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How Cactus survive in the desert
   Most cacti do come from warm weather/low rainfall areas of the world although there are exceptions.  There are no known fossils of cacti, and it is not known with certainty when they developed the unique adaptations that enable them to live in the harsh environments that many of them survive in.  Many scientists believe that cacti developed their physiological traits in response to changing climatic conditions several million years ago.
    The major traits that people see when they first observe cacti is the abundance of spines on many species.  These spines serve several uses, they guard against most browsing herbivorous animals by making the plant difficult and dangerous to chew. The spines also help to shade the plant, helping keep internal heat down. Finally the spines also can channel the infrequent rains to the base of the plant.
    Another easily identified characteristic of many cacti is a waxy coating that surrounds the plant over the skin, this often has a bluish cast to it, and may be damaged by careless handling. This is called a 'glaucus bloom' and help to reduce evaporation by the plant, this holds in more of the precious moisture so rare in the desert.
    All advanced plants have leaf holes called 'stomata', these holes are what open to permit entry and exit of various gases and moistures, in cacti these stomata close tighter than in most plants further reducing the loss of moisture in the heat of the day.
    Another thing these plants can do is to store up lots of water when it does fall, many have the ability to open themselves up accordion style to hold massive amounts of water. Then when the plant is loosing water, the pleats of the accordion fold up shading the plant more and reducing surface area exposed to the sun.
    When the plant is stressed and not getting enough water it will often bend over as the 'hygroscopic'  (water filled) pressure inside the plant diminishes. The hygroscopic pressure is what holds the plant up erect, as it lessens the plant loses its 'turgidity' and can no longer stand erect, as it bends over it's exposure to the sun is reduced, much of it is then shaded by the overhead portion and moisture loss is further lessened.
    Root behavior is another adaptation that cacti have made in order to live, they trend to have roots that spread laterally for quite a ways and may exist in a suspended state until the rains activate them, a small amount of moisture will cause the feeding roots to quickly grow out from the main roots, these feeders are what bring in the water and it's dissolved minerals and nutrients. After the rain is gone and as the soil dries these feeder roots die and disappear enabling the plant to live on it's stored water without having to spend energy and moisture keeping these roots alive that may not be needed for many many months.
 
    Cacti are indeed very vigorous plants  that have made a good go of it in their arid environments, they are joined in this by the old world succulents which are a good example of convergent evolution.

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http://www.rivenrock.com/adaption.htm
 Last updated 27 July 2013