The term �Xerophyte� has been defined and interpreted variously. Morphological features and rates of transpiration have almost failed to explain the true nature of this group of plants. They are sometimes loosely defined as �plants of dry habitats�. But if taken in this sense, most of the mesophytes should be called xerophytes. Daubenmire (1959) in his discussion on the group defined xerophytes as �plants, which grow on substrata that usually become depleted of growth water to a depth of at least 2 decimeters during a normal season�. Thus, in arid zones, all plants not confined to the margins of streams or lakes have been considered as xerophytes, whereas in regions of heavy rainfall the class would be represented only by shallow rooted plants of sandy soils, by plants of dry ridge tops, and by algae, mosses and lichens which grow on tree barks or rock surfaces etc.
The true nature of xerophytism is not clearly understood. For example, it is difficult to decide whether a xerophyte is really xerophilous and occurs only in dry habitats and desserts, or is merely drought resistant. There are instances, for example, that, if desert plants are grown in soils with moderate amount of water, they show better growth as in Artemisia fasciculate, Zygophyllum fabago, and Centaurea solsitialis, the yield of plants increased to 1.5-2 times by increasing water holding capacity of soil from 40% to 60%. Thus, such xerophytes are not xerophilous at least so far as the soil moisture is concerned. Oppenheimer (1960), in a literature survey, favors a much broader interpretation including all the morphological, anatomical and physiological modifications, which may assist the plant to cope with environmental water deficit.
On the basis of their morphology, physiology and life cycle pattern, xerophytes are generally classified into the following three categories:
Ephemeral annuals:
They are also called as �drought evaders� or �drought escapers�. They are mostly found in arid zones. They are annuals, which complete their life cycles within a very short period of 6-8 weeks or so. With their small size and large shoots in relation to roots, they are well adapted to such dry habitats. They actually avoid and not withstand dry seasons, and thus escape dryness in external and internal environments. Some do not prefer to call them true xerophytes. Examples- Argemone Mexicana, Solanum xanthocarpum, Cassia tora, Tephrosia purpurea, etc.
Succulents:
They are the plants that suffer from dryness in external environment only. Their succulent, fleshy organs (stem, leaves, roots) serve as water-storage organs, which accumulate large amount of water during brief rainy seasons. In cacti moreover, the root systems also become shallow. Their root system is shallow, stem swollen and leaves thick, leathery and succulent. Some of the examples are Aloe, Euphorbia and Opuntia and various cacti, Agave and Ceiba parviflora. As the succulents avoid drought, some prefer to exclude them from true xerophytes. This is indeed a unique mode of adaptation.
Non-succulent perennials:
These are actually the true xerophytes or drought resistant, because they possess a number of morphological, anatomical and physiological characteristics, which enable them to withstand critical dry conditions. They are the plants that suffer from dryness both in their internal as well as external environments. Their morpho-physiologic features include rapid elongation and extensive root system, high osmotic pressure and endurance of desiccation, ability to reduce transpiration to an extremely low level during wilting, and reduction in size of leaf blades and cells. Example Calotropis procera, Acacia nelotica, Zizyphus jujuba and Capparis aphylla etc.
Morphological features of Xerophytes:
1. Root system- well developed, extensive, long, taproots, extensive branching spread over wide area, root hairs present with well developed root caps.
2. Stem- stunted, woody, dry, hard, ridged, covered with thick bark. In some it become underground (Saccharum) whie in some it become fleshy, green, leaf like covered with spines. Stems and leaves are covered with hairs or waxy coating.
3. Leaves- much reduced, scale like or modified to spines. Lamina may be long, narrow or needle like as in Pinus or divided into many leaflets like Acacia. Foliage leaves may become thick, fleshy and succulent, or tough and leathery in texture. Leaf surface are shiny and glazed to reflect light and heat.
