How do arid plants store water?

Plants in the arid growth zone are adapted to obtain and retain moisture. In the process of evolution, they have got hold of various adaptations. The trunks of some plants have a waxy coating, while others have leaves modified in the form of thorns, some are completely absent. There are even plants that do not grow up, but down. Such plants tolerate heat and lack of water well, examples of such plants are cacti and velvichia.