388 SPORE GERMINATION 



processes which control germination and of the physiological and bio- 

 chemical changes consequent upon germination. It is reasonable to 

 expect that investigations into these and related topics would in turn 

 throw light on some of the more obscure environmental and ecological 

 problems in spore physiology. 



1. MEASUREMENT 



The most commonly used criterion of spore germination is the frac- 

 tion of spores that in a given time forms a germ tube; the time period 

 is usually chosen so as to allow all viable spores to germinate. Spores 

 are sown on water, on agar, or on nutrient media, incubated under 

 controlled conditions, and counted microscopically; or, the spores may 

 be germinated in an aerated or shaken liquid medium (65). Self-in- 

 hibition and other associative effects (p. 413) make it imperative that 

 the concentration of spores be held constant and be known. It is usu- 

 ally convenient to bring the spore suspension to a known turbidity, 

 measured optically and previously standardized against a direct micro- 

 scopic count (240). Spore concentration can also be estimated by 

 capillary volume (192, 289). Clumping of spores introduces error and 

 must therefore be minimized (130). 



The rate of germination may be determined by periodic counts dur- 

 ing the experiment and often provides valuable supplementary infor- 

 mation. 



A second major criterion is the latent period of germination, the 

 time required for germination to begin or to reach some specified low 

 value (25, 118, 287). Limited evidence suggests that this is fundamen- 

 tally different from total germination, in that temperature affects it 

 differently (p. 410) and toxicants may exert greater or less effect on the 

 latent period than on total germination (287). 



The rate of increase in the length of the germ tube is again a differ- 

 ent criterion of spore activity. In general, it is more sensitive to tem- 

 perature than is total germination, responding more like mycelial 

 growth; similarly, the pH range for optimum germ tube growth in 

 Alternaria solani is rather narrower than that for best total germina- 

 tion (159). The rate of elongation can be expressed as a velocity con- 

 stant (126). 



No one of these criteria can be said to be "best." The method used 

 must take account of the problem which is under investigation. Fun- 

 damental studies on the germination process, for example, might make 

 use of the rate of germination and the total germination. On the other 

 hand, an ecological or epidemiological problem might be served better 



