329 
CHAPTER XXI 
INFLUENCE OF THE ENVIRONMENT ON PLANTS 
Tue 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 
environment 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 neverthe- 
less engage our attention, 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 asso- 
ciated 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, 
or it may be altogether floating. ‘The stems are generally 
long and slender, and easily swayed to and fro in the water. 
