11 2 Rayner.—Obligate Symbiosis in Calluna vulgaris . 
For microtome work, material was embedded in paraffin, and the 
sections stained on the slide by one of the methods mentioned below. Fo r 
sectioning ripe fruits enclosing seeds, material was also embedded in celloidin 
to avoid displacement of the seeds after removal of the paraffin. 
Staining methods. The method found most generally useful for the 
differentiation of mycelium in the tissues was a concentrated solution of 
cotton blue (Baumwollblau 4 B) in lactic acid. Sections stained for some 
hours (8-24) in this solution can be differentiated in lactic acid, and examined 
in this reagent or in glycerine, or they may be dehydrated and mounted in 
balsam. By the use of whole roots, very satisfactory preparations of the 
Fungus in the root-cells can be obtained in this way. 
The method can also be used for microtome sections if precautions are 
taken to prevent the sections floating off the slide. 
The orseillin-aniline-blue method (Strasburger) for differentiating myce¬ 
lium, and various modifications of it, were also used; satisfactory preparations 
of the leaves and shoot were obtained by this means, and likewise by 
use of iron-alum haematoxylin (Benda), and other stains in common 
laboratory use. 
For rapid identification of mycelium in the tissues, slow maceration in 
sulphuric acid is useful, as is also treatment by ammoniacal cupric hydrate 
and chloral hydrate. 
A. The Seedling. 
Infection of the seedling tissues by the Fungus can be readily followed 
in material germinated on blotting-paper, fixed in Carnoy’s fluid, stained 
and differentiated in ‘ cotton blue ’ in lactic acid, and mounted for 
examination in either pure lactic acid or a mixture of lactic acid and 
phenol. 
Infection may begin at the tip of the root, by hyphae forcing their way 
between the cells of the apex; more usually it takes place simultaneously 
at several points, and the mycelium immediately becomes intracellular in 
distribution. 
The hyphae penetrate cell membranes with ease; there is no trace of 
swelling at the point of entrance, and they ramify in the external tissues of 
the root as if cell-walls offered no obstruction to their growth. Infection 
spreads rapidly from cell to cell; some hyphal branches grow out and infect 
fresh rootlets as they develop ; others form a tangled skein of fine hyphae in 
the superficial cells. 
In these early stages, the mycelium is homogeneous throughout, and 
consists of a system of delicate colourless threads which stain deeply with 
‘ cotton blue *, both within and without the cells of the root. 
The tissues of the young root, like those of healthy seedlings growing 
in soil, are quite colourless. In the absence of infection, arrest of growth 
