116 Rayner.—Obligate Symbiosis in Callnna vulgaris . 
leaf; the upper epidermis roofs over an almost continuous air-space, 
traversed only by the branches of the central vascular strand and the tissues 
which surround it (PL VI, Fig. 5). 
In median longitudinal sections parallel to the upper and lower 
surfaces, the relations of the large air-spaces to the vascular strands and to 
the assimilating tissue below the epidermis of the lateral face of the leaf are 
made clear. A few layers of assimilating cells on either side pass internally 
into spongy mesophyll, consisting of strands of green cells, which bridge 
over at intervals the space between the assimilating tissue and the paren¬ 
chyma surrounding the bundles. 
Towards the base of the young leaf there is a continuous tissue of 
thin-walled cells. In the mature leaf this region is filled with the remains 
of these cells intermingled with a dense accumulation of crystal-aggregates 
of calcium oxalate. 
All stages of degeneration are to be observed in these cells and in those 
forming the trabeculae. Empty cells may be seen, each filled by a single 
large crystal-aggregate, or groups of such crystals lie free in the space, 
entangled among the walls of the cells which originally contained them. 
The cells are represented often only by a framework of walls, though some¬ 
times traces of the cell-contents are recognizable (PI. VI, Fig. 6). 
The presence and distribution of the fungal hyphae in these tissues will 
now be described. 
The Distribution of the Fungus. 
Longitudinal sections through the young shoots provide abundant 
evidence of the prevalence, on the outside of the stem and leaves, of a net¬ 
work of fine hyphae, ramifying among the hairs or closely applied to the 
cuticle of the epidermal walls. These hyphae are often excessively fine and 
are easily^overlooked in unstained preparations. 
In fairly thick hand-sections or in serial sections of the shoot, it may be 
determined that this external mycelium is part of a system which pervades 
the tissues of the stem and leaves. Hyphae penetrating the epidermal 
cells can be traced into the deeper-lying tissues of the leaf and into close 
contact with cells of the mesophyll. 
The microphotograph produced in PI. VI, Fig. 7 shows a tangle of 
hyphae between the leaves of a young shoot, branches from which penetrate 
the tissues of the leaves on either side. 
The hyphae show no preference for special points of entrance or 
egress. They penetrate with equal ease the cuticularized cells of the 
epidermis or the base of a hair, and may be occasionally seen emerging 
from the large hair which terminates the leaf apex. 
In sections which have been specially treated and stained, it is possible 
to follow the ramifications of these fine hyphae across the air-spaces 
and into the cells of the mesophyll. They are often of extreme tenuity, 
