Describe adaptations of plants that live in dry conditions, such as the desert.

Plants that live in these areas need to minimise the rate at which they transpire in order to preserve as much water as possible. 

One way they do this is by reducing their surface area:volume ratio, in many cases they are reduced to just spines like on a cacti. This reduces the amount of leaf exposed to the sun, which therefore reduces evaporation and transpiration of water in the leaf. 

in order to get the water in the first place, these plants have two types of roots. Very deep root are required to reach the very low groundwater and shallow roots that spread far to collect the occassoinal rainfall.

In these plants there are often fewer or smaller stomata (the holes in which gas exchange can occur from the air inside and outside the leaf). Fewer stomata means less water will transpire from the leaf. Another adaptation is for the stomata to close during midday when it is hottest. although this means that the plant cannot photosynthesise, it also means the plant has water to survive, so it is a compromise. 

Other adaptations plants can make to reduce transpiration are rolled leaves, leaf hairs and stomata sunken in pits. All of these adaptations trap moist air which increases humidity. This results in a smaller diffusion gradient for the water vapour between the air and the stomata, slowing the diffusion of the water vapour away from the stomata.