The plant Kingdom and the very essential features of it!

THE PLANT KINGDOM 
Having thus indicated the chief distinctions between plants and animals, we must now turn our attention to the different kinds of organisms which belong to the plant kingdom.
For convenience, plants are classified into phyla, of which the following are the most important :-

• Thallophyta (Algae, Fungi and Bacteria)
• Charophyta (Stoneworts)
• Bryophyta (Mosses and Liverworts). 
• Pteridophyta (Ferns and Horsetails). 
• Spermatophyta (Conifers and Flowering Plants). 

The first four phyla are collectively called Cryptogams, because their reproductive organs were for long unknown, as opposed to the Sperma­tophyta or Phanerogams, in which they are easily recognizable in the cones or flowers. Each of these great groups is further subdivided into Orders and Families.
Perhaps the earliest plants, like the earliest animals, lived in water at a time when the dry land was uninhabited. The evolutionary tendency of plants has been, firstly, to migrate from water to land; and secondly, to make the modifications necessitated by the change from an aquatic to a terrestrial habitat. The Algae, which include the seaweeds, are forms which live almost exclusively in water; hence there is little necessity to develop a complex body, since food substances can be obtained directly from the water in which they live. The simpler members consist of a single cell, but in more advanced forms the cells are joined together in groups or in filaments in which every cell retains its individuality and is identical with every other cell. It is only among the more specialized Algae that we find a division of labour and a marked morphological differentiation of the cells, but even then there is nothing approaching the development of the stem, root and leaf of the flowering plant. So long as plants remain supported on all sides by water, their bodies need little or no reinforcing to keep them erect, but in even the simplest land plants some kind of supporting tissue is found to be present. This usually takes the form of hard material, called wood or xylem. In the Bryophyta, which though terrestrial are small plants, e.g., Mosses, supporting tissue is poorly developed and does not consist of wood.
In the Pteridophyta, particularly in the Filicales or Ferns, very large leaves may be developed in association with which a particularly complex woody tissue has arisen.
The Phanerogams have developed both the tree and herbaceous types, but the wood system is more complex than in the ferns. At the present day the herbaceous plant is probably the most successful type of vegetation.
This, then, very briefly is the conception of the range of organisms which are included under the term plants. Many are very different indeed from the wayside herb, or the garden flower with which we are familiar, but they all agree in having one common feature, namely, that they live holophytically.
The essential feature of holophytic nutrition is that food is absorbed entirely in solution, never in solid form. 'Where the food solution contains organic substances derived from the decay of other organisms the plant is called a saprophyte. Where the food is derived from the living body of another organism the plant is called a parasite. Such plants are only a minority. Green plants, on the other hand, take up only inorganic substances from which they build up their own organic material. They are said to be autotrophic, which means" self-feeding." This mode of nutrition is shown by no organisms except plants, and we may briefly refer to the process here.
First of all the green plant combines the free carbon dioxide of the air with \yater from the soil, to form carbohydrates. Every green plant obtains its carbohydrates by means of this process of feeding, which is called carbon assimilation or photosynthesis. This process can take place only in the presence of light and chlorophyll.
Chorophyll is a green pigment of a complex nature, containing carbon, oxygen, hydrogen, nitrogen and magnesium. It is found in all the green parts of plants, particularly in the leaves, where it occurs in specialized protoplasmic structures which are known as chloroplasts. Only in cells containing chloroplasts can the photosynthetic production of carbohydrates take place. The necessary water and some mineral salts in solution are either obtained directly from the water in which the plants live or, in land plants, from the soil through the roots. In the body of the plant certain of these salts containing nitrogen are combined with carbohydrates to form protein and, finally, protoplasm.

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