Adaptation to Environment. 307 



able to absorb much water, it would be fatal to them to 

 permit rapid transpiration. Accordingly we find under 

 these circumstances a diminution in the transpiring surface 

 by a reduction in the number and extent of the branches 

 and leaves, as in the case of cacti, for example. There is 

 also usually an increase in the thickness and waterproofing 

 qualities of the outer wall of the epidermis, a restriction of 

 the stomata to the under sides of the leaves, a depression 

 of the stomata below the surface of the leaf, as in the 

 leaves of the India rubber tree, a reduction in the size 

 of the intercellular spaces, and frequently the above- 

 ground parts are covered with hairs, scales, etc., which 

 retard transpiration and reduce the intensity of illumina- 

 tion. There is also frequently an occurrence of speciaUzed 

 cells and tissues for the storage of water, and of cells con- 

 taining mucilage, which assists in retaining water within 

 the plant. 



189. Conditions affecting Absorption. — There are certain 

 conditions under which plants absorb water with difficulty 

 even when it is present in abundance. Thus, if there is a 

 large percentage of salts dissolved in the water, or a large 

 amount of humic acid produced by the disintegration of 

 organic remains, as in peat bogs ; or if the soil and soil 

 water are cold, plants absorb the water with great difficult}'. 

 Under such circumstances they must be modified to reduce 

 transpiration just as if little water were present. 



190. Effect of Submergence in Water. — We find quite 

 another class of modifications in those plants which are 

 rooted in earth saturated with water, such as the mud at 

 the bottom of ponds, etc. In these plants there is not so 

 much need of retarding transpiration, but access of air to 

 the submerged parts must be provided for. Accordingly 

 we find the stomata nearly all transferred to the upper side 



