Feb. 9,1924 
Resistance of Wheat to Puccinia graminis tritici 
393 
Sections of its stem also are shown in Plate 2, D. In marked contrast to 
figure i, A, the chlorophyll-bearing area is quite extensive and con¬ 
tinuous. The sclerenchyma is confined to the regions between the 
fibrovascular bundles. The surface of the stem is smooth and even. 
Figure 1, C, is of Vernal (White Spring) emmer, C. I. 3686. This variety 
is similar to Sonem emmer in the extensive development of sclerenchyma- 
tous tissues. A tracing of a section of the stem of Marquis, C. I. 3641, is 
shown in figure 1, D. In this variety the collenchyma areas are smaller 
than similar tissues in Little Club, but, nevertheless, they are considerably 
larger than the collenchyma areas of the emmer varieties. Figure 1, E, 
is a tracing of Kota, C. I. 5878. A photograph of the variety is shown in 
Plate 1, F. Figure 1, F, shows a tracing of Einkorn, C. I. 2433, a variety 
of wheat possessing a stem of small diameter, compared with that of 
other varieties. All of the tracings shown in figure 1 have been traced 
on the same scale and show relative size. 
It must be kept in mind that the diagrams shown illustrate the ten¬ 
dency of a variety to produce a certain type of structure and that certain 
fluctuations are to be expected in individuals. Even Little Club, when 
sectioned some distance from the rachis, shows the chlorophyll areas, 
beginning to be divided by sclerenchyma, but never to the extent shown 
in other varieties. Plate 1, F, shows this fact in that the pustules at a 
distance from the rachis are taking on a more elongated appearance. On 
the other hand, out of hundreds of sections made, never has a section of 
emmer shown any tendency of the collenchyma areas to be confluent. 
Considering the established fact that the development of the mycelium 
of P. graminis is practically limited to the collenchyma of the stem, it is 
not difficult to see that the different varieties studied will necessarily not 
show the same amount of stem-rust injury when subjected to a rust 
epidemic. 
STEM STRUCTURE AS RELATED TO RUST DEVELOPMENT 
Sappin-Trouffy (47, p. 93) states that the action of P. graminis on oat 
stems is quite restricted, as the mycelium is unable to extend except 
longitudinally, because it is bounded at the sides by a hemispherical 
sheath of sclerenchyma fiber. The same is true for the action of P. 
graminis on wheat stems. On the Kota stem the uredinia are linear 
each one being confined to a single collenchyma bundle. (PI. 2, G.) In 
the neck of Little Club the mycelium is not limited by sclerenchyma 
strands but is free to extend laterally and therefore produces very large 
pustules. (PI. 2, H.) Comparing these photographs with Plate 1, F 
and G, the importance of the sclerenchyma in relation to the size of the 
pustule is clear. 
If the opportunities for infection are exceptionally favorable and a 
large number of individual infections result, even the varieties with 
extensive sclerenchyma may become severely rusted. Under such con¬ 
ditions more susceptible varieties, such as Little Club or Marquis would 
be seriously injured. It is quite possible, on the other hand, that when 
only a few infections take place, these latter varieties may be severely 
attacked, while the emmer varieties may escape comparatively unharmed. 
The structure of the stem affects only the extent of the spread of the 
organism and its subsequent rupture of the epidermis. Resistance to 
stem rust must be considered as being due fundamentally to a complex 
physiological relationship. The results obtained by Stakman and Levine 
(54) show the difficulty in explaining actual resistance of wheat varieties 
