356 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
cutin. Irregular tooth-like projections alternate with notches and pits of 
a different consistency, so that this region appears more or less porous 
or granular. The cuticular layer is thickest on the outer wall of the cell, 
but it extends down between the epidermal cells some distance, and may 
even occur along the inner wall. 
The epidermal cells of the cedar are oblong or brick-shaped, easily dis- 
tinguished from the palisade mesophyll on the upper or proaxial side of the 
leaf. Other features necessary to the ready understanding of the patho- 
logical aspects of the subject are brought out by figures in the text and 
plate. 
Infection experiments and field observations made during a number of 
years indicate that the first telia of G. clavipes on Juniperus virginiana 
are formed either directly on the leaves or, more commonly, at the margins 
of the decurrent leaf bases or in leaf axils of branches not over two or 
three years old. Later, as cork is laid down, sori break through in the 
ordinary manner. We shall therefore follow here the course of the fungus 
during the first year or two of its life in the host. 
The Mycelium in the Leaf 
Infection takes place on the proaxial side of the young leaf, or directl}^ 
on the young stem at the base of the leaf. Primary infection can not occur 
Fig. I. A to K, portions of longitudinal section of a leaf of red cedar infected with 
Gymnosporangium clavipes; shows two sori and the manner of advance of the mycelium. 
Intercellular hyphae have penetrated little beyond the palisade cells, which in many cases 
have been stimulated to divide. See text for further description. 
