Xerophytes

Xerophytes are plants adapted to live in dry (xeric) conditions, and 'dry' can mean two quite different things here: physical dryness, where the soil itself genuinely holds very little water because of low rainfall, or physiological dryness, where water is actually present in the soil but the plant cannot take it up — for instance because the soil lacks the fine capillary spaces needed to hold water in an accessible form (as in salty or acidic soils). Based on how they cope with dryness, xerophytes fall into three categories. Ephemerals (also called drought escapers/evaders) are not true xerophytes at all in a structural sense — instead they dodge the problem entirely by completing their whole life cycle within a single short favourable season, so they are never actually present during the harshest dry period (e.g. Argemone, Mollugo, Tribulus, Tephrosia). Succulents (drought-enduring plants) instead store water in their tissues during wetter periods and draw on those reserves through the dry season, developing a suite of adaptive traits to resist extreme drought (e.g. Opuntia, Aloe, Bryophyllum, Begonia). Non-succulents (drought-resistant plants, the true xerophytes) face dryness head-on — both externally and internally — and rely on a large toolkit of structural adaptations to survive it (e.g. Casuarina, Nerium, Ziziphus, Acacia). Morphologically, xerophyte roots are extensively developed, often exceeding the shoot system in size, with well-developed root hairs and root caps; stems are typically hard and woody (aerial or underground) and are often coated in wax or dense hairs; and some xerophytes convert entire internodes into fleshy photosynthetic stem structures — phylloclades (Opuntia), cladodes (Asparagus), or convert the petiole itself into a flattened phyllode (Acacia melanoxylon) — while leaves may be leathery and shiny (to reflect light and heat), reduced to spines (from modified stipules, as in Euphorbia, Acacia, Ziziphus, Capparis, or from the whole leaf as in Opuntia), or reduced to small scales (Asparagus); some xerophytic plants that instead grow dense covering hairs on their leaves and stem are called trichophyllous plants (e.g. the cucurbits Melothria and Mukia). Anatomically, xerophytes show a multilayered epidermis with a heavy protective cuticle, a well-developed sclerenchymatous hypodermis, sunken stomata confined to the lower epidermis (often within hair-lined pits to trap humid air and cut transpiration), well-developed vascular bundles with a multilayered bundle sheath, and clearly di …