Classicly

The habit of parasitism, however, which has just been alluded to is a powerful means of inaugurating and maintaining change of life and living in plants. A parasitic being is one which lives in or upon some other living organism. There are degrees of parasitism, however: some parasites are mere ‘lodgers,’ so to speak; others both board and lodge at the expense of their host, and these latter are of course the more typical parasites of the two.

But there are even degrees and differences to be seen in the behaviour of plant-lodgers and boarders. For example, mistletoe is a plant of peculiar habits, in respect that whilst its roots enter the substance of the tree-host to which it is attached, and drink up so much of the sap that host is elaborating for its own use, it also can make food-products for itself. For the green leaves of mistletoe, like the leaves of other plants, take in carbonic acid gas, and decompose it, as already described, retaining the carbon, and setting the oxygen free. On the other hand, a parasitic fungus will not elaborate any food-products for itself; and hence it is, if anything, a more complete and typical ‘boarder’ even than mistletoe.