194 
E. B. MAINS 
the dark throughout the experiment became infected. Their incu- 
bation period was lengthened only two days. The remaining nine of 
'the thirteen, however, remained uninfected. 
Experiment 20.— \n this experiment the procedure was the same as 
in the previous experiments, except that the plants were kept in the 
dark for five days to exhaust them as much as possible of carbohy- 
drates. In this case, two out of the seven plants which were put in the 
dark for seven days did not have their incubation period lengthened 
at all. The other five had their incubation period lengthened from 
2 to 4 days, which is shorter than the time they were in the dark. 
Infections occurred on two out of twelve of the plants in the dark for 
the entire time and in these cases the incubation period was lengthened. 
Since these results more nearly agree with those obtained with oats 
{Avena sativa), it would appear that the reserve food supply of the 
maize is to be considered as the cause of the disagreement. In the 
case of maize, the endosperm furnishes considerable food to the plant 
during the first month and by the time this is exha^asted, the plant is 
of such a size that considerable reserve food is stored up in the stem 
and other organs of the plant. The next experiment was carried out 
with the object of exhausting this reserve as nearly as possible before 
inoculating. 
Experiment 21. — In this experiment, young plants were used. In 
order to control the reserve food supply of the host as much as possible, 
maize was germinated in a moist chamber and after four days the 
endosperms were dissected away. The plants were then planted in 
quartz sand which had been moistened with Knop's solution and were 
left in the light until the leaves were out and the plants had taken on a 
green color. Following this they were removed to the dark for three 
days to exhaust the carbohydrates manufactured during this time. 
All were finally inoculated with uredospores of Puccinia Sorghi and 
four kept under belljars and eight under dark cylinders. Five of the 
latter were removed at various intervals and placed under belljars. 
Three were left under dark cylinders throughout the experiment. 
From the results of this experiment, it is evident that when the 
reserve food supplies of the host are cut down to the minimum, the 
incubation period of the rust is lengthened to a period corresponding 
to the time that the host was placed in the dark and that when the 
host is kept in the dark, there is no development of the rust. Not 
only does this indicate a direct relation to the carbohydrate supply 
