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, &. 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 upona 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. 
