18 LICHEN FLORA OF THE UNITED STATES 



In foliose and crustose lichens the central portion of the thallus often dies and 

 the outer portions form a "fairy ring." The ring often becomes discontinuous, 

 and thus a number of individuals arise from one. The dying at the center may 

 be due to age or to the exhaustion of food substances from the substratum. 

 Lecanora muralis and Lecidea speirea are crustose species in which this method 

 of reproduction is often seen. 



Spores. The spores of lichens have repeatedly been proved to be capable 

 of germination in culture and of producing the usual thalli of their species, both 

 when sown with the algae and when sown without them in favorable nutrient 

 media. But the question still remains whether lichens are often produced from 

 spores in nature. Evidently, except in the instances of germination in certain 

 culture media, the spores of lichens must happen to come into contact with the 

 alga of the same species as the one that forms the algal host in the species of lichen 

 by which the spore was produced. And, in addition, the conditions in the sub- 

 stratum, and otherwise, must be sufficiently favorable for germination and the 

 production of a new lichen individual. Though it has been proved that the fungus 

 can exist for some time without the 'alga, it may be doubted whether the above 

 combination is likely to come about often enough so that, among lichens, spores 

 may be regarded as important agents in reproduction. The chances of reproduc- 

 tion by spores were plainly decreased greatly when the relationship with the 

 alga was taken on, and the result has in all probability been a considerable physi- 

 ological degeneration of the spores in the course of phylogenetic development. 



Sexual Reproduction. The sexual processes have been studied in many of 

 the fungi most closely related to lichens, and discoveries indicate that sexuality is 

 common among them and in the ascomycetous lichens as well. In species of 

 Collema, Stahl and others have found that the ascocarp is preceded by an archi- 

 carp and a trichogyne, which are supposed to be reproductive organs. The research 

 of Baur, Darbishire, Lindau, and Vainio has proved the existence of similar organs 

 in lichens of several genera, and although there is yet much need of research regard- 

 ing nuclear behavior, the general presence of sexual organs in some of the lichens 

 can scarcely be questioned longer. 



The spermagonia have been supposed to constitute the male reproductive 

 organs, and Stahl thinks that he has established beyond reasonable doubt that 

 fertilization actually takes place, the spermatia from the spermagonia becoming 

 attached to the apex of the trichogyne, where a transfer of protoplasm occurs. 

 Strangely enough, Moller, experimenting on lichens not closely related to Collema, 

 has produced, from the spermatia, thalli in all respects similar to those which he 

 obtained from the spores. This would seem to indicate that the spermatia, if they 

 are sexual cells, have become so degenerate in certain lichens as to lose their 

 sexual function, becoming capable at the same time of reproducing vegetatively. 

 It seems necessary to add that some botanists regard the spermagonia as bodies 

 belonging to fungi parasitic on the lichen thallus. Some reject the view that these 

 are sexual organs and regard them as similar to pycnidia in related fungi. 



HABITAT AND DISTRIBUTION 



Lichens on trees usually grow on the trunks and lower branches, and much 

 more commonly on the former. Some especially foliose and fruticose species are 

 more common on old trees with rough bark. Crustose species are more commonly 

 found on trunks or branches with smooth bark. Definite statements regarding 

 these points would be desirable, but they are not usually available. 



