836 VEGETABLE PHYSIOLOGY 
CHAPTER XXI 
INFLUENCE OF THE ENVIRONMENT ON PLANTS 
The ultimate form of a plant is such as to secure the most 
harmonious relations between itself and its environment. 
Such relations are inseparable from a healthy condition. 
It is clear, therefore, that with varied conditions of the en- 
vironment we must expect modifications of both form and 
structure. It is impossible in such a work as the present 
to do more than touch upon so large a subject, full of detail 
as it must necessarily be. It should nevertheless engage 
our attention, though we can do little more than illustrate 
it, for it has a very important bearing upon the power of 
a plant to respond to variations in its external conditions, a 
power which must be associated with a kind of nervous 
system. 
According to the nature of their surroundings and the 
consequent differences in their mode of life, we find in 
many plants certain peculiarities of form and structure in 
which they differ from most of those which we have hitherto 
considered. Of these the vascular plants which live in 
water may be first discussed, as the direct influence of the 
environment is most conspicuous in their case. 
These aquatic plants, most of which are Spermophytes, 
but which include a few of the Pteridophytes, may be 
divided into two chief groups: those which are altogether 
submerged, and those which bear floating leaves as well as, 
or instead of, submerged ones. 
In the former case the plant-body may be attached by 
roots to the bottom of the stream or pool in which it lives, 
