Adaptations in xerophytes are of two types:
(i) Xcromorphic adaptations are those which are inherited whether the xerophyte grows in xeric conditions or not. For example, a Cactus has the same feature, whether it is in a desert or in a normal land.
(ii) Xeroplastic adaptation is the ones that are induced temporarily but disappear when the conditions are favourable.
Xerophytic adaptations may be morphological, anatomical or" physiologiclll.
A. Morphological adaptations
Xerophytes exhibit a number of special features in their morphological organs.
The root system is well developed, extensive and much branched.
Roots of perennial xerophytes reach greater depth to absorb water but some xerophytes
have shallow root system especially when water is available in the surface layers
Root hairs are profuse.
Stems are hard and woody.
Some stems are covered with dense hairs (Calotropis), coated with wax (Opuntia) or silica (Equisetum).
Stems in some xero-phytes are modified to thorns (Duranta).
Succulents have their stems modified into structures like phylloclades (Opuntia.); cladodes (Asparagus) or leaf like structure (Ruscus). All such struc- tures are usually meant for water storage.
Usually leaves of xerophytes are reduced or modified to various kinds of structures to minimise transpiration. The following types of condition are seen:
(i) Microphyllous when the leaves are small scaly (Casuarina: Asparagus) or needle like (Pinus)
Trichophyllous when the leaves are covered with hairs (Nerium, Calotropis (iii) Macrophyllous when the leaves are soft and fleshy (Begonia) Sclerophyllous when the leaves are stiff and hard. (Banksia) Many xerophytes have no leaves (Capparis) or they fall very early (caducous) as in Euphorbia.
Rolling of leaves is observed in some xerophytes like Ammophile where the stomata are directed inwards.
Anatomical adaptations can be conveniently discussed under the headings- epidermis, hypodermis, ground tissue and vascular tissue.
(i) Some xerophytes have multiple epidermises (Nerium).
(ii) Epidermis is with thick cuticle and deposition of waxes, resins etc.
(iii) There are epidermal hairs especially in grooves (furrows) that protect the sunken stomata.
(iv) Mostly stomata are sunken and are in pits.
(v) Stomatal frequency is low.
(vi) Leaves that have the capacity to roll have specialised cells called bulliform cells that help in rolling.
Iiypodermal layers of xerophytes are thick and well developed. Ground tissue
(i) In stems there is abundant mechanical tissue in the form of sclerenchyma as in Casuarina stem.
(ii) Since leaves are reduced, the stems usually have chlorenchyma.
(iii) In succulent plants, cortex is filled with water, mucilage, latex etc.
(iv) In plants that have leaves, palisade parenchyma is well tievelopedO
(v) In Pinus mesophyll cells are modified.
(vi) Intercellular spaces are greatly reduced. Conducting tissue
Vascular tissue (xylem and phloem) is very well developed in xerophytes. C. Physiological adaptations
Xerophytes show a number of physiological features:
Transpiration is well regulated.
Osmotic concentration of the cell sap is high.
Succulents have high pentosans (derived from polysaccharides) resulting in accumulation of water.