382 MINNESOTA BOTANICAL STUDIES 



kept ready for use in a large covered earthenware jar. When filhng 

 the tumblers, they were constantly jarred in order to compact the 

 soil, and enough soil was added to fill them to within 5 mm. of the 

 top. The seeds of the cereals were germinated in a moist chamber 

 and then ])lanted in the soil contained in the glasses. About six seeds 

 were planted in each container. These were then set away under 

 bell jars in a warm place and in diffuse light until the plants were 

 from 3 to 5 cm. high. Small dishes of water were placed with the 

 plants under the bell jars. 



Inoculations with the urcdospores of the various rusts were made 

 when the first leaves had reached a length of from 2 to 4 cm. This 

 was accomplished by transferring the rust spores wnth a bent dis- 

 secting needle, from fresh stock material always kept on hand, onto 

 the new leaves which had previously been wetted and rubbed lightly 

 between the thumb and finger. After inoculation all the containers 

 to be used in the experiment were placed in pans of water and again 

 kept under the bell jars for a period of about 24 to 48 hours. 

 Seventy degrees Fahrenheit was found to be the optimum tempera- 

 ture for infection. 



By the end of the inoculation period the first leaves were mostly 

 full-grown, and since determinations of relative transpiration were 

 made before as well as after the appearance of the rust pustules, 

 the glasses were at once sealed to prevent water loss from the con- 

 tainer except that transpired by the plant. 



Before sealing, the glasses were removed from the bell jars and 

 the plants thinned down to 3 to 5 in each container. The less vigor- 

 ous plants Avere always discarded. Then the soil was smoothed, 

 the top of the glasses wiped clean and the wax seal applied. This 

 consisted of a mixture of 80 per cent paraf^n, with a melting point 

 of 45° C, and 20 per cent petrolatum. The mixture was poured 

 around the plants when it had cooled down to 48" C. This soon con- 

 gealed and formed a seal covering the soil to a depth of 2 or 3 mm. 

 and adhering perfectly to the glass and the plant stems without in- 

 juring the latter. However, considerable care in the application of 

 the seal was necessary. It was found that plants just out of the in- 

 oculating chamber, if at a low temperature, were apt to be injured by 

 contact with the warm wax. Also, under such conditions the wax 

 adhered less perfectly to the glass. A warm knife-blade was some- 

 times used to remelt the edge of the wax seal and make the contact 

 with the glass more certain, while invariably a large number of con- 



