Thick waxy cuticle.
In most plants up to 10% of the water loss by transpiration is actually through the cuticle. Some plants have a particularly thick waxy cuticle to help minimise water loss. This adaptation is common in evergreen plants and helps them survive both hot dry summers and cold winters when water can be hard to to absorb from the hard/frozen ground. Example: Holly
Sunken and reduced number of Stomata
Many xerophytes have their stomata located in pits, which reduce air movement, producing a microclimate of still, humid air that reduces the water vapour potential gradient and so reduces transpiration. Example: Cacti
Many xerophytes have reduced numbers of stomata, which reduce their water loss by transpiration but also reduces their gas exchange capabilites.
Some xerophytes have very hairy leaves that, like the spines of some cacti, create a microclimate of still, humid air, reducing the water vapour potential gradient and minimising the loss of water by transpiration from the surface of the leaf. Some plants (marram grass) even have microhairs in the sunken stomata pits. Example: Borage leaves
By reducing leaf area, water loss can be greatly reduced. The leaves of conifers are reduced to thin needles. These narrow leaves, which are almost circular in cross-section, have a greatly reduced SA:V ratio, minimising the amount of water lost in transpiration. Example: Barrel Cactus
Another adaptation that greatly reduces water loss by transpiration, especially in combination with other adaptions, is the growth of curled or rolled leaves. This confines all of the stomata within a microenvironment of still, humid air to reduce diffusion of water vapour from the stomata. Example: Marram grass
Succulent plants store water in specialised parenchyma tissue in their stems and roots. They get their name because, unlike other plants, they often have a swollen or fleshy appearance. Water is stored when it is in plentiful supply and then used in times of drought. Example: Aloe Vera
Some plants prevent water loss through their leaves by simply losing their leaves when the water is not available. Example: Palo Verde, a desert tree that loses all of its leaves in the long dry seasons. The trunk and branches turn green and photosynthesise with minimal water loss to keep it alive.
Many xerophytes have root adaptations that help them to get as much water as possible from the soil. Long tap roots growing deep into the ground can penetrate several metres, so they can access water that is a long way below the surface. A mass of widespread, shallow roots with a large surface area able to absorb any available water before a rain shower evaporates is another adaptation. Example: Giant Saguaro cacti, which can get enough water to grow to around 12-18 metres metres tall and live for around 200 years.
Avoiding the problems
Some plants are adapted to cope with the problems of low water availability by avoiding the situation entirely. Plants may lose their leaves and become dormant, or die completely, leaving seeds behind to germinate and grow rapidly when rain falls again. Others survive as storage organs such as bulbs (daffodils), corms (crocuses), or tubers (potatoes). A few plants can withstand complete dehydration and recover- they appear dead but when it rains the cells recover, the plant becomes turgid and green again and begins to photosynthesise. The ability to survive in this way is linked to the dissacharide trehalose, which appears to enable to the cells to survive unharmed.