RELATION OF SOME RUSTS TO THEIR HOSTS 21 3 
That these results are due to the prevention of photosynthesis and 
not either to non-inoculation due to the negative heliotropic germ-tube 
of Puccinia coronata or to the effect of the lack of light upon the develop- 
ment of the mycelium of the rust is shown in Experiment ii. Since 
it has been shown that the germ-tubes of the uredospores of Puccinia 
coronata are negatively heliotropic, the explanation of the retardation 
of the incubation period might be an inability of the germ-tube to enter 
the stoma while in the dark. This is not the case, as shown by the 
fact that when plants (C 17.19-C 17.27, Table III) after having been 
inoculated in the light were placed in the dark for a short period, the 
incubation period of the rust on them was also lengthened. The 
demonstration of the presence of mycelium in the leaves of plants 
inoculated and kept in the dark finally establishes this. 
In Experiment ii there is one case (C 17.35) where the retardation 
of the incubation period was greater than the time during which the 
host was in the dark. In this case, the oats were in the dark for eleven 
days and were consequently so starved and their physiological proc- 
esses so disarranged that when returned to the light, they carried on 
their physiological processes poorly and so were able to furnish the 
rust with only a small amount of food. 
The relation of Puccinia Sorghi to the carbohydrates formed in the 
light is not at first glance so striking as in the case of Puccinia coronata. 
Heckel (191 2) and Blackshaw (191 2) have however shown that corn 
plants contain from 6 to 9 percent of sugars. Besides this, corn forms 
starch in the parenchyma sheaths so that under ordinary conditions 
it contains quite a considerable reserve of carbohydrates. Oats, on 
the other hand, contain no such reserve and consequently the rust 
quickly shows the effect when the daily supply is cut off. When means 
were adopted to decrease the sugar content of corn the results compare 
more nearly with those obtained with Puccinia coronata on oats. It is 
very evident from Experiments 12-20 that the rust itself does not have 
any direct relation to light, for infection took place and the rust devel- 
oped to spore formation in the dark. The same is also shown much 
better in experiments where carbohydrates were supplied to the host 
in the dark. 
The relation of the rust to the carbohydrate supply is further seen 
when the host is deprived of its carbon dioxide supply (Experiments 
22 and 23). For when the host is prevented from manufacturing 
carbohydrates by surrounding it with a carbon-dioxide-free atmos- 
phere, there is no development of the rust. 
