PI.ATE I« 
Pucciniastrum americanum (Farl.) Arth. on June X Cuthbert Red Raspberry 
A. —Section through the center of a young uredinium on leaf stalk. Four layers of 
host cells lie above the sorus. Cells next to the sorus have been crushed by the ex» 
pansion of the buffer tissue. Disorganization of cells at the center. Uredospores 
formed irregularly, intercalary cell replaces stalk. 
B. —^Vertical section through a mature sorus on a leaf stalk. Only two layers of host 
cells above. The buffer tissue has been flattened by the pressure from within and will 
persist more or less as a peridium, although the cells at the center are fully collapsed 
and will soon disappear. No corona would have been formed. Uredospores stalked. 
C. —Vertical section of sorus on lower side of leaf. One cell of the corona showing. 
D. —Inverted broadly elliptical sorus on young cane. ' 'Peridium’' or buffer tissue 
facing the cork cambium, which is deep-seated; e, epidermis; ck, cork layers. 
E. —Inverted sorus still more deeply seated. Buffer tissue directly against the 
cork. 
F. —^Two lenticular sori on a cane. The upper one lies just beneath the epidermis 
and is erect; the lower is inverted. 
G. —Erect double sorus on large cane. Peridium of the part at the left has entirely 
disappeared, cavity filled with spores. At the right peridium still evident although 
disorganizing. Degeneration of tissue at the center of the sorus (dark area), normal 
spore production below. Note the position of the epidermis above the sorus and of 
the cork cambium below it; e, epidermis, cp, cortical parenchyma, ck, cork. 
B to F, photographed with Leitz 8 mm. lens and No. lo oc.; A, 4 mm., 6 oc.; G, 
16 mm., 10 oc. 
a The photographs shown in PI. i, and all the sections which were studied in the preparation of 
this paper were made by Miss Ruth Colvin. 
