Quite a large number of plants exist in the world of nature. More than 3, 40,000 plants have been identified, described and named by botanists and a pretty large number still remain unknown.
Attempts to classify plants were made even by the early ancients like Theophrastus in Greece and the Indian sages in the age of the Upanishads.
Plant Kingdom, according to modern classification, is broadly divided into four groups:
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1. Spermatophyta – Flowering plants (Phanerogams)
2. Pteridophyta – Flowerless plants (Cryptogams)
3. Bryophyta
4. Thallophyta
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Flowering plants (spermatophyta):
The so-called higher plants belong to this group. Plant body is differentiated into stems, leaves and roots. Development of flowers and reproduction through seeds are the outstanding features of this group. They are divided into two groups:
1. Gymnosperms:
These are naked seeded plants. There is nothing like ovary, style, stigma on flowers and naturally, there is no fruit.
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2. Angiosperms:
These are the most highly developed plants which bear true flowers and develop fruits and seeds. Seeds remain within the fruits. Angiosperms may be of two types: (a) monocotyledon (embryo with single cotyledon) and (b) dicotyledon (embryo with two cotyledons).
Parts of a Typical Flowering Plant:
A typical flowering plant consists of root, stem, leaves, flowers, fruits and seeds. Each part has a separate function for the plant.
(i) Root:
Root is colourless and grows into the soil. It serves to fix a plant in the soil and to absorb water and mineral salts from the soil. It grows profusely into fine branches called root hairs which are the real absorbing organs of the plant. Root is protected by root cap at the tip. In some plants, roots become swollen and serve as storage places for food manufactured by leaves such as in sweet potato (shakar kandi).
(ii) Stem:
Stem is the aerial part and grows upward towards light. It bears swollen parts called nodes from which leaves arise. The leafless part of the stem between nodes is called internode.
Stem serves as a channel for the transport of water and other dissolved salts from the soil to the leaves. At the same time, it transports manufactured food from the leaves to the places of storage in the plant. In some plants, stems become modified into storage organs such as in potato, colocasia and ginger.
(iii) Leaf:
Leaf is a green structure borne on the stem branches and arises from nodes. The green colour of leaf is due to the presence of chlorophyll which is the main part of photosynthetic process (food-manufacturing). In the presence of sunlight, chlorophyll brings about chemical combination between water and CO2 to manufacture starch (sugar) and proteins. Leaves are also responsible for respiratory and water exudation (transpiration) processes.
During the night, tissues of leaves take in 0} from the atmosphere through the stomata and give out CO2 The excess of water absorbed from the soil evaporates from leaves through the stomata which are generally found on the lower surface of leaves except in the case of water floating plants, where they are present on the upper surface.
(iv) Flower:
Flower is a modified shoot meant for reproduction of seed and fruit. A typical flower consists of a short stalk known as thalamus, on which are inserted floral leaves in four whorls calyx, corolla, androecium and gynoecium.
(v) Fruit:
Fruit is defined as a ripened ovary containing seeds. It is formed as a result of changes which take place in the walls of ovary due to pollination and fertilization. It may remain dry as in pea or become fleshy as in apple,
(vi) Seed:
Seed may be defined as a ripened ovule. Inside the seed is the embryo of the plant. The chief function of seed is the propagation of plants.
The flowering plants show great variation in form and mode of life. Lemna and Wolffia are the smallest flowering plants, while some others may attain large sizes as the Banyan tree in Botanical Garden, Kolkata. There is great variation in their duration of life as well. The famous Bodhi tree at Gaya is calculated to be nearly 2000 years old and the giant Sequoias in California (USA) are several thousand years old.