THE GERMINATION PROCESS 391 



A given population of spores germinates over a period of time; if the 

 population is standardized as to age, there is good evidence that ran- 

 dom variability is the cause of the spread in time of germination. As 

 shown in Figure 1, a plot of per cent germination on a probit scale 

 against the reciprocal of time is linear (provided a correction is made 

 for non-viable spores); i.e., time of germination is normally distributed. 

 Similar data have been reported in studies of Neurospora crassa (240). 

 The over-all rapidity of germination is a species characteristic: Melamp- 

 sora lini begins germination in less than one hour (129), but Phoina 

 apiicola spores require 24—48 hours (13). Other data on rapidity of 

 germination are reviewed by Gottlieb (113) and by Doran (73). 



The three distinguishing morphological events of typical spore ger- 

 mination are: nuclear division (8, 12), swelling of the spore, and emer- 

 gence of the germ tube. Swelling, with one significant exception, 

 appears to be universal in fungi and actinomycetes, having been de- 

 scribed in sporangiospores (41), ascospores (121), basidiospores (32, 

 135), and conidia (25, 114, 125, 192, 255, 260a, 306). The exception— 

 apart from the fact that observations on Sclerotinia fructicola conflict 

 (192, 306) — is found among the powdery mildew fungi; conidia of these 

 do not swell prior to germination (306). The significance of swelling 

 in relation to water requirements will be considered later. 



There is scattered evidence that physiological changes also occur 

 during the germination process, although the data cannot be general- 

 ized to all fungi. Anion toxicity is more marked in the early stage of 

 germination of the resistant sporangium of Blastocladiella sp. (45) than 

 in a later stage; oxygen uptake by ascospores of Neurospora tetrasperma 

 becomes more sensitive to iodoacetamide after appearance of the germ 

 tube (109); and in the same material cation permeability appears to 

 change at the beginning of germination (274). Other evidence of 

 separable physiological states during the germination process appears 

 in studies of oxygen uptake and of temperature responses. 



Germ tube growth presumably follows the same rules as any other 

 mycelial growth. Extension occurs at the tip only (259, 263). For at 

 least some time, growth is exponential, i.e., the logarithm of germ tube 

 length increases linearly with time (220, 259); germ tube growth in 

 Ustulina vulgaris, however, is reported to follow a different course 

 (301). 



Germ tube tropisms toward water and toward nutrients have been 

 reported, but the evidence is conflicting and inconclusive (9, 55, 100, 

 1 17, 203). It is not, in particular, necessary to postulate a chemotropic 

 reaction in the penetration of plant pathogens into host tissue (39, 40, 



