368 REPRODUCTION 



It has often, of course, been found in studies of particular fungi 

 that Klebs' first principle does not hold or is overridden by other 

 factors (78, 184, 185); this is best documented in studies on fleshy 

 basidiomycetes (185, 226), in which the relatively greater mass of the 

 fruit body might be expected a priori to change the usual relation. 



The mechanism by which high nutrient concentrations inhibit spor- 

 ulation is not definitely established. One cause which must be opera- 

 tive in some cultures is simply the accumulation of toxic metabolites; 

 effects of culture age and of concentration have been attributed to 

 this phenomenon (104, 251). It seems, however, unlikely that such 

 accumulation can be a general explanation of the concentration ef- 

 fect. For the present, we may posit, no doubt oversimplifying the 

 picture, that a limitation of growth per se is the stimulus to reproduc- 

 tion, and that the effect of high nutrient levels is merely to postpone 

 the onset of starvation. 



The second principle of Klebs is a corollary of the first. Granted 

 that some minimum amount of growth must precede fruiting, and 

 that too high concentrations delay or inhibit fruiting, then the range 

 of concentration suitable for reproduction must necessarily be nar- 

 rower than that suitable for vegetative growth. 



The third principle, of specificity of requirements, is obvious insofar 

 as different species are concerned. Even within a species, however, 

 different spore forms have different requirements; examples include 

 the sexual and asexual stages of Sporodinia grandis (13, 96) and the 

 alternative asexual stages of Blakeslea trispora (112). 



The concentration of the carbon source affects the type of asexual 

 spore formed in Cytosporina ludibunda and related fungi (218) and 

 the septation of macrospores of Fusarinm sp. (157). 



The type of carbon source used strongly influences the formation of 

 reproductive structures, both sexual and asexual. In general, it is 

 found that oligosaccharides and polysaccharides support more fruiting 

 than do the simple hexoses; cellulose, a very slowly available com- 

 pound for most fungi, is the best carbon source for sporulation of some 

 forms (85, 319). 



The mechanism of specific carbon source effects has been thoroughly 

 investigated by Hawker and her collaborators (133, 137, 141) and by 

 Bretzloff (42). There seems little doubt that the primary mechanism 

 is actually a concentration effect, i.e., it exemplifies Klebs' first prin- 

 ciple. Essentially, provision of a slowly utilizable carbon compound 

 is the same as use of a low concentration of a more available sugar. 

 That is, the fungus hydrolyzes (or phosphorylates) the complex car- 

 bohydrate slowly, the result being a steady maintenance of a low con- 



