214 
E. B. MAINS 
A comparison of the infection of corn when suppHed with car-* 
bohydrates alone and when suppHed with carbohydrates plus Knop's 
nutrient shows varying results. In spite of the varying results, the 
experiments clearly show that in all cases there is a much greater 
infection when carbohydrates are supplied than when there are no 
carbohydrates present. It is probable that with the slow growth of the 
host the carbohydrates taken up by the host united with the nitrogen 
compounds formed in the metabolism of the host and formed proteins 
for the use of both host and fungus and so the rust did not feel the loss 
of the mineral elements to a very great extent. 
It appears from experiments in which the host was deprived of its 
soluble carbohydrates that the rust was not able to live upon the host 
even though the host was alive and consequently did not use the pro- 
toplasm of the host, at least directly, as food. Marshall Ward (1904), 
especially, among the workers upon the rusts has- noticed that the 
protoplasm of the host does not appear to be affected by the rust. In 
his Croonian lecture (1890), he points out that the relations of the 
rusts to their hosts are very different from those of a facultative para- 
site such as Botrytis. The rusts he considers as merely tapping the 
food supply of the host, establishing a relation which approaches 
symbiosis. 
The results obtained agree with Ward's view. The development 
of the rust upon seedlings or cut leaves of the host furnished with 
carbohydrates, and the non-development except in a few cases, upon 
hosts furnished only with distilled water or mineral nutrient indicate 
that the rust is dependent upon the food supply of the host and does 
not live upon its protoplasm. The healthy development of host tissue 
in the infected regions compared with the surrounding dying tissue also 
shows that not only does the rust not live upon the protoplasm of its 
host but it even stimulates it to greater activity. 
It is possible that the lack of carbohydrates might produce prod- 
ucts in the host which are toxic to the rust. Thus amino acids and 
other products of metabolism might form which in the presence of 
carbohydrates would unite with them to form proteins again. These 
products might then inhibit the development of the rust. Experi- 
ments 38-40 indicate, however, that such is not the case. In these 
experiments where cut pieces of leaf were floated upon water and 
mineral nutrient such products would have had plenty of chance to 
diffuse out of the leaf. Yet on those solutions deprived of carbohy- 
drates, there was no infection. 
