Rayner.—■Obligate Symbiosis in Calluna vulgaris. 113 
occurs, and this is invariably accompanied by browning and discoloration 
of the cell contents. Subsequent to infection, rapid elongation of the roots 
takes place. 
In stained and cleared preparations of the young seedling, it may be 
further observed that infection is not confined to the rootlet, but that 
penetration of the tissues of the hypocotyl and cotyledons by hyphae 
likewise occurs and it is often possible to determine that the same mycelium 
is continuous from root to hypocotyl. 
In the sub-aerial parts the mycelium does not develop so extensively 
on the surface of the plant, nor do the hyphae become massed in the super¬ 
ficial cells as in the roots. They are irregularly distributed in the tissues, 
penetrating the cell-membranes with the same apparent ease as in the root. 
Hyphae not infrequently grow out to the exterior, and since this may also 
be observed in seedlings germinated in soil, the tendency to extend outside 
the plant cannot be entirely due to growth in saturated air. 
It is suggested that this frequent outgrowth of hyphae to the outside 
of the sub-aerial parts of the plant is significant in connexion with the 
problem of nitrogen fixation by the Fungus (p. 107). [See also Hiltner ( 33 ).] 
In vertical section, the cotyledons, in contradistinction to the later 
leaves, show typical dorsiventral structure of the assimilating tissue : the 
mesophyll shows marked differentiation into palisade and spongy paren¬ 
chyma ; stomata are present on the lower surface, the guard-cells being 
slightly above the general level of the other epidermal cells. 
In young seed-leaves progressive degeneration of cells of the meso¬ 
phyll, such as is recorded in the mature leaf, is not apparent (p. 117), 
nor is there an accumulation of crystals or crystal-aggregates of calcium 
oxalate. 
Hyphae, though not abundant, can be identified in the tissues of the 
cotyledon under high magnifications. They are often extremely attenuated, 
show a preference for the intercellular spaces, but undoubtedly invade cells 
of the mesophyll, some of which become filled with a tangled mass of 
hyphae (PI. VI, Fig. 3). 
B. The Mature Plant. 
Material for ‘ examination of the mature tissues was collected from 
many sources, and from wild and cultivated plants. In all cases the con¬ 
dition observed was substantially the same, and in no case—so far as my 
observations go—are the green parts of the plant free from mycelium. 
The Root. The distribution and general appearance of the mycelium 
in the cells of the root of ericaceous plants were described many years ago 
by Frank for Andromeda polifolia ( 35 ), and more recently by Coville for 
Vaccmium corymbosum ( 36 ). My observations on the younger parts of 
I 
