RELATION OF SOME RUSTS TO THEIR HOSTS 211 
the high temperature brings about a disorganization of the vital func- 
tions of the plant. Photosynthesis according to Pfeffer (1900) in- 
creases with the temperature up to an optimum, which is approximate 
to that of growth and then it falls. Matthaei (1905), on the other 
hand, gives a curve for photosynthesis which resembles that of respira- 
tion. This curve, however, is a curve of maximal pnotosynthesis for 
each temperature. At high temperatures, the maximal photosyn- 
thesis is maintained only for a short time. The higher the temperature 
the sooner the decline sets in and the steeper its slope. The full 
development of photosynthesis also needs the best conditions of illumi- 
nation while respiration is not much affected. Consequently in 
Experiment 4, the food supply available for the fungus at 30° C. is 
much reduced from that which would be available for it at 20° C, 
since photosynthesis has fallen off and respiration has increased. At 
the lower temperatures, although the respiration is lowered, photo- 
synthesis is also reduced and to a much greater degree so that the food 
available for the fungus is less than at the optimum temperature for 
photosynthesis. 
Although the growth of the fungus is thus influenced to some extent 
by the effect of temperature upon the amount of food supply available 
in the host, it is probably the direct effect of the temperature upon the 
rust itself which is the most important factor in determining the de- 
velopment of the rusts in Experiments 1-4; especially since the tem- 
peratures obtained for growth also correspond very well to those for 
spore germination. The actual temperature of the fungi in these ex- 
periments, however, was undoubtedly higher than that recorded, since 
Matthaei (1905), Ehlers (191 5), and others have shown that the in- 
ternal temperature of leaves is one to ten degrees higher than the sur- 
rounding a'tmosphere, depending upon the amount of illumination, the 
presence or absence of air currents, and the amount of transpiration. 
The evidence concerning the effect of moisture upon the develop- 
ment of the rusts is rather conflicting. Blaringhem (1912, 1913) and 
Stone and Smith (1899) claim that the rusts are favored by a dry soil. 
Buchet (191 3) believes that a wet soil is favorable. Sirrine (1900) 
considers that dew is the controlling factor. Smith (1904) finds that 
a dry atmosphere is unfavorable to Puccinia Asparagi, while a dry 
soil is favorable. From the results of Experiment 5, it is evident that 
for Puccinia Sorghi wet soil and moist atmosphere bring about an 
increase in vigor, as shown by the greater number of pustules and the 
