B36 VEGETABLE PHYSIOLOGY 



CHAPTEK 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, 



