Feb. 9,1924 
Resistance of Wheat to Puccinia graminis tritici 
401 
uredinia would be produced, because each one would be restricted to a 
single collenchyma bundle. There is likely to be relatively little scle- 
renchyma in leaves of plants which have been fertilized with an unbal¬ 
anced excess of nitrogen; and the uredinia which develop on them there¬ 
fore may be larger than those on plants which have been fertilized nor¬ 
mally. While the actual protoplasmic resistance of the plants may not 
have been changed, there may be a greater total area of tissues in which 
the rust mycelium can grow. 
Furthermore, heavy nitrogenous fertilization promotes rank growth 
and dense stands of wheat. Moisture, therefore, is retained and the 
opportunities for spore germination are increased. An excess of nitro¬ 
gen also seems to increase the transpiration of the plants, causing the 
stomata to open and thus facilitate the entrance of the germ tubes. 
The balance of nutrients seems to be much more important than the 
total amount. The results of field experiments on the effect of fertilizers 
on rust development are therefore significant only if the soil type is 
definitely known. 
THE RELATION OF PHYSICOCHEMICAL PROPERTIES OF THE PLANT SAP TO 
RUST RESISTANCE 
As some varieties of wheat are completely susceptible to certain 
biologic forms of P. graminis tritici and are immune from others, it is 
obvious that the sole basis of resistance can not be morphological char¬ 
acters. It is a well established fact, for instance, that Kanred is com¬ 
pletely susceptible to some biologic forms and it is so highly resistant to 
others that not even flecks are developed on plants inoculated artificially. 
It is known, also, that the mycelium of the biologic forms to which Kanred 
is resistant can not develop extensively within the tissues. Therefore, 
there is a real protoplasmic or physiologic resistance. The exact nature 
of this resistance has never been determined. Hurd (24) studied the 
possible relation of hydrogen-ion concentration of wheat varieties to 
their resistance to rust and other pathogenic fungi, and concluded that 
hydrogen-ion concentration probably had little effect on resistance. 
In fact, the concentration varied more in the same variety grown under 
different conditions than it did in different varieties. 
The osmotic concentration of the sap of a host plant probably exerts 
considerable influence on the ability of the rust fungus to absorb nutri¬ 
ment from it. The total solids indicate the amount of moisture in the 
plant tissues and therefore to a certain extent are correlated with the 
succulence of the plant. It is possible, therefore, that differences in total 
solids might affect the development of rust. The importance of avail¬ 
able carbohydrates in the nutrition of rust fungi has been shown clearly 
by Mains (33). Henning (22) and Kirchner (27) were of the opinion 
that the content of reducing sugars was lower in rust-susceptible than in 
resistant varieties. Eckerson ( 11) has shown that the carbon substances 
vary in relative amount in the plant sap at different stages of develop¬ 
ment of the wheat plant. 
The writer made attempts to ascertain whether there were differences 
in the physicochemical properties of different varieties great enough and 
sufficiently consistent to account for differences in their resistance to 
rust. Particular attention was paid to the determination of the depres¬ 
sion of the freezing point, total solids, average molecular weight, hydro¬ 
gen-ion concentration, and sugar content. The following methods were 
