44 BULLETIN 380, U. S. DEPARTMENT OF AGRICULTURE. 
in the temperature of the light and dark plates, but probably not 
sufficient to interfere seriously with the experiments. 
Series 1. On corn-meal agar plates under bell jars—In nine days 
there was no distinguishable difference between the plates in light and 
darkness, a few spore masses occurring near the middle of each. 
In 18 days most of the light plates showed a central ring of spore 
masses and a zone of scattered spore masses near the edge. The dark 
plates showed a few small spore masses near the center, and scat- 
tered about the outer portion were the small masses of mycelium 
which usually constitute the early stages of pycnidial formation. 
In 80 days the number of spore masses had increased somewhat in 
both sets of plates, but more in the darkened plates, so that the 
number of spore masses was about equal in all the plates. The two 
sets of plates were fairly uniform as to the arrangement of the spore 
_Masses. Plate XVIII shows a typical example. 
Series 2. On chestnut-twig agar plates under bell jars.—After nine 
days the cultures in light and darkness were alike. No spores had yet 
appeared in either set. 
In 30 days there were a few spore masses on nearly all of the 
plates, there being no difference between those in light and those in 
darkness either in number or distribution. 
Series 3. On corn-meal agar and chestnut-twig agar under bell 
jars.—In this test the plates were piled alternately, first a corn-meal 
and then a chestnut-twig agar plate, so that the two media would be 
under conditions as nearly identical as possible. The plates were 
inoculated as before and left untouched for 18 days and after that 
were examined daily. After 18 days all the corn-meal plates showed 
spore masses in practically equal numbers, while the chestnut-twig 
agar plates showed no spore masses whatever. There was no ap- 
parent difference in the growth on either medium between the plates 
in light and those in darkness. 
At the end of 25 days the cultures on chestnut-twig agar plates 
showed numerous small masses of mycelium, indicating the forma- 
tion of pycnidia. No difference was perceptible between the dark 
and light plates. 
In 28 days, from 100 to 150 of these pycnidia in each plate were 
extruding spore masses. The light plates showed in general a larger 
mass of spores than the dark plates, but this was not marked, cer- 
tainly no greater than was accounted for by the unavoidable dif- 
ference in radiation and the consequent difference in moisture. This 
difference in the moisture of the medium was clearly shown each 
morning by the greater amount of moisture condensed on the covers 
of the darkened Petri dishes. 
At this time (after 28 days) four corn-meal agar plates which had 
been wrapped in four layers of heavy black photographic paper and 
