Plants are multi-cellular and mostly photosynthetic organisms which found essentially everywhere, both in water and on land.
The aquatic plants include red, brown and green algae and the land plants include mosses, ferns, gymnosperms and angiosperms.
The angiosperms or flowering plants are dominated the earth the last 70 million years.
There are over 3, 00,000 angiosperm species growing on the earth which show enormous diversity in size and form. In India there are about 45,000 plant species are known that represent about 7% of the world’s flora. Nearly, 4900 species of angiosperms are endemic to India.
The diversity in plant life can be understood from the following headings:
I. Diversity on The Basis of Habitat:
Plants grow in a variety of habitats.
On the basis of habitat, plants may be classified into following groups:
1. Hydrophytes:
The plants growing near water or submerged under water are called hydrophytes. Such plants have poor root system, soft stem and poor vascular tissue. The bulk of the tissue is spongy provided with air spaces.
These plants may be:
(i) Submerged (e.g., Vallisneria, Hydrilla, Potamogeton etc.),
(ii) free-floating and fixed-floating (e.g., Wolffia, Utricularia, Salvinia, Ceratophyllum, Lemna, Pistia, Eichornia, Trapa, Azolla Nymphaea etc.), and
(iii) amphibious (only partly submerged, e.g., Ranunculus aquatilis, Alisma plantago, Sagittaria, Limnophylla etc.).
Two angiosperms are also marine, e.g., Zostera and Thalassia.
2. Hygrophytes:
These plants grow in moist and shady habitats. Their stem and roots are soft and spongy and show stunted growth. The leaves are well-developed, provided with stomata. Common examples are Ferns, Begonias, Aroid and certain grasses.
3. Halophytes:
These plants grow in saline soil or saline water. They can tolerate a relatively high concentration of salts (Nacl, MgCl2, and MgSO4). They have characteristic negatively geotropic breathing roots called pneumatophores. Common examples are mangrove vegetations like Rhizophora, Ceriops, Avicennia, Sonneratia etc.
4. Mesophytes:
The majorities of angiosperms grows in places of moderate water supply and are known as mesophytes. They are usually large and fast growing. They have well developed roots and leaves. There stem may be herbaceous or woody. There are certain mesophytes, such as deciduous trees (viz., shedding leaves at a certain season), which are mesophytic during the summer and xerophytic during the winters.
5. Xerophytes:
The plants which grow in xeric or dry conditions or where water availability is negligible, are known as xerophytes, e.g., Euphorbia, Acacia, Argemone, Amaranthus, calotropis, Nerium, Ziziphus etc. some xerophytes store water in their stem (opuntia), leaves (Aloe. Agava, Bryophyllum) or in roots (Asparagus) and are called as Succulents.
Xerophytes may be further divided into following types:
i. Lithophytes – Plants growing on rocks
ii. Psammophytes – Plants growing in sandy soils
iii. Oxylophytes – Plants growing on acidic soil.
6. Epiphytes:
Are the plants that grow on the trunk or branches of other plants, e.g., an orchid or lichen growing-as an epiphyte on a mango branch. The epiphytes are considered as space parasites. However, the interaction between the orchid (a commensal) growing on a tree (host) would be an example of commensalism where the host is unharmed, while the commensals benefits,
7. Parasitic plants:
These plants live on other plants as parasites, e.g. Cuscuta, Striga (grows on roots of jowar)
II. Diversity On The Basis Of Habit:
On the basis of habit (i.e., shape, size and form), angiosperms are classified into four groups:
1. Herbs (Herbaceous):
The stem of those plants is green, delicate and short. Usually their life is short e.g., wheat (Fig 1.2A), gram. In some herbaceous plants, the underground part of stem is greatly reduced but the aerial branch with flowers at the top arises from underground parts at the time of reproduction. Such a stem is called scape e.g. onion (fig. 1.2B)
2. Shrubs (= Shruby or Fruticose):
These plants are woody, branched and larger than herbs. Usually with several stems but no main axis e.g., China rose, rose, henna. (Fig 1.2C).
3. Trees (Arborescent):
The plants are longer or larger than shrubs, hard and woody, very well developed and thick. Possess a prominent trunk.
These are of the following types:
(a) Caudex:
The stem is un-branched and usually bears a crown of leaves at the apex e.g., Date-palm (Fig. 1.3A).
(b) Excurrent:
The lower part of the stem is thicker which gradually tapers above. The plant appears conical due to acropetal arrangement of branches on the main stem (Fig. 1.3B).
(c) Deliquescent:
The apical bud of the main stem dies after some time and branches and sub- branches spread in different directions e.g., Tamarindus, Ficus.(Fig. 1.3C).
4. Culms:
In these plants, nodes and internodes are extremely prominent, lnternodes of such plants are usually hollow: These plants are grasses but cannot be considered as herbs or shrub or tree, e.g., Bamboo. (Fig. 1.4)
III. Diversity of Angiosperms on The Basis of the Nature of Stem:
On the basis of the nature of stem, the angiosperm plants may be classified as given below:
1. Erect:
These plants grow upright. Most trees, shrubs and some herbs have strong stem (axis) and thus can stand erect on the soil.
2. Creepers:
These plants have trailing stem having roots throughout its length. Since these plants have weak, long and thin stem they creep on the surface of the soil. Leaves emerge from nodes, from the axil of which branches arise. Adventitious roots arise from nodes throughout the length of the stem, e.g., Oxalis; Cynodon (doob grass) etc.
3. Trailers:
Stem sprawling on the ground with the help of adventitious roots. These plants are like creepers with the difference that here adventitious roots do not arise from nodes. A trailer may be procumbant or decumbent. In procumbent trailer, the stem lies completely horizontal (e.g., Basella), while in decumbent, the apical part of the stem is raised above the ground (e.g., Lindenbergia).
4. Climbers:
These plants with weak stem climb on some support by means of tendrils, petioles, spines, adventitious roots etc., e.g., pea, betel etc.
IV. Diversity on The Basis of Life-Span:
On the basis of life-span, angiosperms are classified into following four groups:-
1. Ephemerals:
Such plants complete their life span within a very short period before the approach of actual dry conditions. These are not true xerophytes, and often called as drought evaders or drought escapers, e.g., Argemon mexicana, Solanum xanthocarpum, Cassia tora, Artemesia etc.
2. Annuals:
They complete their life-cycle within one year and die after producing seeds, e.g., wheat, rice, gram.
3. Biennials (or Biannuals);
These plants complete their life-cycle in two years. During first year they show only vegetative growth, and during second year they develop flowers, fruits and seeds. These plants are usually herbs, e.g., Radish, Turnip, and Carrot.
4. Perennials:
These plants have long life, and once established continue to live for many years. The great banyan tree (Ficus bengalensis) in the Botanical Garden, Kolkata is more than 200 years old. The Bodhi tree (ficus religiosa) at Gaya is about 2500 years old.
Most perennials, after attaining maturity, bear flowers and fruits in a particular season of each year. They are called polycarpic, e.g., coconut, mango, Acacia etc. Some perennials (e.g., Bamboos, Agave) are monocarpic i.e., they bear fruits only once in their life. All annuals and biennials are monocarpic.
V. Diversity on The Basis of Size:
The angiospermic plants show great variation in their size. The smallest angiosperm is a rootless aquatic Wolffia. It has a diameter of 0.1 mm. Aquatic Lemna has a diameter of 0.1 cm. The tallest angiosperm plant it is Eucalyptus regnans. It is over 100 meter tall. Some of the Eucalyptus trees attain a height of 130 meter. The largest-sized plant is Banyan tree (Ficus bengalensis). It can spread over an area of 2 – 5 acres with more than 200 prop roots.
VI. Diversity on The Basis of Nutrition:
On the basis of mode of nutrition, plants are classified as follows:-
1. Autotrophic plants or autotrophs. Most of the plants are autotrophs as they are green and manufacture their own organic food from inorganic raw materials (viz. CO2 and H2O).
2. Heterotrophic plants or heterotrophs. These plants obtain a part or whole of their nourishments from outside sources. Heterotrophs may be parasites, saprophytes, symbionts and insectivorous.