PLANTS AND WATER 133 
may be incidental to the particular kind of 
habitat in which a plant is growing. For 
example, plants which live on bare rocks, or 
on tree trunks, are evidently exposed in a 
greater degree to intermittence in water 
supply than those which are rooted in the 
soil. We find that such lithophytic and epi- 
phytic vegetation is especially rich in species 
that exhibit wonderful adaptations to their 
own particular environment, adaptations 
which enable them successfully to cope with 
the difficulties and disadvantages that so 
obviously face them. 
A somewhat wider survey of the water 
problem as it affects vegetation generally, 
shows that it is necessary to distinguish clearly 
between that kind of drought which is merely 
physical, i.e. is due to actual scarcity of water, 
and another kind which may be more properly 
described as physiological. In the latter case 
a plant, however favourably it may seem to 
be situated so far as access to water is con- 
cerned, may nevertheless be unable to absorb 
it in sufficient quantity. 
This may happen when the temperature 
of the medium is too low; for the active 
absorption by roots is only possible within 
a rather narrow range of temperature, the 
limits varying somewhat for different plants. 
Or the water itself may contain substances 
in solution which prejudicially affect the 
exercise of the absorptive functions. Thus 
the water of salt marshes, as well as that 
