888 
Journal of Agricultural Research 
Vol. XXIV, No. II 
however, that the two forms are distinct and reports the work of one of 
his students who failed to infect R, strigosus with uredospores from 
R. trifiorus, whereas R. strigosus and R. occidentalis were infected with 
spores from R. strigosus. The reader is not informed just what was done 
in the way of attempting to infect R. trifiorus with spores from either 
host. The leaves of R. arcticus and R. trifiorus, practically devoid of 
tomentum on the lower surface, are certainly quite different in texture 
from those of our common raspberries. No one would question that 
there may be, as we have shown, certain hosts that are more susceptible 
than others, and it is possible that differences in texture of tissues at¬ 
tacked by the rust could very well account for the differences or modi¬ 
fications in the form and size of sori which originally led Farlow to sug¬ 
gest that the form on R. strigosus, etc., be called P. arcticum var. ameri- 
canum. The presence of P. americanum at Bell, Md., where pustules 
occurred so abundantly on leaf blades, midribs, petioles, canes, floral 
envelopes, and even on the fruit, offered a rare opportunity to study the 
morphology of the sorus. 
Our sections show that there is a great variation in the form of 
the uredinia developing on the same plant. Some are sharply conical 
with coronate peridia; others are low and broad. Pustules were 
found to vary in size from 50 to i mm. in diameter. The peridia of 
uredinia developing on the under side of the leaf (PI. i, C) conform to 
the type described by Farlow. On the larger veins and especially on 
the petioles they tend to grow much larger and round out, sometimes 
becoming nearly globular (PI. i, B). The corona is not conspicuous in 
such peridia, especially where ^ey are rather deep-seated, originating 
beneath three or four layers of cells. 
Sori on canes are much flattened, elliptical or lenticular (PI. i, D, E, F, 
G, and 3, A). In such cases the heavily cutinized epidermis effectively 
prevents the protrusion of the peridium (PI. i, G), so that the peristomal 
cells and most of the peridium often disappear through disorganization. 
Spores are discharged through the passageway opened by the crushing 
and thrusting aside of the peridial and host cells above (PI. i, A). 
Deep-seated sori are apt to occur in the calyx and fruit where the apex 
of the peridium seldom appears through the epidermis. Adjacent sori 
merge by the dissolution of side walls, large saclike cavities which are 
packed with spores resulting. Sori that do not lie much over two oi; 
three cells deep in the tissues beneath the epidermis are erect (PI. i, F), 
broadly elliptical, or lenticular. While individual sori on canes are large, 
they increase in length by coalescence (PI. i, G). Very frequently the 
vegetative hyphae mass together, crushing aside the host cells of the 
cortex. If these primordia lie near the forming cork layers, or lie deep 
down in the cortex, the uredinia will be inverted, the “peridium” being 
formed on the inner side (toward the center of the cane) (PI. i, D, E). 
Two sori may originate from the same primordium, one having a 
peridium on the side toward the epidermis, the other on the side next to 
the cork cambium, which lies quite deeply embedded in the cortex. The 
sori shown in Plate i, F, originate from separate primordia; the one above 
is erect, the other inverted. 
ORIGIN OF THE SORUS AND ITS PERIDIUM. 
The origin of the sorus is most easily studied from sections of the rust 
as it occurs on leaf stalks. The vegetative hyphae are not well provided 
with a granular or stainable cytoplasm, but the cells giving rise to the 
