400 
Journal of Agricultural Research voi. xxvii. No. 6 
and M, shows sections of a leaf from a plant grown in a culture deficient 
in nitrogen but containing an excess of phosphates. The contrast with 
Plate 2, L, is very marked. The epidermis, particularly on the lower 
surface, is composed of small, highly cutinized, thick-walled cells. Asso¬ 
ciated with the fibrovascular bundles is a relatively large amount of 
sclerenchyma, which extends from surface to surface. The chlorenchyma 
is compact and the intercellular spaces are small. The leaf is compact 
and firm. These sections were made from leaves of the same age, of the 
same wheat variety, and grown under the same conditions, the difference 
being only in the nutrient salts which the plants received. 
Kraus and Kraybill (28) have shown that feebly vegetative tomato 
plants, grown in soil with a small amount of nitrogen, contain practically 
no nitrate nitrogen and but little total nitrogen. In these plants the dry 
matter and free reducing substance are comparatively high. The bast 
fibers and xylem tissues are greatly increased. On the other hand, 
plants growing under favorable vegetative conditions have higher total 
nitrogen and nitrate nitrogen and lower free reducing substance and dry 
matter. The bast and xylem tissues are comparatively greatly reduced. 
Such studies suggest the possible function of potassium and sodium 
nitrate in the physiology of the plant. These salts appear to be concerned 
chiefly with carbohydrate utilization. If carbohydrates are not normally 
utilized there would tend to he an increase in the production of cell wall, 
crude fiber, and pentosans. The deficiency of nitrogen in the plants 
reported in these studies appears to result in thickened cell walls, as 
shown in Plate 1, D, and in an increase of fibers, as shown in Plate 2, 
L and M. 
Plate 2, J, illustrates the typical restriction of rust development by the 
morphology of a wheat leaf. The sclerenchyma cells extend from the 
upper epidermis to the lower epidermis and permit the development of 
mycelium in only the longitudinal direction. Plowright (41) states that 
there is a tendency for the mycelium at the base of uredinia to spread in 
a centrifugal manner, but that many causes operate to prevent this, the 
chief being the lack of uniformity in the tissues of the host plant; and in 
a leaf with strongly marked venation this tends to exert a directive 
influence upon its extension. Eriksson and Henning (72, pi. 70, fig. no) 
show a uredinium of P. dispersa on the seedling leaf of Bromus sp., and 
call attention to the concentric nature of the secondary uredinia, showing 
that the mycelium has spread in all directions from the original point of 
infection. The same condition occurs also in the wheat seedling infected 
with P. graminis. However, as figure 112 of the same plate shows, there 
is no such secondary infection on the older leaves. The sclerenchyma in 
the seedling leaf does not extend from epidermis to epidermis and, conse¬ 
quently, does not restrict the growth of the mycelium but permits it to 
grow in all directions. In the older leaves so many bundles of fibers have 
developed that the growth of the mycelium is restricted to the areas 
between the fibrovascular bundles. 
Referring again to Plate 2, K, L, M, and N, it is possible to understand 
the influence of a nitrogenous fertilizer in increasing the size of the ure- 
denia on the host plant. If the succulent leaf, Plate 2, L, becomes in¬ 
fected, the growth of the fungus would probably be rapid and extensive 
because there are no sclerenchyma fibers between the bundles and the 
epidermis to prevent its growth. The uredinia therefore would be large. 
But the growth of the distributive hyphae in such a leaf as that shown in 
Plate 2, K, would be restricted by the sclerenchyma fibers. Small 
