33 



quite common upon lichens. Although most of the fungi parasitic 

 upon lichens belong to the Ascomycetes, there are also a few from 

 other groups. Not unfrequently some species of Mucor entirely de- 

 stroys the lichen-thallus ; it is, however, quite probable that this oc- 

 curs only when the vitality of the lichen is very much reduced and 

 the plant exposed to a high degree of moisture. 



The method of infection is much the same as in higher plants. A 

 spore of the parasite falls upon the surface of the thallus, where it 

 develops a mycelium which destroys the hyphal tissue of the host, or, 

 at least, very materially retards growth, so that in time the parasite is 

 more or less completely enclosed by the upper layers of the lichen- 

 thallus. The line of demarcation between host and parasite is some- 

 times abrupt and distinct, owing to a difference in color, but, as already 

 indicated, it is very difficult to demonstrate the relation of the hyphae 

 of the parasite to those of the host. It is very desirable that this 

 form of symbiosis should be investigated more carefully. Some ex- 

 periments solving the problem of the true physiological relationship 

 of fungal parasites to lichen-hosts would be especially interesting. 



Physaruni mticoroides, a Myxomycete, occurs quite constantly upon 

 Pertusaria communis and Sticla globifera ; other Myxomycetes oc- 

 cur less constantly. 



2. ANTAGONISTIC SYMBIOSIS OF LICHENS WITH LICHENS. 

 (SYNTROPHY.) 



That different species of lichens should occur in symbiotic associa- 

 tion is not surprising when we consider the fact of their close prox- 

 imity in nature ; there is a continual struggle for space. Very 

 frequently we find foliose or fruticose lichens spreading over and 

 crowding out the crustaceous forms. We also find that one form of 

 crustaceous lichen will encroach upon another form and gradually 

 cause it to disappear This is readily understood in the case where one 

 lichen is of more rapid growth than another. As Minks has shown 

 in his communication on the syntrophy of lichens (65, 67), one species 

 of crustaceous lichen may form the lower thalloid portion, while a 

 second parasitic species may develop the apothecia. Not only one, but 

 several, different species may form successive layers upon the lower- 

 most host-species. It frequently happens, doubtless, that the host- 

 lichens are sooner or later entirely destroyed, in which case we can 

 find only the parasitic form growing upon the fragmentary remnants 



