454 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8, 
cannot withstand the extremely low temperatures of winter. In the field, 
even before winter had set in, all urediniospores had disappeared and only 
the teliospores were in evidence. Two pots of heavily rusted oat plants 
were allowed to remain outside during the winter. The plants were winter- 
killed and when removed to the greenhouse in early April did not revive. 
The urediniospore-producing mycelium, if still alive, which one would 
naturally doubt, produced no new crop of spores. 
From this more or less limited observational evidence, then, it seems 
improbable that under Minnesota conditions a perennial mycelium exists 
which is capable of producing a new crop of urediniospores the following 
spring after overwintering on the infected oat host, though Bolley and 
Pritchard (5) consider it in general quite possible, even though no experi- 
mental data are offered to substantiate the opinion. It seems equally 
improbable that the urediniospores themselves can overwinter and cause 
infection the following spring. Just what possibility there is of the existence 
of a perennial mycelium or of the overwintering of the urediniospores 
among the wild grasses, has not been determined. 
IV 
Mains (6), working with Puccinia coronata Cda., has shown that low 
temperatures, lack of moisture in the moist chamber, and the absence of 
light retard the development of the leaf rust of oats. 
These same observations have been made in the present work, though 
only one definite experiment was performed and that to determine the 
effect of light upon the degree of infection and the rate of pustule formation. 
Four pots of oats of the same seed lot, grown under the same conditions, 
were inoculated on the same date with inoculum from the same source. Two 
pots were placed in a pan of water and covered with a glass bell jar; the 
other two pots were given similar moisture and temperature conditions 
though covered by a glass-topped metal moist chamber from which light 
was excluded. 
All four pots were removed from the moist chambers after forty-eight 
hours but retained for two days more under the light and dark covers. At 
the end of this period the plants in the dark had become spindly and dis- 
tinctly yellowed. Flecks appeared on all the seedlings in all the pots at 
about the same time. Pustules ruptured within ten days upon the plants 
kept in the light and within twelve days on the plants kept in the dark. 
Infection, one hundred percent in each case, appeared normal on all the 
seedlings, though not so heavy on the plants grown in the dark. The plants 
that had been kept in the dark, after several days' exposure to the light 
showed nearly normal growth, though the effect of etiolation was evidenced 
by dead areas at the tips of the leaf blades. 
Twenty-eight days after inoculation pigment appeared on one of the 
plants that had been exposed to the light, while twenty-six days after 
