330 VEGETABLE PHYSIOLOGY 



CHAPTER XXII 



INFLUENCE OF THE ENVIRONMENT ON PLANTS 



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 Spermaphytes, 

 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. 

 Some have however very short stems which give rise to 

 numerous elongated ribbon like leaves. These flexible 

 stems depend for their support upon the nature of the 

 medium in which they live, and though they possess a cer- 

 tain rigidity, this is not associated with any great develop- 

 ment of woody tissue. Generally the latter is reduced to a 

 minimum ; the fibro-vascular bundles are usually few and 

 contain few lignified elements. Their substance is largely 

 parenchymatous and the cells have thin walls. The inter- 

 cellular space system is often very complex, large lacunae 

 filled with air occupying considerable space in the distribu- 

 tion of the tissues (fig. 141). Their rigidity is secured by 



