June i6,1923 
Pucciniastrum americanum 
889 
primordium increase in size and their contents become granular and 
stainable and the nuclei show very distinctly (PI. 2, A). Cells become 
arranged side by side in a compact plectenchyma. The terminal 
cells which eventually constitute the peridium elongate rapidly, lose 
their granular cytoplasm and nuclei by disorganization, and act as buffer 
cells to push aside and crush the host cells above. In Plate 2, B, two or 
three terminal cells at each end of the section are more heavily shaded in 
the drawing to indicate that they were more deeply stained than the 
others; the final divisions by which peridial cells arise will soon take 
place. In this way the sorus continues to increase in breadth for 
some time. In the formation of the teleutospores of species of 
Gymnosporangium {8) terminal cells of a sorus primordium function 
purely as buffer cells and entirely disappear as the sorus matures. It is 
the subterminal cells from which the teleutospore buds are formed. 
This is not the case in Pucciniastrum americanum on petioles, where the 
buffer peridium persists more or less and is usually recognizable in an old 
sorus, while the subterminal cells disappear. 
Later the author will consider spore formation as found in P, agri- 
moniae, the species studied by Ludwig and Rees, but if Plate 2, A and JB, 
of this paper is compared with their figure i in Plate VIII it will be seen 
that the only essential difference is in the layer of somewhat shorter or 
more flattened subterminal cells which the author found in various stages 
of disorganization, indicated by the way these cells take the orange G. 
In their figure the subterminal cells in the four central rows are not as 
thick as those below them. Sections presenting such features as are 
shown in Plate 5, A, of this paper certainly can be interpreted to mean 
that the layer of cells, b, which extends across the sorus, is composed of 
active basal cells, since these cells take the gentian violet stain rather 
deeply, while the cells of the “hyphal plate’' and vegetative cells below 
take scarcely any stain. The cells s take the orange and safranin and 
certainly appear to be degenerating. The cells u will develop into spores. 
Toward the center of this sorus space has been formed and the intercalary 
cell below the spore is elongating into a stock as it disorganizes. If this 
method of spore formation were continued, the uredospores would be 
borne in chains, and essentially there is very little difference between the 
two methods. In very compact deep-seated primordia, conditions may 
be such as to necessitate the formation of the first spores in this way. A 
greater upward thrust against the overlying tissue would result if the 
basal cell elongates as a whole or at its upper end in preparation for 
division. In Plate 5, B, are shown the first spores formed in a sorus where 
the pressure was soon relieved at the center by a break in the overlying 
tissues. Large wedge-shaped uredospores with perfectly definite stalks 
can be seen. Nearer the margin of this sorus there was still considerable 
resistance at the left and above, so that the spore initial buds developing 
from the basal cells, b, are pushing out to the right where the tissue was 
less compact. The relationship of peridial cells, p, intercalary cells, i, 
and basal cells, b, is very evident in sections of this sorus. Further 
evidence that pedicellate spores occur in more mature sori is scarcely 
necessary. At the right in Plate 5, C, is shown a perfectly typical pedi¬ 
cellate spore. At the left three adjacent basal cells are budding; nuclear 
division is occurring in the cell at the middle. 
If the tangled mass of hyphae constituting the sorus primordium is 
exceptionally large the development of a clear-cut peridium is more or 
