PLANTS AND WATER 139 



photosynthesis) by another factor which also 

 influences the process of nutrition as a whole. 



The reduction of leaf surface, which is of 

 intermittent or annual occurrence in our every- 

 day vegetation, may become the normal state 

 of a more specialised xerophyte. The leaves 

 may be very small, or they may even be 

 practically absent so far as photosynthetic 

 function is concerned. In such plants, how- 

 ever, this office may continue to be discharged 

 by the stems, which remain green, and thus 

 to some extent may take the place of the 

 leaves. Their special advantage in this con- 

 nection is mainly due to their structure, and 

 to the relatively small number of stomata, 

 which enables them to check the escape of 

 water from the plant. 



It is a singular fact that when a species 

 or race has once exhibited a tendency towards 

 the loss or atrophy of an organ, e. g. the leaf, 

 the descendants commonly appear to be un- 

 able to check it. If any of them vary in such 

 a way as to increase their green surface, this 

 is effected not by enlarging their diminished 

 leaves, but by flattening and specialising 

 some other organ. Sometimes the process 

 may even be seen to accompany the diminu- 

 tion of the leaves, as in some species of acacia. 

 In Acacia melanoccylon, for example, we find 

 the leaf stalk gradually flattening, and 

 assuming the functions generally undertaken 

 by the blade, which becomes completely 

 atrophied. Other species of acacia show the 



