PLANTS AND WATER 147 



The spiny plants are similarly built on lines 

 calculated to limit the output of water, 

 though why the reduced branches and leaves 

 should so commonly assume the form of 

 spines it is not easy to say. The supposed 

 function of the spines in keeping off browsing 

 animals is a ready, but not very satisfying, 

 explanation. 



Observation teaches that both classes of 

 plants, the spiny and succulent, are of com- 

 paratively slow growth. But it is not quite 

 correct to assert of xerophytes generally that 

 in their habits and rate of growth they com- 

 pare unfavourably with the mesophytes. The 

 truth rather is that they are able if need be 

 to support life on a very limited income, by 

 cutting down their expenditure in various 

 directions. It is by this faculty of exercising 

 economy that they are enabled to flourish in 

 regions from which the less hardy mesophytes 

 are excluded. A large number of xerophytes 

 are, however, by no means solely adapted to 

 a life of austerity. Transplanted and grown 

 under ameliorated conditions, they often 

 respond to the change by a rapid and vigorous 

 growth. It seldom happens, however, that 

 they are able to hold their own in competi- 

 tion with the mesophytes in a natural en- 

 vironment suitable for the latter, and they 

 commonly become killed out sooner or later 

 by their more vigorous rivals. 



There are a few of the highly specialised 

 xerophytes, such as cacti, which are so 



