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 
