44 



MASS. EXPERIMENT STATION BULLETIN 202. 



order to bring the data into shape for plotting a constant curve of tempera- 

 ture and germination. 



PLATE III. 



PercenI 

 germination. 



Curve showing temperature limits for germination of urediniospores of Puccinia Antirrhini. 



The optimum temperature for the germination of the urediniospores of 

 P. Antirrhirji was found to be 10° C, the minimum, 5° C, and the 

 maximum, 20° C. When the data are plotted, a curve is obtained that 

 is nearly symmetrical. A most striking fact is that if the temperature is 

 varied 2° C. above or below the optimum, the germination falls off 50 per 

 cent. In the following table each relative germination is the mean of 

 five experiments: — 



Table 1. — Relative Germination of Urediniospores of P. Antirrhini Diet, 

 and Holw., compared to the Germination at 10° C. taken as 100. 



Conditions affecting Longevity of Urediniospores. 

 The spores used in the previously described germination tests were taken 

 fresh from growing plants. It was noticeable that their viability grad- 

 ually diminished if the leaves dried out long in the room. To determine 



