114 Rayner.—Obligate Symbiosis in Calhma vulgaris . 
the mature roots of Calhma agree with these records, and need only be 
briefly described. 
The outside of the young root consists of a single layer of rather large 
cells; the cortical tissues are much reduced in extent, and there is a slender 
central vascular strand. Root-hairs are absent, but a papillate outgrowth 
of cells may occur in the radicles of young seedlings. 
The surface of the root is traversed by a network of hyaline, septate 
hyphae, brownish yellow in colour, and irregularly branched. 
These hyphae are closely applied to the outer cell-walls and frequently 
grow between the cells, forcing them apart; they usually penetrate near 
the corners, where a cluster of branches is formed, some of which pierce the 
cell-wall. Within the cells they develop several stout coils, from which, 
at intervals, crowded clusters of short thick branchlets are given off, com¬ 
pletely filling the cell (PI. VI, Fig. n). Almost every cell of the younger 
part of the root is infected in this way, and in surface view such cells 
present an appearance as if filled with dense coils of closely interwoven 
hyphae. 
In addition to this characteristic infection, a system of finer hyphae is 
present, the branches of which are often especially abundant around the 
root-tip, penetrating cells in the same way, and forming coiled masses 
within them. 
With the exception of the characteristic 1 clusters ’ in root-cells described 
above, the mycelium within the plant tissues is distinguished from the same 
mycelium outside only by the smaller diameter of the hyphae composing 
it, the two systems of hyphae being continuous. Outside the cells, the 
mycelium sometimes exhibits the structure shown in PI. VI, Fig. 3. The 
hyphae figured are very transparent, of rather large diameter, and show 
characteristic swellings at intervals. This photograph may be compared 
with PI. VI, Fig. 4, which shows hyphae, identical in structure, from a pure 
culture of the Fungus extracted from the ovary, growing on Calluna -extract 
gelatine (p. 129). 
These swellings have not been observed on the hyphae of the inter- or 
intracellular mycelium. They do not seem to correspond to the ‘vesicles’ 
of Gallaud and other observers, but are apparently formed by the vegetative 
hyphae when growing saprophytically, either in proximity to the root, or 
on certain artificial media. 
The Shoot. Historical. The members of Ericaceae possess anatomical 
peculiarities—especially with regard to leaf structure—which have led to 
repeated investigation by plant anatomists. 
In particular, the ericoid leaf has attracted attention as a characteristic 
xerophytic type, and the variations of structure shown by the leaves of the 
several members of the group have been recorded in great detail, and used 
as a basis for classification. 
