46 
vironal soil factors as temperature, moisture, reaction and physical 
condition, are determining as to whether penetration will take 
place. ‘These factors, of course, may exert an influence on the 
germination and early growth of the smut fungus as well as of the 
host plant and, in any given case, it may be impossible to determine 
which is most influenced. Our investigations have developed 
methods which have proved very successful in securing infection 
of susceptible varieties, since 100 per cent. of the plants are 
regularly infected. The favorable combination of conditions is 
a soil with a re 
— 
atively low moisture content and a temperature of 
about 20° C. Sand of a uniform texture has proved an excellent 
medium. 
2. Influence of the subsequent growth of the host plant on smut 
development. Following penetration, the parasite must establish 
a definite relation with the cells of the host in order to complete 
its life cycle. In such fungi as t 
_— 
— 
re rusts and powdery mildews, 
the period necessary for the development of the parasite from the 
time of penetration to spore formation is comparatively short, 
requiring only a few days. his period, however, is greatly in- 
fluenced by external conditions. In the oat smuts, the smut fungus 
grows in the oat plant tissues until the latter develops its flowers, 
a period of weeks or months from the time of the germination of 
the seed. Thus the two organisms are developing together through 
a relatively long period of time. 
During the past year extensive experiments have been made on 
the second phase of these problems. The influence of external 
factors on penetration were eliminated by germinating the seed at 
20° C. with a moisture content of about 20 per cent. ‘The seed- 
lings germinated and pushed through the sand in about four days 
and in six to seven days were ready for transplanting. “The prob- 
lem then was to determine whether alterations in the rate and 
extent of growth of the host plant would have any effect upon the 
ultimate development of smut. 
Definite physiologic races of both loose (Ustilago avenae (Pers. ) 
Jens.) and covered (U. levis (IX. & S.) Magn.) smuts of oats were 
used for inoculating strains of oat varieties whose behavior to- 
wards them had been well established. The Missouri race of 
loose smut was used to inoculate Gothland and Scottish Chief, the 
