RELATION OF SOME RUSTS TO THEIR HOSTS 
215 
Thus the food material for which the rust depends upon the host 
appears to be the sugars or some of the compounds which they enter 
into during the formation of proteins, or what is more Hkely both. 
Such being the case it should be feasible to grow the rusts saprophyt- 
ically. So far as I know only two attempts have been made to do 
this. These have been made by Carleton (1903) who obtained negative 
results and Ray (1901, 1903) who reports having grown several in 
culture. As has been pointed out above Ray's results are open to 
criticism due to the incomplete account of his methods and material. 
In the case of Puccinia Sorghi, I have not been able to find that 
various nutrient media had any appreciable effect towards developing 
a mycelium. Various carbohydrates and organic nitrogenous prod- 
ucts with and without mineral salts and at different concentrations 
showed an effect only upon spore germination and the length of the 
germ tube (Experiments 41-43). Sterilized pieces of leaf upon solu- 
tions such as were used successfully with living pieces of leaf, sterile 
decoctions of the host and water extracts of the host gave no better 
results. In all cases, the germ-tube produced by the spore lived only 
a few days. 
Since Puccinia Sorghi does not appear to be able to use the sugars 
directly, but must have them supplied to the host for their develop- 
ment, it appears that it is not the stable carbohydrates or proteins 
which are to be considered as essential for the metabolism of the rusts. 
Rather, it is probable that the rust is dependent upon some transitory 
products in the formation of these substances, as Fromme (191 3) has 
suggested, or it may be that the rust is able to utihze such compounds 
only in their nascent state, so to speak, when these complex organic 
compounds are not in a condition of equilibrium. Even among the 
saprophytic fungi, we have many which prefer certain stereoisomers 
and it would not be at all surprising to have in the rusts a group of 
fungi which needs certain isomers for their development. It is in 
some such explanation very likely that the obligate parasitism of the 
rusts is to be sought. 
V. SUMMARY 
1. The optimum temperature for the development of Puccinia 
coronata and Puccinia Sorghi is situated at about 20° C. and the maxi- 
mum for Puccinia Sorghi is about 30° C. 
2. While Puccinia Sorghi is not prevented from developing upon 
the host under conditions of dry air and soil, moist soil and a humid 
