THE FLORAL ENVIRONMENT 61 



barely be distinguished from them until they flower. 

 This is an example of convergence, the production of 

 very similar structures by plants which are not at 

 all related to one another, as a response to similar 

 environmental influences. On the other hand, succu- 

 lents are rare in the Great Palaearctic Desert, speaking 

 generally, and the cause is unknown, for in many 

 places the conditions of weather and soil are such 

 as appear to favour succulents in American and other 

 deserts. The only noteworthy exception is the salt 

 parts of the Great Palsearctic Desert : in these such 

 genera occur as are familiar in saltings in many 

 parts of the world. 



Nearly every part of the plant body, but most 

 commonly stems and leaves, may be used for the 

 storage of water. In the Cactus family, for instance, 

 the stem is greatly thickened and often nearly 

 globular : it is used for the storage of water, and 

 MacDougal states that tree cactuses may contain as 

 much as a thousand gallons. Succulence of the stem 

 occurs also in spurges and many other families : in 

 Southern Mexico there is an Ipomsea, a plant closely 

 alHed to the convolvulus, which has a soft thick 

 trunk containing water, and Beaucarnea, a relative 

 of the Yuccas, is so charged with water that its 

 trunk is 7 or 8 feet in diameter, though the tree 

 itself is not above 25 feet high. An example of 

 a plant with succulent leaves, Calotropis procera 

 (Asclepiadacese) is shown in Fig. 27. 



It will be seen that there is a most interesting 

 contrast between the second and third groups of 

 desert plants. Members of the second group have 



