PAEASITISM 483 



Phanerogams which are completely parasitic show a similar 

 degradation of structure. They possess no chloroplasts, their 

 leaves are absent or reduced to the condition of scales, while 

 their stems are often thick and succulent. Their roots are 

 the so-called haustoria, which penetrate into the tissues of their 

 hosts and often complete fusion of the tissue of the host and 

 the parasite takes place. Such parasites are represented in the 

 British flora by the Cuscutas and the Orohancliacece, 



Many of the plants belonging to the Santalacece and the 

 ScrophulariacecB show a partial parasitism of this kind. They 

 have short stems bearing green functional leaves, but in addition 

 their roots become attached by curious sucker-like bodies to the 

 roots of other plants growing near them, and from these suckers 

 absorbing cells are developed which penetrate into the substance 

 of the hosts and draw nourishment from them. The Mistletoe 

 behaves similarlj', striking its haustoria into the tissue of the 

 branches of the apple, oak, poplar, &c. ; but here the parasitism 

 is partly compensated by the fact that its leaves remain green 

 when the host has lost its foliage, and by their activity the}^ 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. 



The habit of capturing msects, 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, as Drosera, grow upon a substratum largely 

 composed of Sphagnum plants, which yield to them a very 

 limited supply of nitrogenous matter ; others are fomid 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 

 bodies yielded by the decomposing bodies of the captured 

 insects may therefore form a valuable supplement to the ordi- 

 nary sources of nitrogen. 



Besides these influences of the environment, 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 reacting to the different external conditions 

 which they meet. These will be considered in subsequent 

 chapters. 



1 I 2 



