344 VEGETABLE PHYSIOLOGY 



remain green when the host has lost its foliage, and by 

 their activity they to some extent assist the tree on which 

 the mistletoe is growing. The relationship seems to be 

 almost one of symbiosis rather than of parasitism. Pro- 

 bably the relationship of the root-parasites and their hosts 

 is also one of mutual assistance rather than true para- 

 sitism. 



The habit of capturing insects, which we have seen to be 

 characteristic of several plants of very different forms, may 

 also be looked upon as connected with their environment. 

 Many of them, e. g. D rose ra, grow upon a substratum which 

 is largely composed of plants of Sphagnum, and which yields 

 to them a very limited supply of nitrogenous compounds ; 

 others are found growing on the surface of rocky mountains, 

 into the chinks of the stones of which their roots penetrate ; 

 others again flourish in the sandy soil of deserts ; in all of 

 which situations compounds of nitrogen exist only in very 

 small amount. The organic substances yielded by the 

 decomposing bodies of the captured insects must therefore 

 form a valuable supplement to the ordinary sources of 

 nitrogen. 



Besides these responses to influences of the environ- 

 ment, which are very far-reaching, and modify very largely 

 the shape and structure of the plants exposed to them, 

 ordinary terrestrial plants also show great power of react- 

 ing to the different external conditions which they meet. 

 These will be considered in subsequent chapters. 



