INFLUENCE OF ENVIRONMENT ON PLANTS 343 



absorbing cells are developed which penetrate into the 

 substance of their hosts and draw nourishment from them. 

 They are generally described as root parasites. The 

 Mistletoe behaves similarly, striking its haustoria into the 

 tissue of the branches of the apple, oak, poplar, &c. The 

 parasitism is partly compensated by the fact that its leaves 

 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 parasitism. 



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. Drosera, 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. 



These illustrations of the modification of structure and 

 general habit serve to show us that there is, throughout 

 the vegetable kingdom, a constant effort on the part of the 

 plant to adapt itself to its surroundings, so as to make the 

 best of the external conditions. This struggle, though 

 perhaps most easily realised by a survey of large groups 

 which are affected, is really carried out by the individual 

 organisms, and the comparatively striking effects seen are 

 the result of the cumulative efforts of a long series of indi- 

 viduals, each of whom has possessed in different degrees 

 powers of reacting to varying external conditions. These 

 powers will be considered in subsequent chapters. 



