78 MYCOLOGY 



existence (Fig. 25). "The gills are partially or completely divided 

 down their median planes into two vertical plates. While desiccation 

 is proceeding, the two plates of each of the longer and deeper gills bend 

 apart and spread themselves over the shorter and shallower gills. 

 When desiccation is complete, the whole hymenium is hidden from 

 external view and the fruit body is covered both above and below with a 

 layer of hairs (Fig. 25). The closing up of the fruit bodies at the 

 beginning of a period of drought serves to protect the hymenium. A 

 fruit body which retains its vitality even when dry for two years will 

 revive again in a few hours and spores are discharged" (Fig. 25). 



As it is not the purpose of this book to consider the so-called Hchens 

 in the classification which follows as distinct entities in which the 

 lichen fungus and the lichen alga are in symbiosis forming a 

 lichen thallus, it is important to describe the ecology of the actual 

 relationship of the two plants to each other, as a matter of botanic 

 interest. Danilov, Elenkin, Peirce and Fink have shown that the dual 

 hypothesis, or that of mutuahstic symbiosis, is untenable. A lichen 

 is a fungus belonging to the orders ASCOMYCETALES, or BASIDIO- 

 MYCETALES, which lives during all or part of its life in parasitic 

 relation with an algal host and also sustains a relation with an organic 

 or an inorganic substratum. Having squarely assumed this position 

 as to the true nature of what currently passes for a lichen, it is interest- 

 ing to note that there are ten algae known as Hchen hosts: Chlorococcum 

 (Cystococcus) humkola, Pahnella botryoides, Trentepohlia (Chroolepus) 

 umbrina, Pleurococcus vulgaris, Dactylococcus infusionmn, Nostoc lichen- 

 oides (?), Rivularia nitida, Polycoccus pimctijormis, Gleocapsa polyderma- 

 tica and Sirosiphon pulvinatus. It is important to note, that although 

 the larger number of the above are blue-green algae, yet the two species 

 of green algae. Chlorococcum humicola and Trentepohlia umbrina form 

 the hosts of many more lichens than all the others combined. Hence 

 the student of these plants can study the algicolous fungi, mainly 

 ASCOMYCETELES, a few BASIDIOMYCETALES, those parasitic 

 upon algje, as the lichens, while the non-algicolous fungi can be over- 

 looked by the lichenologists. We can do no better than quote Bruce 

 Fink,^ who sums up the main arguments against mutualism and the 



1 Fink, Bruce: The Nature and Classification of Lichens. I. Views and Argu- 

 ments, Mycologia, iii: 231-269, September, 1911; II. The Lichen and its Algal Host, 

 Mycologia, iv: 97-166, May, 1913. 



