406 
Journal of Agricultural Research 
Vol. XXVII, No. i 
narrow uredinia often are formed on varieties which appear to be generally 
resistant in the field. 
The morphological structure of a stem can be changed by environ¬ 
mental conditions. It has been shown that the ratio of sclerenchymatous 
tissue to collenchymatous tissue can be reduced by unbalanced nitrogen 
nutrition of the plant. The effect of large amounts of nitrogenous fer¬ 
tilizers, therefore, especially when their effect is not counterbalanced 
by the addition of other fertilizers, probably is to increase the area in 
which the rust fungus can grow. In this way, fertilizers appear to sensi¬ 
tize plants to infection. We should not overlook the fact, however, that 
the fundamental protoplasmic resistance of the plant probably has not 
been changed. The individual cells probably are just as susceptible in 
plants fertilized with nitrogen as they are in those which have not been 
so fertilized, but the number of cells in which the rust can develop is 
greater. Conversely, phosphates and potassium fertilizers may cause 
an increase in the amount of sclerenchymatous tissues and thus limit the 
area in which the rust mycelium can grow. 
It is quite likely also that the reason why seedlings sometimes seem to 
be more susceptible than older plants is that the morphology of the seed¬ 
lings is somewhat different from that of the older plants. It has been 
observed frequently that varieties which are susceptible in the seed¬ 
ling stage behave in a peculiar manner in the field. Either very few 
uredinia are produced on the plants in the field or they remain small. 
The explahation for this fact probably is that the amount of sclerenchyma- 
tous tissue in proportion to the other tissues is greater m the stems of 
older plants than in the leaves of seedlings. While the basic resistance, 
therefore, of the plant is not different from what it was when the plant 
was young, still the area in which the rust can develop is limited mechan¬ 
ically. 
No satisfactory explanation ever has been given for the basic or pro¬ 
toplasmic resistance of wheat varieties. It seems quite obvious that 
this must be due to physicochemical relations between the host and the 
pathogene. As the pathogene can not be grown in artificial culture 
media, a study of the problem is somewhat difficult. The most promis¬ 
ing method of attack would seem to be to ascertain whether there are 
consistent differences in the physicochemical properties of different va¬ 
rieties. Determinations therefore were made of hydrogen-ion concen¬ 
tration, sugar content, etc. None of the observed differences seemed to 
be consistent or great enough to account for the differences in resistance. 
While it may be significant that the sugar concentrations in different 
varieties varied somewhat, too much importance should not be attached 
to these results. A study also was made of the reaction of spores of 
different biologic forms to certain physicochemical factors. It was found 
that the spore germination of two biologic forms was affected differently 
by hydrogen-ion concentration and by temperature. The form with the 
widest host range was able to withstand a wider range of variation than 
was the form with the narrower host range. This fact probably is of 
some significance, but just how much it is impossible to say. 
While the question of the fundamental nature of resistance of wheat 
varieties to P. graminis was not completely solved, it at least has been 
shown that many of the rather puzzling and conflicting observations on 
apparent change of resistance can be explained easily on the basis of 
morphologic changes which occur within the host. It should be kept 
clearly in mind, however, that this is not a change in the fundamental 
