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 melanoxylon, 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 
