o92 THE SUBDIVISIONS OF THK VEGETABLE KINGDOM. 



and dissolved salts are absorbed and supplied it by the Fungus. It is also protected 

 by the Fungus, and able to exist in places where it could not live alone. Thus, 

 for the Lichen, we speak of symbiosis, a living together. 'Tis true the Fungus is 

 the predominant partner in this association, but it is not a parasite in the common 

 acceptation of that term. Further, as predominant partner it is the Fungus which 

 determines the form of growth and takes the initiative, the Alga following after. 

 But even to this rule an exception has been found, and very likely others exist. 

 For where two organisms live together, as in the Lichen, it may well happen that 

 conditions may exist under which the Fungus can only control the Alga with 

 difficulty, and that the Alga, attempting as it were to escape, compels the Lichen- 

 fungus to follow it, not to lead. This indeed seems to be the case in one of the 

 forms of that most remarkable of all Lichens, Cora pavonia, to be referred to 

 below. 



The conception of the Lichen as a dual organism, compounded of Fungus and 

 Alga, is of relatively modern origin. Its establishment is due to the researches of 

 Schwendener, which date back some thirty years, and to those of Bornet, which 

 shortly followed them. Since those days the continued study of Lichens has tended 

 only to secure for the " Schwendenerian theory " (as it was formerly termed) a 

 more wide and universal recognition. Previous to the Schwendenerian epoch the 

 Alga was regarded as a definite portion of the Lichen-thallus, its cells as arising 

 from the hyphse of the Fungus; indeed the Algse were termed " lichen-gonidia ". 

 And for many years was the new view opposed by the majority of professed 

 Lichenologists; but into this old controversy we have not space to enter here. It 

 is sufficient to say that the Algse of Lichens are referable to known genera and 

 species of free-living Algse, and that they have been determined for a number of 

 cases. The Alga freed from the Lichen-fungus pursues its normal mode of life, and 

 can then be identified ; this is not always possible so long as it remains within the 

 Lichen, owing to the change which the Fungus calls forth in it. It is a noteworthy 

 fact that hitherto no Alga has been found so completely adapted to lichenism that 

 it could not attain to its normal development outside the Lichen-thallus. On the 

 other hand, Lichens have been raised from the spores of the Lichen-fungus allowed 

 to germinate on free-growing Algse. In this way a number of Lichens have been 

 synthesized; and it has been shown that one and the same species of Alga could 

 serve for several Lichens. Finally, the spores of Lichen-fungi have been grown on 

 nutrient solutions, and have attained to advanced stages of development. In 

 nature, however, with one exception {Cora, see below), it is not certainly known 

 that any Lichen-fungus can grow independent of its Alga as substratum. We 

 must regard the Lichen-fungi as being members of various Fungus- families which 

 have become so specialized to a peculiar form of nutrition that under ordinary 

 circumstances they do not develop upon anything except their Algje. The Fungus- 

 forms which occur in Lichens are vastly more numerous than are the Algse ; indeed 

 the latter are drawn from relatively few families — from the Chroococcacese and 

 and Nostoccacese of the Blue-green Algse, and from the Protococcoidese, Confervoidese, 



