liCay X9, xQaj 
Spore Germination of Ustilago Avenae 
585 
With these three considerations in mind, it may be concluded that the 
spores begin to germinate between 4"^ and 5° C., and that the maximum 
temperature for germination lies between 31° and 34°. If the optimum 
be arbitrarily chosen as that range of temperature over which germination 
with usual sporidial production will take place under these conditions 
in 24 hours, it is 15° to 28®. The minimum and optimum for sporidial 
production are the same as for germination, but the maximum is some¬ 
what lower, ranging from 29® to 30°. 
A few interesting observations may be noted here. In the broth 
solution the spores most commonly germinate into promycelia from which 
sporidia are cut off, in contrast wiQi the germination on agar placed in 
the soil, where hyphae usually are produced in place of the sporidia. The 
effect of very high temperatures is evident in the type of germ tubes. 
At the lowest temperatures the tubes appear to be normal, whereas, at 
temperatures around 29® and 30° C., the sporidia sometimes seem not to 
be completely formed and the tubes tend to be thin and transparent. At 
temperatures around 32° the tubes are barely visible and never grow to 
the usual length. 
TH^ INFI.U^NCE Olf MOISTURE ON SPORE GERMINATION 
The object of this phase of the work was to determine the effect of the 
moisture content of the soil on the spores of Ustilago avence. Various 
methods, whereby spores could be placed in the soil under conditions 
similar to those obtaining in nature and recovered for examination under 
the microscope, were considered. The following technique finally was 
adopted. About 30 cc. of 2.5 per cent water* agar of known acidity 
(Ph 6.4 or H-o. 4 Fuller's scale) was poured into open petri dishes and 
dried thoroughly at a temperature not above 60° C. From the dried 
agar, smooth pieces i cm. square were cut. To distribute the spores 
evenly over the pieces of agar, a surface of paper was covered as evenly 
as possible with the spores. The pieces of agar were then laid one by 
one upon this surface and pressed gently to bring the whole of the surface 
in contact with the spores. Pieces of filter paper then were clipped 
above and below the agar with a fine wire. The spores thus prepared 
were sown under the same conditions as the oat seeds, i inch below the 
surface of the soil. 
The soil moistures were made up on the basis of the moisture-holding 
capacity, as in the experiments of Bartholomew and Jones (-2), and 
the acidity determined by Truog's (21) method. To insure uniform 
moisture distribution in the soil, either water was added to the soil 24 
hours previous to sowing, or moistened soil was mixed before using, 
except that the 80 per cent was made originally to 60 per cent and aft^ 
seeding was brought to 80 per cent. Uniform temperatures were main¬ 
tained by incubators. The germination of the spores on the agar layer 
could be counted under the microscope after the removal of the filter 
paper. 
At this point the question will arise as to how closely the conditions 
to which the spores were subjected in these experiments approximate the 
actual conditions to which the spores are exposed when naturally dis¬ 
tributed in the soil. The availability of water to the spores is probably 
not the same when the spores are in contact with the agar as when they 
are in contact with the soil. In order to determine whether the amount 
of water absorbed by the agar varies directly with the moisture content 
39364—23- i 
