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 Orobanchacea. 
Many of the plants belonging to the Santalacee 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 ag 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 
