular eukaryotes were moved to other kingdoms). This chapter describes the major groups of plants — algae, bryophytes, pteridophytes, gymnosperms and angiosperms — with their distinguishing features and examples. It also explains the systems used to classify plants (artificial, natural and phylogenetic). NEET asks two to three questions from this chapter, usually on pigments and stored food of algal classes, the dominant phase in each group, heterospory and seed habit, and examples of each group.

2. Systems of Classification

• Numerical taxonomy, now easily carried out with computers, is based on all observable characteristics. Numbers and codes are assigned to all characters and the data are processed. Each character is given equal importance, and hundreds of characters can be considered at the same time.
• Cytotaxonomy is based on cytological information such as chromosome number, structure and behaviour.
• Chemotaxonomy uses the chemical constituents of the plant to resolve confusions.

3. Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both freshwater and marine) organisms. They occur in a variety of other habitats too: moist stones, soils and wood. Some occur in association with fungi (lichens) and animals (e.g. on sloth bear). The form and size of algae is highly variable, ranging from colonial forms like Volvox and filamentous forms like Ulothrix and Spirogyra to massive plant bodies such as kelps.

Reproduction in algae

• Vegetative reproduction is by fragmentation; each fragment develops into a thallus.
• Asexual reproduction is by the production of different types of spores, the most common being zoospores. They are flagellated (motile) and on germination give rise to new plants.
• Sexual reproduction is through fusion of two gametes:
  • Isogamous — gametes similar in size; may be flagellated (Ulothrix) or non-flagellated (Spirogyra).
  • Anisogamous — fusion of two gametes dissimilar in size, e.g. species of Eudorina.
  • Oogamous — fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete, e.g. Volvox, Fucus.

Usefulness of algae

• At least half of the total carbon dioxide fixation on Earth is carried out by algae through photosynthesis. Being photosynthetic, they increase the level of dissolved oxygen in their immediate environment.
• They are of paramount importance as primary producers of energy-rich compounds, which form the basis of the food cycles of all aquatic animals.
• Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food.
• 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, a unicellular alga rich in proteins, is used as food supplement even by space travellers.

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

• Chlorophyceae (green algae): plant body may be unicellular, colonial or filamentous. They are usually grass green due to the dominance of chlorophyll a and b. Pigments are localised in definite chloroplasts, which may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species. Most members have one or more storage bodies called pyrenoids located in the chloroplasts; pyrenoids contain protein besides starch. Some algae store food in the form of oil droplets. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
• Phaeophyceae (brown algae): found primarily in marine habitats. They show great variation in size and form, ranging from simple branched, filamentous forms (Ectocarpus) to profusely branched forms, as represented by kelps, which may reach a height of 100 metres. They possess chlorophyll a, c, carotenoids and xanthophylls. They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin. 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. The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus. The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe, and leaf-like photosynthetic organ — the frond. Gametes are pyriform (pear-shaped) and bear two laterally attached flagella.
• Rhodophyceae (red algae): the members are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin, in their body. The majority are marine, with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates. The red thalli of most red algae are multicellular. Some have complex body organisation. Food is stored as floridean starch, which is very similar to amylopectin and glycogen in structure. Red algae usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Sexual reproduction is oogamous and accompanied by complex post-fertilisation developments.

4. Bryophytes

Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills. Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction. They usually occur in damp, humid and shaded localities. They play an important role in plant succession on bare rocks/soil.

• The plant body of bryophytes is more differentiated than that of algae. It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves; they may possess root-like, leaf-like or stem-like structures.
• The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte.
• The sex organs are multicellular. The male sex organ is called the antheridium; it produces biflagellate antherozoids. The female sex organ, called the archegonium, is flask-shaped and produces a single 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 a 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. These spores germinate to produce gametophytes.

Economic importance

• Some mosses provide food for herbaceous mammals, birds and other animals.
• Species of Sphagnum, a moss, provide peat that has long been used as fuel and as packing material for trans-shipment of living material because of its capacity to hold water.
• Mosses, along with lichens, are the first organisms to colonise rocks and hence are of great ecological importance. They decompose rocks, making the substrate suitable for the growth of higher plants.
• Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

Bryophytes are divided into liverworts and mosses.

5. Pteridophytes

The pteridophytes include horsetails and ferns. Pteridophytes are used for medicinal purposes and as soil-binders. They are also frequently grown as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues — xylem and phloem. They are found in cool, damp, shady places, though some may flourish well in sandy-soil conditions.

• In bryophytes the dominant phase is the gametophyte. In pteridophytes, the main plant body is a sporophyte, differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues.
• The leaves are small (microphylls) as in Selaginella, or large (macrophylls) as in ferns.
• 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. The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus. 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 and restricted to narrow geographical regions.
• The gametophytes bear male and female sex organs called antheridia and archegonia, respectively. 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 a zygote. The zygote thereafter produces a multicellular, well-differentiated sporophyte, which is the dominant phase of the pteridophytes.
• In the majority of pteridophytes all the spores are of similar kind; such plants are called homosporous. Genera like Selaginella and Salvinia, which produce two kinds of spores — macro (large) and micro (small) spores — 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 zygote into young embryos takes place within the female gametophytes. This event is a precursor to the seed habit, considered an important step in evolution.

6. Gymnosperms

The gymnosperms (gymnos: naked, sperma: seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation are not covered, i.e. they are naked.

• Gymnosperms include medium-sized trees or tall trees and shrubs. One of the gymnosperms, the giant redwood tree Sequoia, is one of the tallest tree species.
• The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots 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. Their thick cuticle and sunken stomata also help to reduce water loss.
• The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls, which are arranged spirally along an axis to form lax or compact strobili or cones.
• The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation, which is highly reduced and confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia.
• The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male or female cones may be borne on the same tree (Pinus). However, in Cycas male cones and megasporophylls are borne on different trees.
• 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 the female cones. The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. The multicellular female gametophyte is also retained within the megasporangium.
• Unlike bryophytes and pteridophytes, in gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes. The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharges its contents near the mouth of the archegonia. Following fertilisation, the zygote develops into an embryo and the ovules into seeds. These seeds are not covered.

7. Angiosperms

Unlike the gymnosperms, where the ovules are naked, in the angiosperms or flowering plants the pollen grains and ovules are developed in specialised structures called flowers. In angiosperms, the seeds are enclosed in fruits. The 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 us with food, fodder, fuel, medicines and several other commercially important products. They are divided into two classes: the dicotyledons and the monocotyledons.

• The male sex organ in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The anthers, following meiosis, produce pollen grains.
• The female sex organ in a flower is the pistil or carpel. The pistil consists of a swollen ovary at its base, a long slender style and a stigma. Inside the ovary, ovules are present. Generally each ovule has a megaspore mother cell that undergoes meiosis to form four haploid megaspores. Three of them degenerate and one divides to form the embryo sac.
• 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, after dispersal from the anthers, are carried by wind or various 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 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 (syngamy) to form a zygote. The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). Because of the occurrence of two fusions, i.e. syngamy and triple fusion, 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 the endosperm, which provides nourishment to the developing embryo. The synergids and antipodals degenerate after fertilisation. During these events the ovules develop into seeds and the ovaries develop into fruit.

8. Master Summary Table

9. Mind Map

10. Tips and Tricks

11. Important One-liners

• Numerical taxonomy gives equal importance to every character.
• Bentham and Hooker gave a natural system of classification.
• About half of Earth's CO₂ fixation is done by algae.
• Chlorella is used as a protein supplement by space travellers.
• Algin comes from brown algae; carrageen and agar from red algae.
• Kelps can reach 100 metres in height.
• Protonema is the juvenile stage of the moss gametophyte.
• Gemmae are asexual buds of liverworts formed in gemma cups.
• Pteridophytes are the first terrestrial vascular plants.
• The pteridophyte gametophyte is called the prothallus.
• Sequoia is one of the tallest tree species.
• Double fertilisation is unique to angiosperms.

12. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

13. Quick Revision Summary