350 VEGETABLE PHYSIOLOGY 



penetrate into the tissues of their hosts, complete fusion of 

 the tissue of the host and the parasite frequently taking 

 place. We have representatives of such parasites in the 

 British flora in Cuscuta and the Orobanchacece. 



Many of the plants belonging to the Santalacece and 

 the Scrophulariacece show a partial parasitism of this kind. 

 They have short stems which bear green functional leaves, 

 but are peculiar in that their roots become attached by 

 curious sucker-like bodies to the roots of other plants 

 growing near them (figs. 145, 146), and from these suckers 

 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 para- 

 sitism 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. Probably 

 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 



