COMMENSALISM. 



393 



order to be nourished by them, they obtain a freedom in the choice of their dwelling- 

 place which is enjoyed by no other Fungus. 



There is however yet another remarkable consequence which results from 

 the commensalism of Fungus and Alga, in that the external form of the body 

 of the Lichen generally no longer remains that of the ordinary Fungus, but 

 behaves rather like that of non-parasitic plants which contain chlorophyll. 

 It is true there are many so-called crustaceous Lichens which grow closely 

 attached to the substratum. But where larger growths more separated from the 

 substratum are formed among the Lichens, the restrictive significance of the chloro- 

 phyll for the whole configuration of the vegetable world at once makes itself 

 prominent again. The thallus of the Lichen then becomes developed either in the 

 form of a flat leaf-like extended plate, as in the so-called foliaceous Lichens, or in 



Fro. 251.—^. Usnea !<arba/,i. a fniticnse I.iclien (natural size), 

 (natural size) seen from below, a fructification ; / the disc by me: 



the form of a much branched shrubby plant. In both cases the purpose is attained 

 of presenting the chlorophyll elements of the Lichen body to the light in thin laminae, 

 in order to accomplish the function of assimilation ; for, as was shown in the first 

 lectures, it is this principle which dominates the relations of configuration of all 

 shoots containing chlorophyll, and as among the higher plants there are massive and 

 succulent as well as graceful forms, so also among the Lichens succulent forms, the 

 so-called gelatinous Lichens, are found, the translucent bodies of which allow the 

 light to penetrate deeper, whereby a looser but more homogeneous distribution of 

 the cells containing chlorophyll is possible in them. Thus as we have repeatedly 

 found already in Phanerogams as well as Cryptogams, that in correlation with the 

 disappearance of the chlorophyll, on the one hand dependence upan organic substrata, 

 and, on the other, massive forms of body are produced ; so, conversely, we find here 

 in the Lichens that typical Fungi devoid of chlorophyll, in correlation with their 



