May, 1922] RAINES VEGETATIVE VIGOR OF THE HOST 219 



advantage of such a method of maintaining stock cultures of cereal rusts 

 is that it eliminates the most technical operation, that of sowing or applying 

 the fungous spores to the new host, and reduces the problem of maintaining 

 cereal rusts in culture in the greenhouse to a non-technical routine such as 

 can be entrusted to the average gardener or greenhouse marh 



In growing the rust under aseptic conditions on seedlings in test tubes, 

 the method developed was to treat the seed with chlorine water (cf. Wilson, 

 1915), put the seed to germinate on filter paper in Petri dishes, and transfer 

 the germinated seed to a test tube plugged with cotton. Half an inch of 

 sterile water was put into the test tube with the plant. The reserve food 

 materials of the endosperm are capable of bringing the seedling to the third 

 leaf stage, which is sufficient to raise a generation of the rust on it. P. 

 coronifera was cultivated for 10 generations in this manner, transfers being 

 made once a month ; P. Sorghi for 8 generations ; P. triticina for 8 generations ; 

 and P. secalina for 6 generations. A small platinum spatula was employed 

 for making transfers; spores were applied to the upper surface of the first 

 leaf in each case, and material for inoculum was taken from the under 

 surface. That a cereal rust can thus be grown under conditions free from 

 accidental contamination was indicated by the total absence of organic 

 growth, bacterial or fungous, when a rust-infected seedling was deposited 

 on sterile beef-peptone agar. 



Studies on the Incidence of Infection by Measured Doses of Uredospores 

 of Puccinia Sorghi on Zea Mays 



An effort was made to determine the minimal dose of uredospores of 

 P. Sorghi that (i) can possibly, and (2) will certainly produce infection in 

 Zea Mays. 191 tests were made on corn seedlings growing under aseptic 

 conditions in twelve-inch test tubes. 



The method employed to determine the dose and to inoculate was as 

 follows: A dilute suspension of uredospores was made in a vial of sterile 

 water. A small drop from this suspension was put on a piece of sterile 

 cover slip, and the number of uredospores in the drop was counted under 

 the microscope. The piece of cover glass was then inverted and deposited 

 on the upper surface of the first leaf of the young corn seedling in the test 

 tube, about one half inch below the tip, bringing the drop of water con- 

 taining the known number of uredospores in contact with the host tissue. 

 The work was done in the winter in the laboratory, with no rust growing 

 free anywhere in the building, so that the danger of accidental contamina- 

 tion was negligible. No infection ever developed on the seedlings except 

 on the spot where the plant had been inoculated. 



The inoculated seedlings were kept under observation for 21 days. 

 If the inoculated leaf yellowed or withered before 15 days, the plant 

 was discarded. The data on 191 tests are shown in table 4. 



