256 ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY. 
their relations to animals as are occasioned by the endeavour to acquire nutriment 
are extremely various and often linked together and complicated or deranged by 
one another in the most curious manner. Cases occur of a particular plant being 
socially connected with another, and at the same time also beset by vegetable and 
animal parasites. The absorption-roots of the Black Poplar are covered with a 
dense mycelial mantle, so that this tree is associated for purposes of nutrition with 
the fungus to which the mycelium belongs. Such parts of the roots of the Black 
Poplar as are left free from the mycelium are fastened upon by suckers sent forth 
by Toothwort plants, which withdraw from the roots the juices absorbed by the 
latter from the earth through the instrumentality of the mycelial mantles clothing 
them. Meantime, in the cavities in the leaves of the Toothwort various small 
animals are caught and made use of as nitrogenous food. Again, the poplar-tree 
bears Mistletoe on its boughs, and its presence there is due to the missel-thrush. 
The thrush takes the Mistletoe-berries for food, and, in return, renders the plant 
the service of dispersing the seeds and establishing them on other trees. The para- 
sitic Mistletoe takes its liquid nutriment from the wood of the poplar-tree; but, on 
the other hand, its own stems are covered with lichens, and these lichens are them- 
selves a symbiotic community of alge and fungi. Within the wood of the poplar- 
stems spread the mycelia of certain Basidiomycetes (Panus conchatus and Poly- 
porus populinus), whilst the foliage-leaves are covered with a little orange-coloured 
fungus, Melampsora populina. In addition, no less than three gall-creating species 
of Pemphigus live on the leaves and branches of the Poplar, and a number of 
beetles and butterflies are nourished by them. Certain lichens, mosses, and liver- 
worts regularly settle on the bark of old trunks, and included amongst these may 
be the species of liverwort which is inhabited by rotifers. If all the plants and 
animals which live upon the poplar-tree, within it or in association with it, are 
counted, the number turns out to be not much fewer than fifty. 
