92 James G. Horsfall 



The range of conditions suitable for germination seems narrow in many 

 respects. Urediniospores refuse to germinate well if they are scraped from 

 the sori, whereas, if they are washed from the leaf with a stream of distilled 

 water from a wash bottle, germination is good. This may be due to the 

 fact that the latter method removes mostly the more mature spores and 

 does not injure them (Doran, 1922). 



The urediniospores either have a specific gravity of less than unity or 

 they are not easily wettable, for they float on the surface of the water drops. 

 When the drops are placed in van Tiegham cells, the spores collect in the 

 lower part of the drop, and then their tubes intermingle so freely that the 

 percentage of germination is almost impossible of determination. If, on 

 the other hand, raised drops are placed on slides in moist chambers, the 

 spores float down to the periphery and appear as a ring of tangled tubes 

 and spores after germination has occurred. Spores that happen to be 

 immersed in the drops do not germinate very well. This indicates a high 

 oxygen requirement. 



The best results have been obtained by placing the drops of spore sus- 

 pension on clean glass slides, and, after the spores have germinated, remov- 

 ing the surplus water with a pipette. The surface tension tends to separ- 

 ate the ring of spores at the edge of the drop so that the individuals are 

 more readily discerned. 



Difficulty has been experienced also in allowing too long a time to elapse 

 before counting. After twenty-four hours the tubes become hyaline at 

 the point of emergence from the spore and possess an index of refraction 

 essentially the same as that of water. For this reason, counts mus be 

 made as soon as possible after the last spore has germinated. Usually this 

 is within three hours at room temperature. Thus it will be seen that an 

 incredibly short length of ime is necessary for complete germination. 

 Another advantage which accrues from making the counts early is that 

 the tangling of tubes is at a minimum. 



Even when all these conditions have been complied with, germination 

 of the checks is sometimes still low. A few of the reasons for this are: 

 (1) imperfectly cleaned slides, (2) staling products in the moist chambers, 

 (3) toxic tap water (distilled water should be used), (4) parasitized spores, 

 and (5) immature or overripe spores. 



No satisfactory time curve has been obtained, but certainly it must be 

 relatively steep since the peak is reached within three hours. 



Temperature has a minimum effect upon germination if sufficient time 

 be allowed. Germination is a slower process at low temperatures. A 

 glance at the graph in figure 21 shows, however, that germination after 

 eighteen hours is just as good at 3° C. as it is at 26° C. Unfortunately a 

 time curve for finding the minimum time for each point was not made on 

 the same day for each temperature chamber. Had that been done, and 



