Xerophytes are adapted to grow in dry habitats. Daubenmire (1950) defined xerophytes as "plants which grow on substrata that usually become depleted of available water to a depth of atleast 20 cms during a normal season". On the basis of their morphology and life cycle generally classified into the following four categories.
- Drought escapers or drought evaders (Ephimerals) - These plants are drought escaping species which complete their life cycle in a very short period. They are generally found in arid zones and are characterized by their small size and larger shoots than roots. These plants are also called as rought evaders or drought escapers, Common examples are Cassia tora, Argemone mexicana, Solanurn xanthocarpum, etc.
- Annuals or drought evading - Drought evading plants require small quantity of water for their growth and development. These plants are similar to ephemeral xerophytes but grow for longer periods. They have modifications to reduce transpiration and thus conserve moisture. These modifications are spines, thorns, leathery leaves and latex. Examples are Echinops echinatus, solanum, surattense, etc.
- Succulents or drought resisting - The succulents are perennial plants dry areas especially those with sandy soils, like sandy beaches. They assess water storage regions made up of thin walled parenchymatous cells. Their succulent, fleshy organs (stems, leaves and roots) serve as water storage organs which accumulate large amounts of water during rainy season. Among cacti and Euphorbias, the leaves are absent or modified into spines. Examples Aloe, Bryophyllum, Opuntia, Euphorbia etc.
- Non-succulent perennials or drought enduring - They are also called true xerophytes or euxerophytes because they are capable of enduring long periods of drought and permanent wilting without having any mechanism of storing water. Examples are Calotropis procera. Nerium, Zizyphps jujuha, etc.
1. Root - The roots in such plants are extensively branched, so that they can catch hold of as much water as possible. The roots are elongated and richly covered with root hairs so that plants may absorb water from deeper layers of soils.
2. In some herbaceous xerophytes the roots become fleshy in character, so that they are able to accumulate large quantities of water, e.g., Dandelion etc.
3. Stem - In some plants the axis or stem become short and thick e.g., Euphorbia, Stapelia and on several members of cactaceae. In certain plants the stem are found to be filled with mucilage sap. In few plants like Ephedra they become slender, dry, bittle and hard to prevent the loss of water.
4. Mostly xerophytic plants are prostrate in habit e.g., Euphorbia prastrata, Sucoda jlucticosa etc.
5. In stem of certain xerophytic plants the chlorophyll tissue is developed e.g., Cactus, Ruscus, Capparis.
6. Leaf - The leaves are densely coy erred with hairs and cuticle in Xerophytic plants, so as to reduce excessive evaporation of water and also to reduce light.
7. The leaves of most of xerophytes are highly reduced because -
- The leaves are folded or rolled so that some of the stomata are packed in side and thus save loss of water.
- Leaves are smaller in size and also the number of leaves on plant is greatly reduced.
- In some cases the leaves are highly dissected into linear or thread like lobes and are separated apart.
- In some places the leaves are totally absent and they become modified into spines example - Cactus. In species of Euphorbia and some other plants the leaves fall off too early.
9. In some plants leaves become thickened, leathery in texture i.e., hard and stuff. They store a lot of water due t abundance of mucilage and gums. e.g., Salvadora, Aloe, A gave, etc.
10. The leaves and leaflets assume a vertical position or at any angle to the sun rays so that large surface of the leaf is prevented from being exposed to light.
11. The leaves are usually dull, greenish in colour.
Anatomical features
some Anatomical characteristic features of xerophytes are-
- Mostly the xerophytes have a somewhat specialized epidermis, usually the eqidermal cells become cutinized and radically elongated. Some times a waxy substance is produced-on the cuticle e.g., Calatropis.
- The stomata are very minute openings formed in the epidermal layer in green aerial parts of the plants. The stomata are mostly on inner side, very much reduced in size and number. To reduce excessive transpiration, usually the stomata sunken in pits are produced and commonly termed as "Sunken stomata".
- Mostly a multilayered, lignified hypodermis is present immediately beneath the cutinized epidermis.
- The xerophytes commonly have a large proportion of sclerenchyma in their leaf structure than the mesophytes. The tissue is either found in groups or continuous sheets. The selerenchyma layers check excessive transpiration to some extenty and also strengthen the plant body. The xerophytes which possess heavy sclerification on the leaves may be easily called "sclerohyllous" as for example: Dasylirion.
- Presence of hairs on epidermis or over the entire aerial part of the plant is a feature of xerophytes. The hairs prevents rapid evaporation through stomata. Xerophytes which have abundance of hairs are called 'trichophyllous'.
- Many xerophytic genera have the stomata confined to ventral surface, uid when there is a deficiency of water the leaves roll up so that all the stomata are on the inner surface of rolled leaf, example- Ammonphila arenasia.
- Many xerophytes possess very reduced neddle like or sealy leaves exampl - Casuarina, Asparagus etc. Hence the reduction of leaf surface prevent evaporation because of the relatively small total surface of the leaf, Xerophytes, which reduced leaves are termed as "microphyllous".
- Many fleshy xerophytes possess water storage tissue in them. The xerophytes possess succulent stems or leaves, which consist largely of special parenchymatous tissue used for water storage as for example- Cactus, Aloe, Opuntia etc.
- The palisade tissue in stem of many xerophytes plants remain present as for example - Capparis. In the leaves the palisade is compact and found in abundance.
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