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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 

 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. 



