The ecological groups of plants are indeed much broader without any airtight boundaries between them. This is a classification or rather a very broad grouping of plants based on their water relationships. Warming (1909) classified plant communities on the basis of plants dependence upon and relation to water, as done earlier by Grabner (1898, 1901, 1908). Water as an ecological factor occupied the foremost position in distribution of vegetation and its structure. On the basis of their water requirements, Warming primarily recognised three major groups of plants:
Aquatic plants (Hydrophytes): living in abundance of water with their lower parts i.e., roots, rhizomes etc., and leaves immersed in water.
Land or terrestrial plants: living in normal water conditions with their assimilatory organs adapted to existence in air. Land plants exhibit many grades of adaptation to their mode of life. Thus, those which encounter greatest difficulties in securing water are the xerophytes, while others as Mesophytes, which in some respects occupy an intermediate position between the two extremes, the hydrophytes and the xerophytes.
They are very extensive on the land surface. They are such land plants, which grow in moist habitats and need well-aerated soils. They prefer soil and air of moderate humidity and avoid soil with standing water or containing a great abundance of salts. In some respects they stand in between the hydrophytes and xerophytes. Broad-leaved trees growing in wet depressions, along lakes and rivers, are mesophytes. They generally lack the special structural and physiological adaptations found in hydrophytes and xerophytes. For example, these plants may suffer from temperature stress in extreme hot temperature. These plants do not have any special adaptation to trespass this. But in the presence of sufficient water in soil these plants can survive by increasing the rate of transpiration by opening stomata. It means evaporating the water will dissipate the heat. Also this plant can stand the water-saturated soil for limited time when the temperature is not very high. But in dry weather these plants will suffer from water stress. Here also no adaptation is present to fight this condition and the only way is to close the stomata. As the stomata gets closed, there is decrease in surface are of leaf exposed to environment, so the rate of transpiration also decreases. This long period of dehydration can lead to permanent wilting, cell plasmolysis and death. Most crops are mesophytic plants examples are: corn (maize), privet, lilac, goldenrod, clover, and oxeye daisy.
The general morpho-anatomical features of mesophytes are as follows:
1. Root system is well developed. Roots are generally fairly branched, with root caps and root hairs.
2. Stems are generally aerial, solid and freely branched.
3. Leaves are generally large, broad, thin and varied in shapes; generally oriented horizontally, green, without hair or waxy coatings.
4. Cuticle in all aerial parts moderately developed.
5. Epidermis well developed, without any hair or waxy coatings and cells without chloroplast.
6. Stomata generally present on both surfaces of leaves. Guard cells show frequent movements.
7. Mesophyll in leaves is differentiated into palisade and spongy parenchyma, with many intercellular spaces.
8. Vascular and mechanical tissues fairly developed and well differentiated.
9. They may show temporary wilting during noon hours.
