48 
of potash, phosphate or nitrate. Again, there were decided differ- 
ences in the time of heading and extent of development of the 
plants in the different series, those supplied with nitrate being 

exceptionally luxuriant. 
3. A third series was grown in sand cultures to which were 
added the various chemicals, nitrate, potash and phosphate. In 
one set of cultures the full nutrient solution was supplied while 
in the other there was a deficiency of potash, phosphate or nitrate. 
These cultures also showed striking differences between the plants. 
Those grown without the nutrient solution were very short with no 
stooling, while those supplied with a full nutrient solution or an 
excess of nitrate were tall, some stooling out to a slight extent. 
In all these experiments, then, there were very decided differ- 
ences in the rate of growth and development of the host plants. 
The interval of time between planting and heading also varied. 
It was found, however, t 
— 
at the varieties fully susceptible to a 
particular race of smut showed practically complete infection in 
every series. There were minor variations in the percentage of 
— 
infection of the moderately susceptible varieties, but no definite 
indication that these were correlated in any way with particular 
changes in the growth of the host. 
The investigations on the influence of the growth of the host 
on smut development are supported in part by a grant from the 
Penrose Fund of the American Philosophical Society. 
Phystologic Races of the Oat Smuts 
collections of loose and covered smut of oats are similar in their 
It is very important to avoid the erroneous assumption that a 
ability to infect oat varieties. Variations occur in the physiologic 
behavior of different collections which are of equal or greater im- 
portance than morphological characters such as size, shape, spore 
Coy 
surface, and so on. These physiologic differences are recognize: 
by the difference in the ability of collections of the parasite to 
attack species and varieties of oats. 
This phenomenon of host specialization is very widespread 
| a 

among the parasitic fungi. Such races have been demonstrated in 
practically all of the groups, and show great differences in their 
physiologic behavior. During the past year we have published 
