218 AMERICAN JOURNAL OF BOTANY [Vol. 9 



The last two sowings, of August 5 and August 25, never reached the heading- 

 out stage. The barleys were rust-free until the middle of September, 

 when a few uredo pustules were to be discovered on the leaves and sheaths 

 of the three older plots. No teleuto was found. 



As has been noted for the plots of wheat and rye, even more strikingly 

 in the case of the oats, the rate of development of the parasite differs with 

 the host. It is much more rapid in the case of the oats than in that of the 

 other cereals. For example, in the seedling stage the amount of rust on 

 the plants may appear less on the oats than on the rye and wheat, although 

 at the time of heading out the same plants will show the reverse condition, 

 the oats being much more severely infected. 



The rust history of plots of cereals sown at successive intervals through 

 the summer may be taken as indicating that the age and maturity of the 

 host is a factor in the progress of the disease, and that the action of this 

 factor differs with the identity of the host plant. 



LABORATORY AND GREENHOUSE STUDIES 

 Culture Methods 



Four cereal rusts Puccinia coronifera Kleb., P. secalina Grove, P. triti- 

 cina Eriks., and P. Sorghi Schw. were successfully grown for periods of 

 time on the host in pots on greenhouse benches as described by Melhus 

 (1912) and Fromme (1913), and under aseptic conditions on host seedlings 

 growing in test tubes as described by Ward (19020) and Mains (1917). 

 Variations were introduced in both methods. 



Fromme (1913) reviews the problem of growing cereal rusts in the green- 

 house. The method recommended by him includes sowing rust spores on 

 new host plants every few weeks by applying them with a scalpel or camel's 

 hair brush, or spraying on in suspension in water with an atomizer, and 

 then putting the host plants into a moist chamber for from 24 to 48 hours 

 to provide the conditions of high humidity necessary for spore germination 

 and infection. Tests, however, indicated that the first part of the method 

 recommended by Fromme, artificially sowing rust spores on the new host, 

 was not necessary under the conditions obtaining in the Columbia green- 

 house. It was found that when new host plants are grown beside infected 

 plants in the greenhouse, rust spores will be sown on them by natural 

 agencies, such as convection and other atmospheric currents, sufficient to 

 produce abundant infection if conditions of high humidity are provided 

 occasionally to render possible the germination of the spores. 



Accordingly, the method adopted for maintaining stock cultures of the 

 cereal rusts in the greenhouse was to introduce new host plants alongside 

 the infected plants every third week and to cover the cultures with a moist 

 chamber every second or third night. The fungus maintained itself self- 

 sown in this manner, and no artificial inoculations were needed. The 



