XIV] AQUATICS : DESERT PLANTS 143 



We meet with indications of this everywhere, and in 

 some of the situations referred to the modified leaf- 

 structure is so remarkable that we must examine a few 

 examples and compare them with the type. 



We have already seen that floating leaves have their 

 stomata above, and submerged ones have usually none at 

 all ; and that the venation of aquatic plants generally is 

 inconspicuous, because there is no active ascent of water 

 to meet the requirements of transpiration in dry air 

 and which would demand a voluminous system of pipes 

 (vessels); also that there is no need for strong supports 

 (fibres, &c.) to carry the weight of the leaf and keep its 

 lamina stretched in the air. Similarly the tissues of 

 water-plants are thin and soft, and no thick cuticle or 

 other protection against excessive evaporation is needed. 



If we contrast the leaves of plants adapted to the 

 other extreme of environment i.e. situations liable to 

 prolonged drought it is suggestive how common are 

 leaves closely rolled up and exposing little surface, or 

 covered with dense blankets of hairs, wax, thickened 

 cuticle or other protection against the access of solar 

 rays ; or such as have close-set thick-walled tissues and 

 few stomata, or provided with stores of water held fast by 

 special water-holding cell-contents ; or leaves exhibiting 

 simultaneously two or more of these and other peculiari- 

 ties, which suggest adaptations against difficulties of ob- 

 taining or keeping their water-supplies in time of drought. 



Many such plants especially those inhabiting per- 

 sistently arid regions such as true deserts have given 

 up forming leaves at all e.g. many Cacti but I am here 

 concerned with plants which do form leaves, and such 

 leaves, to be able to exist at all, must be adapted in some 

 way for protection against excessive transpiration. 



A word of warning is here necessary, however. It is 



