Chapter-3: Plant Kingdom

Classification of Plants into Major Groups

The plant kingdom encompasses a diverse group of organisms that are multicellular, eukaryotic, and mostly autotrophic. Over time, various systems of classification have been proposed to understand this diversity.

Artificial Systems of Classification

The earliest systems of classification used only superficial morphological characters such as habit, color, number, and shape of leaves. These were mainly based on vegetative characters or a few gynoecium characters.

• Proponent: Linnaeus.
• Limitation: They separated closely related species since the systems often gave equal weightage to vegetative and sexual characteristics. Vegetative characters are easily affected by the environment.

Natural Systems of Classification

These systems developed, which were based on natural affinities among the organisms and considered not only the external features but also internal features like ultra-structure, anatomy, embryology, and phytochemistry.

• Proponent: George Bentham and Joseph Dalton Hooker.

Phylogenetic Systems of Classification

Current classifications assume that organisms belonging to the same taxa have a common ancestor. Phylogenetic classification systems are based on evolutionary relationships between various organisms.

• Advantage: This ensures that information is gathered from many other sources as well, not just external morphology.

Numerical Taxonomy, Cytotaxonomy, and Chemotaxonomy

Modern taxonomical tools assist in plant classification:

• Numerical Taxonomy: Carried out using computers, based on all observable characteristics. Number and codes are assigned to all characters and the data is processed. Hundreds of characters can be considered equally.
• Cytotaxonomy: Based on cytological information like chromosome number, structure, and behavior.
• Chemotaxonomy: Uses the chemical constituents of the plant to resolve confusions.

Algae

Salient and Distinguishing Features of Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (both freshwater and marine) organisms. Some occur in association with fungi (lichen) and animals (on sloth bear).

• Plant Body: The form and size of algae are highly variable. They range from colonial forms like Volvox to filamentous forms like Ulothrix and Spirogyra. A few marine forms such as kelps form massive plant masses.
• Reproduction: Occurs by vegetative, asexual, and sexual methods.
• Vegetative reproduction: By fragmentation, where each fragment develops into a thallus.
• Asexual reproduction: By the production of different types of spores, the most common being flagellated zoospores which on germination give rise to new plants.
• Sexual reproduction: Takes place through fusion of two gametes. These gametes can be flagellated and similar in size (isogamous, e.g., Ulothrix) or non-flagellated (non-motile) but similar in size (e.g., Spirogyra). Fusion of two gametes dissimilar in size is termed anisogamous (e.g., some species of Eudorina). Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete is termed oogamous (e.g., Volvox, Fucus).

Economic Importance of Algae

• At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis, increasing dissolved oxygen levels in their immediate environment.
• 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, obtained from Gelidium and Gracilaria, is used to grow microbes and in preparations of ice-creams and jellies.
• Chlorella and Spirulina are unicellular algae rich in proteins and are used as food supplements by space travellers.

Major Classes of Algae

Bryophytes

Salient and Distinguishing Features of Bryophytes

Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills. They are often called the amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction.

• Plant Body: The plant body 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, stems, or leaves. They may possess root-like, leaf-like, or stem-like structures.
• Thallus Nature: The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte.
• Sex Organs: The sex organs in bryophytes are multicellular. The male sex organ is called antheridium. They produce biflagellate antherozoids. The female sex organ is flask-shaped called archegonium and produces a single egg.
• Fertilization: Antherozoids are released into water to reach the archegonium and fuse with the egg to produce the zygote.
• Sporophyte: Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. The sporophyte is not free-living but 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 gametophyte.

Economic Importance and Ecological Significance

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

Divisions of Bryophytes

• Liverworts: Grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees, and deep in the woods. The plant body of a liverwort is thalloid (e.g., Marchantia). Asexual reproduction in liverworts takes place by fragmentation of thalli, or by the formation of specialized structures called gemmae. Gemmae are green, multicellular, asexual buds, which develop in small receptacles called gemma cups located on the thalli. During sexual reproduction, male and female sex organs are produced either on the same or on different thalli. Sporophyte is differentiated into a foot, seta, and capsule.
• Mosses: The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages. The first stage is the protonema stage, which develops directly from a spore. It is a creeping, green, branched, and frequently filamentous stage. The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axes bearing spirally arranged leaves. They are attached to the soil through multicellular and branched rhizoids. Sex organs are produced at the apex of the leafy shoots. Sporophyte in mosses is more elaborate than that in liverworts.

Pteridophytes

Salient and Distinguishing Features of Pteridophytes

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.

• Habitat: They are found in cool, damp, shady places though some may flourish well in sandy-soil conditions.
• Plant Body: In pteridophytes, the main plant body is a sporophyte which is differentiated into true root, stem, and leaves. These organs possess well-differentiated vascular tissues.
• Leaves: The leaves in pteridophytes are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
• Reproduction: 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 (e.g., 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.
• Requirements for Fertilization: Prothalli require cool, damp, shady places to grow. Because of this specific requirement and the need for water for fertilization, the spread of 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 transfer of antherozoids to the mouth of archegonium. Fusion of male gamete with the egg present in the archegonium result in the formation of zygote. Zygote thereafter produces a multicellular well-differentiated sporophyte which is the dominant phase of the pteridophytes.

Heterospory and Seed Habit

• Genera like Selaginella and Salvinia produce two kinds of spores, macro (large) and micro (small) spores, 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 considered an important step in evolution.

Classes of Pteridophytes

• Psilopsida: Psilotum
• Lycopsida: Selaginella, Lycopodium
• Sphenopsida: Equisetum
• Pteropsida: Pteris, Adiantum, Dryopteris

Gymnosperms

Salient and Distinguishing Features of Gymnosperms

Gymnosperms are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilization.

• Plant Body: Gymnosperms include medium-sized trees or tall trees and shrubs. The giant redwood tree Sequoia is one of the tallest tree species.
• Roots: The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (e.g., Pinus), while in some others (e.g., Cycas) small specialized roots called coralloid roots are associated with N2-fixing cyanobacteria.
• Stems: The stems are unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves: 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 surface area. Their thick cuticle and sunken stomata also help to reduce water loss.
• Reproduction: 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 is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
• Pollination and Fertilization: Unlike bryophytes and pteridophytes, in gymnosperms the male and female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophyte. Pollen grains are carried by 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 their contents near the mouth of the archegonia. Following fertilization, zygote develops into an embryo and the ovules into seeds. These seeds are not covered.

Preliminary Idea of Angiosperms

Salient and Distinguishing Features of Angiosperms

Unlike gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialized structures called flowers. In angiosperms, the seeds are enclosed in fruits.

• Size Range: Angiosperms are an exceptionally large group of plants occurring in wide range of habitats. They range in size from the smallest Wolffia to tall trees of Eucalyptus (over 100 meters).
• Classes: 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.

Dicotyledons versus Monocotyledons

Flower Structure

• The male sex organ in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. Within the anthers, the pollen mother cells divide by meiosis to produce microspores which mature into pollen grains.
• The female sex organ in the flower is the pistil. Pistil consists of an ovary enclosing one or many ovules, a long neck called style, and stigma. Inside the ovule, the megaspore mother cell divides by meiosis to form four haploid megaspores, out of which three degenerate and one divides to form the embryo sac.

Embryo Sac Structure

• 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 definitive nucleus.

Pollination and Double Fertilization

• Pollen grain, after dispersal from the anthers, is carried by wind or various other agencies to the stigma of a pistil (pollination).
• The pollen grain germinates on the stigma and the resulting pollen tube grows through the tissues of stigma and style and reaches the ovule.
• The pollen tube releases two male gametes into the embryo sac.
• Syngamy: One male gamete fuses with the egg cell to form a zygote (develops into an embryo).
• Triple Fusion: 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 viz., syngamy and triple fusion, this event is termed double fertilization, 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.
• The synergids and antipodals degenerate after fertilization. During these events the ovules develop into seeds and the ovaries develop into fruits.

Exam-Note: Double fertilization involving syngamy and triple fusion is the most distinctive hallmark characteristic of Angiosperms.