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 algae 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. 



