394 
Journal of Agricultural Research voi. xxvii, no. 6 
on morphologic grounds alone. Actual resistance is in its finality a 
matter of the individual cell. We may picture the collenchyma cells of 
the neck of Little Club wheat as so many easily rotted fruits in a large 
container where the fungus may spread quickly throughout the entire 
mass, infecting them all. The collenchyma cells in the stem of such a 
variety as Kota or Vernal (White Spring) emmer, when attacked by a 
biologic form to-which it is susceptible, would be like the same fruit 
packed in small baskets, so that when one container was infected the 
fungus could not pass to the next. Thus a large part of the fruit would 
remain uninjured. On the other hand, if one conceives two kinds of 
fruit, one resistant and the other susceptible, both of them packed alike, 
the amount of actual injury sustained is placed on an entirely different 
basis. Of the two bases, the latter is a more fundamental type of resist¬ 
ance, and it is the type emphasized by Stakman and Levine in identifying 
biologic forms. Acme, C. I. 5284, a wheat variety reported by Stakman 
and Levine to be highly resistant to only 1 biologic form in 37, suffers 
very little from rust attacks in the field at University Farm, St. Paul. A 
section of the neck of this variety is shown in Plate 2 , 1 . It has a strong 
tendency to produce sclerenchyma fibers between the epidermis and the 
collenchyma, a characteristic not observed in any other variety studied. 
The amount of sclerenchyma shown in Plate 2, I, is unusually large for 
Acme. 
The diameter of the stem may have some effect on the amount of injury 
caused by rust. Other factors being equal, the diameter of the stem alone 
would tend to determine the resistance to rupture, etc. The smaller 
diameter gives the greater surface curvature with the greater resistance to 
rupture. That this resistance to rupture may be of some significance is 
shown by the fact that the mycelium may develop within the collenchyma 
areas of certain small-stemmed varieties with abundant sclerenchyma, 
and yet produce no uredinia. This condition is quite common in the 
etnmers. Within a given variety, any factors which would increase the 
size of the stem or the proportion of collenchyma to the sclerenchyma 
would also tend to increase the actual injury resulting from a rust attack. 
The many statements by investigators concerning the heavier rust 
infections due to manuring with nitrogenous fertilizers may be in some 
way partly connected with this fact. 
RELATION OF NUTRIENT SALTS TO THE DEVELOPMENT OF STEM RUST 
A series of sand cultures was set up for the purpose of studying the 
relation of certain nutrient salts to the morphology and physiology of 
the wheat plant, and the possible indirect influence of these salts on the 
development of stem rust. 
EXPERIMENTAL PROCEDURE 
A clean, white quartz sand was secured from the division of soils of the 
Minnesota Agricultural Experiment Station. This was best Ottawa 
quartz and contained so little nutrient material as to be unable to support 
growth. All the salts used in making up the nutrient solutions were 
chemically pure but were not recrystallized. 
