18 BULLETIN 759, U. S. DEPARTMENT OF AGRICULTURE. 
fruited at 8° C. in one of the trials.. At and below 16° P. trifolit 
requires a shorter time for the production of spores than P. medi- 
caginis, while above this temperature the condition appears to be 
reversed. But the most striking differences in the behavior of the 
two fungi are not shown in the table. These differences lie in the 
length of the spore-producing period and in the abundance of the 
spores produced. Although P. trifolii is under most circumstances 
the more prolific spore producer, this preponderance is greatly in- 
creased at and below 18° C. Here abundant fruiting may occur for 
two weeks and longer, while cultures of P. medicaginis fruit very 
meagerly and only for a short period at these temperatures. Above 
16° P. medicaginis begins to fruit more abundantly, reaching its max- 
imum at temperatures a little above 20° C. Thus, the optimum tem- 
peratures for the fruiting of these fungi may be judged roughly as 
about 13° to 22° C. for P. trifolit and 16° to 24° for P. medicaginis. 
RESISTANCE OF THE SPORES TO DESICCATION, 
In order to determine whether ascospores discharged from the 
ascus might be able to live over winter on seed or débris carried with 
the seed, it was desirable to test the resistance of discharged asco- 
spores to periods of drying of such duration as they would be obliged 
to endure on the seed. Owing to the slow growth of the fungus after 
germination and the limitations of conditions under which spores 
will germinate at all, it is obvious that the spores must be dried and 
germinated under such conditions that all other organisms will be 
excluded; that is to say, the entire process must be carried out under 
conditions of pure culture. 
Obviously, the preferable method would be to dry the spores on 
the seed itself. In order to do this, sterile seeds were necessary. The 
difficulty of obtaining such seeds which were certainly free from any 
residual effect of the sterilizing agent on the seed coat was so great 
that it was finally abandoned. 
Preliminary tests with plaster of Paris blocks as the conveyor for 
the spores during desiccation were so satisfactory that they were used 
exclusively. Thin blocks small enough to slip into a test tube easily, 
were sterilized by heat and placed beneath cultures which were dis- 
charging spores actively. After a period of 8 to 12 hours the blocks 
were placed in sterilized test tubes. These were stored, some in a 
glass case in the laboratory and some outside a north window. 
From time to time one or more of these blocks were placed on 
water agar to which a small amount of alfalfa-leaf decoction had 
been added. The amount of the decoction appeared to make no ma- 
terial difference up to any amount that could be added to 3 per cent 
agar without causing it to lose its ability to solidify upon cooling. 
Other culture media were tried, but none gave as prompt a result 
