398 MINNESOTA BOTANICAL STUDIES 



the press of other duties was such that the work here was Hmited 

 to a few determinations on cockleburs, sunflowers, and carnations. 

 The methods used are in general those already described, and the 

 differences only need be pointed out. 



Experiments with Cockleburs 



Fruits of Xanthium couunime Britton were collected from a 

 nearby field on February 20, submerged in water for 24 hours, and 

 planted in a sandy-loam soil in a flat on February 22. This was 

 kept in a warm place in the greenhouse and when the cotyledons 

 were just out of the soil, on February 27, the plants were potted, one 

 in each glass, in soil prepared in the usual manner. Forty-five cc. 

 of water was added to each glass at this time and the plants were 

 placed under bell jars. Six days later the first pair of leaves was 

 inoculated with teleutospores of Puccinia xanthii Schw., secured 

 the previous autumn from cockleburs of the same species by Mr. 

 Walter Muenscher at Lincoln, Nebraska. After inoculation all the 

 plants were placed in pans of water under the bell jars for 36 hours, 

 after which they were kept in a moist place until February 9. The 

 glasses were then sealed, the cotyledons cut off, and the stems also 

 cut just above the first pair of leaves. Then, by removing one end 

 from a small printing frame and cutting a notch to admit the petiole, 

 leaf prints were made and the leaf area was determined in the usual 

 manner. This was done in the morning, and in the afternoon of the 

 same day the first weighings were made. The plants were about 

 10 to 12 cm. high and the leaves about 2.5 cm. wide and 5 cm. long, 

 exclusive of the petioles. At this time no external evidence of rust 

 was apparent, nor had the conditions changed at the time of the 

 second weighing 22 hours later. But soon after, yellowish patches 

 began to appear which gradually increased in size and number. 

 Simultaneously with their appearance a falling off in transpiration 

 was noted. These data are shown in figure 9. Here may be seen 

 a great decrease in the transpiration of the diseased plants. The 

 difference in the rate of transpiration of the dift'erent diseased plants 

 may be readily accounted for by the difference in the amount of in- 

 fection. On March 15, the pustules were breaking out in various 

 places but were not yet abundant. The experiment was discontin- 

 ued because the tips of the leaves on two of the rusted plants showed 

 signs of drying up. The rusted leaves were variously distorted. 



