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AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
lie above the sorus. The corrosive effect of the hyphae, especially of the 
buffer cells, on the cuticularized layer is such that indentations or notches 
are left in the layer which, as noted, clings to the cuticle finally raised 
above the sorus. These markings are all the more misleading inasmuch 
as the outlines of the epidermal cells also persist in the raised portion, 
because the cuticularized part of the walls extends down between the 
epidermal cells, forming a sort of cap. One can readily show that there 
are two sets of markings or notches in the portion raised above the sorus 
by photographing oblique sections of a mound-shaped sorus. Such a sur- 
face view is shown in Plate XXII, figure 8. Unquestionably additional 
nourishment for spore formation may come from the mycelium in the 
mesophyll, and it is not impossible that hyphae present in this region may 
push up between the epidermal cells to form the telial stroma in the cuticu- 
lar layer. Such a sorus would not, however, be subepidermal. The red 
cedar leaf is strengthened by stereome tissue at the edge as shown below A 
in figure 4. Nothing but epidermal cells lies above this stereome, and the 
epidermal layer is unbroken under this sorus from A to E, although at C 
and D its cells are hypertrophied. 
The Sorus in Young Stems 
When young stems are infected, sori appear at the margins of the 
leaf bases or in the axils. It is unnecessary to describe in detail the de- 
velopment of the sorus, as the epidermis, provided with stomata, persists 
for two or three years and the sorus is formed in the cuticular layer just 
as it is in the leaf. It is much more difficult to trace the unbroken epi- 
dermis beneath the sorus because of the great irregularity of the cells in 
the region of the leaf axils where the epidermal cells are normally very 
large, often pointed, and frequently separated by intercellular spaces. In 
an infected stem these cells are generally still further enlarged and come 
to look like cortex cells, especially as they are pushed aside or displaced 
by the invading strands of mycelium. In no case has there been found 
the slightest evidence that the sorus is subepidermal. Figure 7 shows a 
portion of a sorus on a young stem, the raised cuticle carrying a part of 
the cuticularized wall of the epidermal cells appearing at the right. 
It is an important point to determine just where the sorus originates, 
although it is the morphology of the fungus and not the host-parasite relation 
that determines its place in a classification. This relationship, however, may 
very often indicate special adaptations developed in the course of evolution, 
and as such may be characteristic of a number of species in a given group. 
The writer is indebted to Professor J. C. Arthur and Dr. J. F. Adams 
for criticisms written after an examination of some of the preparations, and 
in line with their suggestions there has been an attempt to describe still 
more clearly the exact nature of the origin of the telial sorus of this unique 
Gymnosporangium. 
