Plant Kingdom
From simple algae to complex flowering plants — exploring the diversity of Kingdom Plantae
In this chapter
3.1 Introduction
In the previous chapter, we looked at the broad classification of living organisms under Whittaker's Five Kingdom system. In this chapter, we will deal in detail with further classification within Kingdom Plantae — popularly known as the 'plant kingdom'.
Our understanding of the plant kingdom has changed over time. Fungi, and members of Monera and Protista having cell walls have now been excluded from Plantae. So, the cyanobacteria (blue-green algae) are not 'algae' any more. In this chapter, we will describe Algae, Bryophytes, Pteridophytes, Gymnosperms and Angiosperms under Plantae.
Classification Systems
The earliest systems of classification used only gross superficial morphological characters such as habit, colour, number and shape of leaves. They were based mainly on vegetative characters or on the androecium structure (system given by Linnaeus). Such systems were artificial; they separated closely related species since they were based on a few characteristics.
Against this, natural classification systems developed, which were based on natural affinities among the organisms and considered not only the external features, but also internal features — like ultrastructure, anatomy, embryology and phytochemistry. Such a classification for flowering plants was given by George Bentham and Joseph Dalton Hooker.
At present, phylogenetic classification systems based on evolutionary relationships between the various organisms are acceptable. This assumes that organisms belonging to the same taxa have a common ancestor.
Numerical Taxonomy — now easily carried out using computers, based on all observable characters. Number and codes are assigned to all the characters and the data are then processed. Each character is given equal importance and hundreds of characters can be considered.
Cytotaxonomy — based on cytological information like chromosome number, structure, behaviour.
Chemotaxonomy — uses the chemical constituents of the plant to resolve confusions.
3.2 Algae
Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms. They occur in a variety of other habitats: moist stones, soils and wood. Some also occur in association with fungi (lichen) 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. A few marine forms such as kelps form massive plant bodies.
Reproduction in Algae
- Vegetative reproduction — by fragmentation. Each fragment develops into a thallus.
- Asexual reproduction — by 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 — through fusion of two gametes. Three types: isogamous (similar gametes), anisogamous (dissimilar in size), oogamous (large non-motile female + small motile male).
Economic Importance
• At least half of total CO₂ fixation on earth is carried out by algae through photosynthesis.
• They increase the level of dissolved oxygen in their immediate environment.
• They are primary producers of energy-rich compounds — the basis of food cycles of all aquatic animals.
• Porphyra, Laminaria and Sargassum are among 70 species of marine algae used as food.
• Commercial products: algin (brown algae), carrageen (red algae), agar (from Gelidium and Gracilaria).
• Chlorella — unicellular alga rich in proteins, used as food supplement even by space travellers.
Three Classes of Algae
3.2.1 Chlorophyceae (Green Algae)
Commonly called green algae. Plant body may be unicellular, colonial or filamentous. Usually grass green due to dominance of pigments chlorophyll a and b. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped. Most members have one or more storage bodies called pyrenoids located in the chloroplasts — they contain protein besides starch. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.
3.2.2 Phaeophyceae (Brown Algae)
Found primarily in marine habitats. Range from simple branched filamentous forms (Ectocarpus) to profusely branched kelps, which may reach a height of 100 metres. They possess chlorophyll a, c, carotenoids and xanthophyll. Food is stored as complex carbohydrates — laminarin or mannitol. Vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin.
Plant body is usually attached to the substratum by a holdfast, has a stalk (stipe) and leaf-like photosynthetic organ — the frond.
Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.
3.2.3 Rhodophyceae (Red Algae)
Commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body. Majority are marine with greater concentrations in warmer areas. They occur in both well-lighted regions close to the surface and at great depths in oceans where relatively little light penetrates. Food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.
Usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Sexual reproduction is oogamous.
Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.
Table 3.1 — Divisions of Algae and their Main Characteristics
| Class | Common Name | Major Pigments | Stored Food | Cell Wall | Flagella | Habitat |
|---|---|---|---|---|---|---|
| Chlorophyceae | Green algae | Chlorophyll a, b | Starch | Cellulose | 2–8, equal, apical | Fresh/brackish/salt water |
| Phaeophyceae | Brown algae | Chlorophyll a, c, fucoxanthin | Mannitol, laminarin | Cellulose and algin | 2, unequal, lateral | Mainly marine |
| Rhodophyceae | Red algae | Chlorophyll a, d, phycoerythrin | Floridean starch | Cellulose, pectin & poly sulphate esters | Absent | Mainly marine |
3.3 Bryophytes
Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills. They 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 in bryophytes are multicellular. The male sex organ is called antheridium — they produce biflagellate antherozoids. The female sex organ called archegonium is flask-shaped and produces a single egg.
The antherozoids are released into water where they come in contact with 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 attached to the photosynthetic gametophyte and derives nourishment from it.
• 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 their 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.
3.3.1 Liverworts
The 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. The thallus is dorsiventral and closely appressed to the substrate. The leafy members have tiny leaf-like appendages in two rows on the stem-like structures.
Asexual reproduction in liverworts takes place by the production of gemmae — green, multicellular, asexual buds which develop in small receptacles called gemma cups located on the thallus. The gemmae become detached from the parent body and germinate to form new individuals. During unfavourable conditions, they produce spores. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule.
3.3.2 Mosses
The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages:
- Protonema stage — develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage.
- Leafy stage — 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. This stage bears the sex organs.
Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema. In sexual reproduction, the sex organs antheridia and archegonia are produced at the apex of the leafy shoots. After fertilisation, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. The sporophyte in mosses is more elaborate than that in liverworts. Common examples: Funaria, Polytrichum and Sphagnum.
3.4 Pteridophytes
The Pteridophytes include horsetails and ferns. They are used for medicinal purposes and as soil-binders. 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 in sandy-soil conditions.
In bryophytes the dominant phase in the life cycle is the gametophytic plant body. However, 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. The leaves in pteridophyta 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 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.
In majority of the pteridophytes all the spores are of similar kinds; 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 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.
The pteridophytes are further classified into four classes: Psilopsida (Psilotum); Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum).
3.5 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., 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.
Reproduction in Gymnosperms
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.
- Male strobili (microsporangiate) — bear microsporophylls and microsporangia. The microspores develop into pollen grains.
- Female strobili (megasporangiate) — bear megasporophylls with ovules or megasporangia.
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 divides meiotically to form four megaspores. One of the megaspores develops into a multicellular female gametophyte that bears two or more archegonia.
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. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharges their contents near the mouth of the archegonia. Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered.
3.6 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 wide range of habitats. They range in size from the smallest 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.
Dicotyledons
Embryo with two cotyledons. Leaves show reticulate venation. Tap root system. Flowers usually tetramerous or pentamerous. Vascular bundles in stem are numerous and arranged in a ring.
Monocotyledons
Embryo with single cotyledon. Leaves show parallel venation. Fibrous root system. Flowers usually trimerous. Vascular bundles in stem are scattered.
3.7 Summary
• Plant kingdom includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
• Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic. Classified into three classes: Chlorophyceae, Phaeophyceae and Rhodophyceae based on pigments and stored food.
• Bryophytes are amphibians of the plant kingdom — dependent on water for sexual reproduction. Plant body is gametophyte. Divided into liverworts and mosses.
• Pteridophytes — first terrestrial plants with vascular tissues. Main plant body is sporophyte. Includes ferns and horsetails. Homosporous or heterosporous.
• Gymnosperms — naked-seeded plants. Ovules not enclosed by ovary wall. Heterosporous. Produce male and female cones. Pollen tube carries male gametes to archegonia.
• Angiosperms — flowering plants with seeds enclosed in fruits. Divided into dicotyledons and monocotyledons. Most diverse and widespread group of plants.