PATHOGENICITY OF H. SATIVUM 21 



the tubes formed such a mat of mycelium by the end of 24 hours that 

 it was often difficult to determine the percenteige of germination. At 

 the lower temperatures a longer time was required for the germ tubes 

 to appear, and they increased in length very slowly. 



In redistilled water, therefore, spores of H. sativum germinate 

 about equally well at temperatures from 6 to 39 C. No very 

 definite optimum temperature for germination is apparent. The char- 

 acter of the germ tubes and the length of time in which they appear, 

 however, would indicate that an optimum temperature lies between 

 22 C. and 32 C. From these results it would seem that for the 

 above-ground parts of the host, temperature is not a limiting factor 

 in infection so far as spore germination is concerned. 



From the data in Table VII it will be seen that in most cases the 

 percentage of germination of the submerged spores is only slightly less 

 than that of those on the surface. In all cases, however, the germ 

 tubes produced under water were very short and abnormally branched 

 in comparison with the long straight tubes produced on the surface. 



The germ tube first appears as a hyaline tip at the apex of the 

 spore. It is difficult to determine whether the tube breaks through 

 the wall or emerges through a pore. After the tube has increased in 

 size, the delicate exospore is split, sometimes for a third of the length 

 of the spore. A second tube soon appears at the base of the spore, 

 just to one side of the scar where the spore was attached to the! sporo- 

 phore. The connection between the two tubes is continuous through 

 the spore, showing the false nature of the septation in the endospore. 

 The endospore is frequently drawn away from the exospore and forms 

 a constricted tube through the latter. Two germ tubes are not always 

 formed from each spore. In one lot of spores germinated in redis- 

 tilled water at 22 C., it was found that 52 per cent of the spores pos- 

 sessed germ tubes at both ends and 39 per cent at only one end. Nine 

 per cent of the spores did not germinate. Very rarely lateral tubes 

 are found. One spore was observed with a lateral germ tube from 

 each of five adjacent cells at one end of a ten-celled spore. In a few 

 other cases, one or two lateral tubes were observed, usually arising 

 from central cells. Fusions between germ tubes are very common. 



EFFECT OF HYDROGEN-ION CONCENTRATION AND TEMPERA- 

 TURE ON SPORE GERMINATION 



As so many spores always germinated in redistilled water at the 

 various temperatures which permitted germination at all, the effect of 

 hydrogen-ion concentration on germination was studied. Culture solu- 

 tions, based on Clark and Lub's (4) titration curve for ortho-phosphoric 

 acid, were made by adding varying quantities of n/5 KOH to 50 cc. 



