196 SECTIONAL ADDRESSES. 



Saltation. 



In this, as in other characters, the fungi are capable of marked varia- 

 tion. Often the varieties grade into one another ; in some cases they 

 are dependent on the content of the substratum and revert to the original 

 form when the original food material is supplied ; in some they return 

 gradually, even under unchanged conditions, to the character of their 

 precursors. Stable variants, however, are common, and their sudden 

 origin in species under observation has often been recorded. Barnes,^* 

 in Eurotium herbariorum, found that they could be induced by the applica- 

 tion of heat to the spores, and Brown,^^ in Fusarium, reported their 

 survival on media which combined high concentration with minimal 

 staling capacity, so that, growth being long continued, the altered hyphse 

 had a chance to develop. 



It is possible that some of these variants may arise in nature as a result 

 of conditions which the fungus barely survives, and that some may be due 

 to mutations comparable to those of animals and green plants. But 

 account must be taken in the higher fungi of the multinucleate character 

 of the vegetative cells, and of the occurrence of mycelial fusions which 

 bring together unrelated nuclei. We are profoundly ignorant of the 

 effect on development of a nucleus surrounded by unfamiliar cytoplasm, 

 or of two or more nuclei in an environment to which only some of 

 them belong. These problems will demand intensive study before the 

 phenomenon of saltation begins to be understood; but it is already 

 established that saltation affects both physiological and morphological 

 characters, that many saltants are stable, and that their peculiarities are 

 inherited. 



Food and Heterothallism. 



How, then, is a nutritive explanation applicable to the heterothallism 

 of Humaria ? We know that the production of fructifications is de- 

 pendent on appropriate food, and that new strains, differing in their food 

 relation, readily arise. Suppose that the (-f ) mycelium be a saltant 

 possessing, as an hereditary character, the capacity of rapidly extracting 

 from the substratum a food substance. A, essential to ascocarp formation, 

 but is lacking, or weak, in the power to accumulate the equally necessary 

 material, B. Suppose, similarly, that a ( — ) strain can obtain B, but not 

 A. If two (+) or two (— ) strains meet, the nutritive conditions for 

 fruiting are not fulfilled, but, if (— ) hyphse fuse with (+) hyphse, all 

 requirements are met, and a row of ascocarps is the result. 



In the great mass of work on other heterothallic forms, information is 

 available which seems to support this hypothesis. In some of the smuts, 

 fusion does not occur if the mycelia have been grown on media rich in albu- 

 minous compounds. In Glomerella the ( — ) strain forms fertile spores only on 

 an appropriate substratum. Both in the rusts and in the Hymenomycetes 

 species occur which can develop fruits from a (+) or a (—) mycelium alone, 

 though more slowly than from the combination of both. In such cases, 

 the hetero-homothallic forms, each mycelium may be inferred gradually to 

 acquire the material which the other can rapidly obtain. 



Again, in the Hymenomycetes, we have species, such as Aleurodiscus 

 polygomts and Coprinus lagopus, which are described as quadrisexual. It 



