Rayner.—Obligate Symbiosis in Calluna vulgaris. 117 
and in the absence of the mass of cumulative evidence available doubt 
might arise as to their true nature. 
The staining reactions of the hyphae in contact with the mesophyll 
cells become affected and the appearance of many of these cells is consistent 
with an active disintegration and digestion of the invading mycelium. 
The difficulty of finding active mycelium in the leaf seems to be 
due chiefly to the fact that in the unaltered condition it is present in 
the intercellular spaces only, these being relatively of such large extent that 
the contained hyphae are usually torn out or displaced in sections. 
It is possible, however, to obtain preparations which provide conclusive 
evidence of the presence of functional hyphae in the leaf tissues and of their 
invasion of mesophyll cells. 
In the chlorophyllous cells, progressive stages of degeneration may be 
observed ; fragments of mycelium, irregular in outline but still stainable, 
can be found in close contact with, and penetrating such cells ; many of 
them obviously contain hyphae in various stages of degeneration, while the 
same process of disintegration is affecting the cells themselves. 
Plasmolysis in various degrees, alteration of staining properties, 
degeneration of the chloroplasts, and finally of the whole contents of the 
cell, mark progressive stages in the process. By suitable staining methods, 
strands of very attenuated mycelium can be differentiated, bridging across 
the spaces, sending branches to the cells, and demonstrably continuous 
in places with those on the outside of the leaf (PL VI, Fig. 13). 
Calcium oxalate is present in remarkable abundance in the leaves 
in the form of large crystal-aggregates. 
The crystals accumulate chiefly in the mesophyll tissue towards 
the base of the leaf, and are conspicuous macroscopically in small pieces of 
shoot, cleared in cedar-wood oil. 
In leaf sections, the crystals are most abundant in the cells of the 
spongy mesophyll in the lower half of the leaf; single aggregates are 
present in the empty cells of the trabecular tissue, completely filling them, 
and occur also in groups intermingled with the cells and cell-remains in the 
central part of the basal region of the leaf (PI. VI, Fig. 6). 
Hyphae may be recognized in close contact with the crystals; some¬ 
times free in the intercellular spaces; more often in the form of strands or 
fragments of greenish membrane mingled with the remains of the cell-walls 
(PI. VI, Fig. 13). 
A network of fine mycelium with looped masses of hyphae may often 
be found in the cells of this part of the mesophyll, and can be identified 
after treatment of the section with concentrated sulphuric acid. 
There is no doubt that the region of the leaf which contains most 
abundant evidence of the presence of mycelium is also the region in which 
the greatest accumulation of calcium oxalate occurs. 
