LICHENS. 243 
6. SYMBIOSIS. 
Lichens.—Cases of symbiosis of Flowering Plants having green leaves with the mycelia of Fungi 
destitute of chlorophyll. Monotropa.—Plants and Animals considered as a vast symbiotic 
community. 
LICHENS. 
In describing the vegetation of a limited area botanical writers are apt to desig- 
nate the various species of plants as “denizens” of the country in question. The 
conditions under which the plants live are likened to political institutions, and the 
relations existing amongst the plants themselves are compared to the life and strife 
of human society. By no means the least important factor in the suggestion of 
these analogies is the circumstance that often as a matter of fact one has 
opportunities of seeing how the species of plants which live together in a locality 
are dependent in various ways upon one another; how they exist in continual con- 
flict for the food, the ground, for light and air; how some are preyed upon and 
oppressed by others, whilst others are supported and protected by their neighbours; 
and how, not infrequently, quite different species join together in order to attain 
some mutual advantage. 
As regards the preying of one upon another the subject has been treated in 
detail in a previous chapter, and it was also stated then that the term parasite can 
only be applied to those plants which withdraw materials from the living parts of 
other organisms without rendering a reciprocal service in return. The host attacked 
by a parasite supplies food and drink without being in any way compensated. One 
might suppose that nothing would be simpler and easier than to ascertain the 
existence of this relationship, and yet many difficulties are encountered in the 
determination of parasitism in individual cases. The main difficulty is due to the 
fact that one cannot always say with certainty whether the host does not perhaps 
get some advantage from the parasite which drains its juices. Should this be the 
case, however, the latter would be no longer a parasite, and the relationship between 
the two would rather be that of simple commerce and mutual assistance, an ami- 
cable association for the benefit of both. 
Whilst discussmg the second series of parasites, the fact was mentioned that the 
plants upon which the various species of Eyebright fasten their suckers suffer no 
apparent injury as a consequence of this connection. The rootlet organically 
united to the suckers does, it is true, die away in the autumn; but the Eyebright 
also withers at that season, and it is not inconceivable that the useful substances 
existing in the green leaves of the Eyebright may be transferred, shortly before the 
latter withers, to the host-plant and deposited there at a convenient time in the 
permanent part of the root as reserve-material, and that in this way the host-plant 
ultimately derives benefit from the so-called parasite. The idea here suggested as a 
possibility for the case of Eyebright and the grasses connected with it is an ascer- 
tained fact in the case of some other plants. For plants are known which unite to 
