110 SPORE GERMINATION 



and 12°, respectively; in this organism the age of endoconidia has a 

 pronounced effect on the shape of the temperature-germination curve 

 (168). 



So far, we have considered the response of germination, i.e., initiation 

 of the germ tube, to temperature. The further growth of the germ tube 

 may be plotted against temperature. When such curves are compared 

 to germination curves, there is often, perhaps usually, a close corre- 

 spondence (42, 86, 93, 103, 146, 241). With many fungi, the optimum 

 is rather narrower than that for germination; examples include Ery- 

 siphe graminis (317), Physalospora obtusa (92), Clasterosporiarn car- 

 pophilum (103), and several rust fungi (57, 20, 266). In still other 

 fungi, the shape of the curve or the position of the optimum, or both, 

 are different (84, 102, 116, 148, 260). In general, it appears that germ 

 tube growth is closer in its temperature response to mycelial growth 

 than it is to spore germination; the data of Snell (260) on Lenzites 

 sepiaria are especially instructive in this connection. For this reason, 

 there are many problems, especially in plant pathology, in which the 

 temperature characteristics of germ tube growth are more illuminating 

 than those of germination. 



The preceding discussion emphasizes the limitations of the conven- 

 tional temperature study of spore germination. Results often depend 

 upon the criterion used, and upon such sometimes uncontrolled vari- 

 ables as age of spore, humidity, and pH. The studies are not, however, 

 to be dismissed as useless; thus, it is often possible to correlate the eco- 

 logical niche of a parasitic fungus with the response of its spores to 

 temperature (44, 57, 317). 



In this discussion we have tacitly assumed that temperature affects 

 all processes up to the appearance of the germ tube in the same way. 

 There is indirect evidence, from two types of temperature reaction, that 

 this assumption is not valid, that the processes which initiate spore ger- 

 mination have a different sensitivity from later stages. 



Although it might be expected a priori that at lower temperatures the 

 rate of all processes is slower, the data of Figure 8 show a more complex 

 situation: to be sure, the rate of germination at suboptimal tempera- 

 tures is somewhat less than the rate at the optimum (18°), but the prin- 

 cipal effect of low temperature is to delay the onset of germination, i.e., 

 to increase the latent period. Once germination has begun, it is almost 

 as rapid at low temperature as at high. Similar data may be found in 

 studies of Fusarium moniliforme (241), Sclerotinia jructicola (299), 

 and Magnusia spp. (276). Both rate and latent period are strongly 

 affected by low temperature in Phytophthora infestans (63) and Unci- 

 nula necator (67). The latent period, and by inference the metabolic 



