Here is your essay on the Plant kingdom

Here is your essay on the Plant kingdom

Plants vary in size and form. in their size they vary from structurally simple, microscopic organisms to plants which are several metres long. The Californian Redwood attains a height of over 115 metres, diameters, diameter of 10 metres and weighs about 2,100 tons. The life span of plants may vary from a few minutes (20 to 30 minutes in bacteria) to about 3,000 years in Californian Redwood.

The number of plants:

Quite a large number of plants exists in this world. More than nearly 347,000 distinct species of plants have been identified, described and named. Still a large number of species remain unknown.

Conventional Classification of the plant kingdom:

There are two main divisions of the plant king dom, viz,

Cryptogams comprising of over 140,000 species

And,

Phanerogams having 200,000 species.

The cryptogams are known as ‘ seedless’ or ‘flowerless’ plants. Kryptos(=hidden) and gamos (=marriage) are the two words to which the word Cryptogam owes its origin. Phanero (-open) and gamos(=marriage) combine to form the word phanerigam which means ‘flowering’ or ‘seed-bearing’ plant.

A. Cryptogams are lower groups of plants.

B. Phanerogams are higher groups of plants.

A. Cryptogams:

There are three main groups of cryptogams which can be arranged from the lower types to the higher types as on next page:

(1) Thallophyta:

(thallus= undifferentiated, phyton= plant). Plants belonging to this group are the simplest ones. The plant body is not differentiated into root, stem and leaf. Such an undifferentiated body is known as Thallus. The thallus-bearing plants are called Thallophyta. The thallophyta are exceedingly variable in structure and many of them are microscopic in size where as others reach lengths of many metres. The chief groups of Thallophyta are

(i) Algae

(ii) Fungai

And

(iii) Bacteria.

(i) Algae:

These green thallophytes are principally water plants numbering about 20,000 species and commonly containing the green pigment cholorophyll. The common examples are seaweeds, Chlamydomonas, Spirogyra etc.

(ii) Fungai:

These plants lack chlorophyll. As these cannot manufacturetheir own food they mostly grow as parasities (growing on other living plants) or saprophytes (growing on dead organic remains). The fungai include organisms like yeasts, molds, mushrooms, smults, rusts etc. numbering about 90,000 species.

(iii) Bacteria:

These are unicellular and non-green and various shapes. They occur in air, soil, water and in living bodies as parasities or on dead matter as saprophytes.

(2) Bryophyte:

(bryon=moss, phyton=plant). This group consists of small green plants lacking vascular tissues but structurally more advanced than thallophytes. This group includes about 23,000 species consisting of Liverworts (leafless bryophytes) and mosses (leaf bryophytes). Mostly they are limited to moist habitats.

(3) Pteridophyta:

(pteris =feather). This group includes the vascular cryptogams. Pterdophyta are the highest group of cryptogams with specialized and well-developed conducting tissues and well-differentiated plant bodies constiting of root, stem and leaves. About 9,000 species of plants belonging to Ferms, club-mosses and Horsetails are included under this group.

B. Phanerogams:

These are ‘flowering’ or ‘seed-bearing’ plants.

Otherwise called the Spermatophytes. The higher plants belong to this group. The reproduction through seeds is the outstanding feature of this group. Depending on the mode of enclosure of the seeds this group is broadly divided into

(a) Gymnosperms

And

(b) Angiosperms.

(a) Gymnosperms:

(gymnos =naked, sperma=seed). These are naked-seeded flowering plants, i.e., the seeds are not enclosed in a fruit. The flowers are unisexual (male or female), simple in construction, without accessory whorls and are arranged usually in cones. A little less than 1,000 species are reported under this group. Cycads (e.g. Cycas ) and conifers (e.g. Pinus) are the common examples.

(b) Angiosperms:

(angeon= a case, sperma = seed). These are close-seeded flowering plants, i.e., the seeds are enclosed in fruits. In other words, the seeds remain with in the fruits. Angiosperms are the highest form of plants comprising nearly 200,000 species, which are sub-divided into two catagories, viz. Monocotyledons and Dicotyledons.

(i) Monocotyledons:

In this group the embryo of the seed bears only one cotyledon. The cereal plants, grasses, bamboo, maize, sugar-cane, palm and coconut etc. are the examples of monocot plants. About 40,000 species of plants are recorded under this group.