CONCLUSIONS 379 



too drastically poisoning its metabolism. Studies on the elTect of light, 

 especially ultraviolet light, are the most important evidence, in view ol 

 the well-established lethal action of radiation. Gross mutilation by 

 scraping or cutting induces sporulation in a few fungi (195, 206, 233); 

 in scraped cultures of Alternaria solani Kunkel (179) observed that 

 conidiophores arise from broken hyphac. Initiation of sporulation as 

 the result of temperature changes, exhaustion of nutrients, and various 

 treatments of the host plant of a pathogen are all consistent with the 

 injury hypothesis. 



Granted that a check in growth initiates sporulation, we can only 

 speculate on the intimate mechanism. Perhaps the most promising 

 suggestion is that injured or moribund cells release substances which 

 act on surviving cells and divert them into a new developmental path. 



It must be noted that a contrary hypothesis, that sporulation requires 

 specific positive stimuli rather than the negative stimulation of injury, 

 could easily be built up. In particular, the ready sporulation of many 

 fungi on the living host plant or on cold-sterilized plant materials ar- 

 gues for nutritional factors which must be present in some threshold 

 amount to permit sporulation or which must be synthesized by the 

 fungus before reproductive activity can begin. 



Less inclusive, but important and more firmly based, generalizations 

 may be listed briefly in conclusion: 



1. Reproduction occurs, in general, over a narrower range of envi- 

 ronmental conditions than does growth; this is especially clear in find- 

 ings on temperature, acidity, mineral, and oxygen requirements. The 

 optimum for reproduction may be the same as or different from that of 

 mycelial growth. 



2. Klebs' principle, that reproduction is favored by depriving an 

 established mycelium of one or more nutrients, is valid in general, 

 especially for carbon and nitrogen sources. It must be qualified only 

 to take account of the fact that for some nutrients — certain vitamins 

 and minerals — the absolute amount required for sporulation is higher 

 than that which supports growth. This qualification essentially only 

 changes the definition of the "established mycelium" to indicate that it 

 must have certain materials in high concentration. 



3. There is no conclusive evidence for a specific nutritional factor 

 required for sporulation and not required at all for growth. Specific 

 endogenous hormones are, however, established as vital to reproductive 

 activity, or at least to the orderly development of reproductive struc- 

 tures once these are initiated. 



4. The requirements for sexual and asexual reproduction in a given 



