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 



