Deserts are formed in regions with less than 25 cm of annual rainfall, .or sometimes in hot regions where there is more rainfall, but unevenly distributed in the annual cycle.
Lack’ of rain in the mid latitude is often due to stable high pressure zones; deserts in temperate regions often lie in “rain shadows”, that is where high mountains block off moisture from the seas.
The climate:of these biomes is modified by altitUde and latitude. At greater distance from the equator the deSerts are cold and hot near equator and tropics.
As the large volume of water passes through the irrigation system, salts may be left behind that will gradually accumulate over the years until they become limiting, unless means of avoiding this difficulty are devised
Adaptations
(i) These plants conserve water by following methods:
They are mostly shrubs. Leaves are absent or reduced in size.
Leaves and stem are succulent and water storing.
In some plants even the stem contains chlorophyll for photosynthesis.
Root system is well developed and spread over large area.
The annuals wherever present germinate, bloom and reproduce only during the short rainy season, and not in summer and winter.
(ii) The animals are physiologically and behaviorally adapted to desert conditions.
They are fast runners.
They are nocturnal in habit to avoid the sun’s heat during day time.
They conserve water by excreting concentrated urine.
Animals and birds usually have long legs to keep the body away from the hot ground.
Lizards are mostly insectivorous and can live without drinking water for several days.
Herbivorous animals get sufficient water from the seeds which they eat.
Mammals as a group are poorly adapted to deserts
Indian Desert — Thar desert (hot)
The climate of this region is characterised by excessive drought, the rainfall being scanty and , irregular.
The winter rains of northern India rarely penetrate into the region.
The proper desert plants may be divided into two main groups.
- i) depending directly upon on rain and
- ii) those depending on the presence of subterranean water.
The first group consists of two types:
the ‘ephemera’s’ and the rain perennials’.
The ephemera’s are delicate annuals, apparently free from any xerophilous adaptations, having slender stems and root-systems and often large Flowers.
They appear almost immediately after rain, develop flowers and fruits in an incredibly short 
time, and die as soon as the surface layer of the soil dries up.
The rain perennials are visible above the ground only during the rainy season, but have a perennial underground stem.
The second group – depending on the presence of subterranean water
By far the largest number of indigenous plants are capable of absorbing water from deep below the surface of the ground by means of a well-developed root system, the main part of which generally consists of a slender, woody tap root of extraordinary length.
Generally, various other xerophilous adaptations are resorted to such as reduced leaves, thick hairy growth, succulence, coatings of wax, thick cuticle, protected stomata, etc., all having for their object of reduction of transpiration.
Fauna
It is home to some of India’s most magnificent grasslands and sanctuary for a charismatic bird, the Great Indian Bustard. Among the mammal fauna, the blackbuck, wild ass, chinkara, caracal, Sandgrouse and desert fox inhabit the open plains, grasslands, and saline depressions.
The nesting ground of Flamingoes and the only known population of Asiatic wild Ass lies in the remote part of Great Rarm, Gujarat.
It is the migration flyway used by cranes and flamingos.
Some endemic flora species of Thar Desert includes Calligonum Polygonoides, Prosopis cineraria, Tecomella undulate, Cenchrus biflorus and Sueda fruticosa , etc
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