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 discussing 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 



