Warm desert and semi-desert biomes exist where available moisture is so low that all plants and animals must possess special drought resistant mechanism in their bodies or their behavour. Warm deserts are located around the tropics because of the high temperatures and calm descending air found there.
Such deserts also form in continental interiors, or along sea coasts that have an upwelling cold current offshore as in case of Chile, Western Sahara, and Namibia. These true deserts experience summer fog that mists the plant and animal population with needed moisture.
As regards vegetation, only such plants exist in deserts that have evolved mechanism to combat drought. Such plants are known as xerophytes. They are perennial shrubs whose root systems below the ground are much more extensive than their visible parts; shrubs have evolved tiny leaves with a waxy covering to check transpiration.
Xerophytic plants have their trunks and limbs that photosynthesize like leaves or that have expandable tissues. Shrubs shed their leaves until moisture makes them reappear.
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Desert plants like cactus adapt themselves to arid conditions by storing moisture in their stems or leaf cells. There are various types of flowering plants. The plant cover in these arid lands undergoes wide variation from season to season and year to year.
The semi-desert shrub is found on the dry side of the tropical thorn forest. Actually the semi- desert vegetation is considered to be a transitional habitat between true deserts and savanna.
The bushes in these semi-desert regions are of fair size. Whatever grasses are in existence. They are very short, wiry, and reduced to isolate bunches. Herbs tend to be leathery and fleshy-leafed. Cacti in the Americas and cactus like plants on other continents are a characteristic feature of the usually widely-spaced vegetation of a semi-desert region.