Feb. 9, 1924 
Resistance of Wheat to Puccinia graminis tritici 
4°3 
the amount of solids in the sap and rust resistance. The hydrogen-ion 
concentrations of the different varieties differed but little, an observation 
agreeing with that of Hurd (24). The data presented in Table X show 
that there is no consistent correlation of any of the sap properties of 
different varieties and their relative degree of resistance to biologic 
forms of Puccinia graminis tritici . 
The data given in Table XI indicate that, under the conditions of the 
experiment, varieties show a difference in the amount of both reducing 
sugar and total sugars. No sucrose was present in Little Club, Marquis, 
and Kanred, all of their sugars analyzing as reducing sugar. These 
varieties also contained higher concentrations of reducing sugars than 
did Khapli and Mindum. In Khapli and Mindum 17 and 25.8 per cent, 
respectively, of the total sugar was sucrose. Khapli is resistant to all 
of the biologic forms described by Stakman and Levine; Mindum is 
resistant to some and susceptible to others. From the data presented 
in Table XI, we are not justified in drawing any definite conclusions as 
to the relation of sugar content to rust development. It can be pointed 
out that wheat varieties do differ considerably in their sugar content and 
that detailed investigations may give significant information both on the 
general nature of the relation of rust development to sugar content of the 
host sap and to the difference in behavior of biologic forms. Such an 
analysis is of little value, however, without a complete analysis of the 
carbon metabolism, changes in plant acids, etc. 
The difficulty in attempting to explain the nature of resistance to 
stem rust on the basis of specific physicochemical properties of the sap 
of a variety is because of the difference in behavior of a particular variety 
to a number of biologic forms of Puccinia graminis tritici . Khapli is 
quite resistant to all of the biologic forms studied by Stakman and Levine 
and it is conceivable, though not proved, that this resistance may be 
due to certain physicochemical properties. 
Little investigation has been made to determine physiological differ¬ 
ences between biologic forms of P. graminis tritici. The writer previously 
has reported that a distinct physiological difference can be demonstrated 
in biologic forms apart from the differential hosts used in identifying 
them. It was shown (25) that urediniospores of two biologic forms 
germinate differently under different conditions of temperature and 
hydrogen-ion concentration. Furthermore, the urediniospores of a 
biologic form which attacks only a few differential hosts could not ger¬ 
minate well under such a wide range of environmental conditions 
studied as could those of a biologic form which can attack many hosts. 
That is to say, the potentiality or possibility for germ-tube development 
is greater in the biologic form with a wide host range. An organism 
restricted in its activity to a narrow temperature range might possibly 
also have a narrow range of adjustment to other environmental variables. 
In seeking for the significance of this fact in relation to rust resistance, 
however, we must turn to the physiology of the host. 
We know that there are variations in the morphologic and physiologic 
characters of varieties within a species. Vavilov (56) has concluded 
that these variations may occur as homologous series within related 
taxonomic groups. It is probable, therefore, that there also are physio¬ 
logic variations. Since resistance is in part physiological in nature, or 
expresses itself as such, we see that different wheat varieties present to 
a rust organism a range of physiologic reaction with which it must com¬ 
pete successfully if it is able to infect. It would appear, then, that a 
