188
THE ESSEX NATURALIST.
discoloured wherever it comes within the influence of the hyphal
threads.
The attack of last year, although wide-spread, was not so
fatal as that of twenty-two years ago, but before anything more
definite can be said respecting its virulence the course of the
epidemic must be followed through another twelve months.
The disease has not at any time been entirely absent from the
Forest during the period 1899-1922, but on some occasions it
has been difficult to locate it.
It is perhaps merely a coincidence that each of the epidemics
occurred after a period of exceptional drought. As regards the
phenomenal absence of rain during the year 1921, the occasion
remains a vivid recollection with us all. The official figures
relating to rainfall, on the Forest area, for the months March—
October 1899-1900 show a considerable deficiency when com-
pared with normal conditions.
The long drought on these two occasions makes it possible to
infer that considerable damage was done to the superficial root-
system of the birch by the prolonged absence of rain. On an
examination of these roots in September 1922, it was abun-
dantly evident that all the mycorrhiza, and a majority of the
roots to which they were attached, had been killed. The soil,
so late as the end of that month, was perfectly desiccated at the
level where the mycorrhiza had functioned. The water supply
with the contained salts had been cut off almost completely,
consequently, the vital energy of the birch was lowered, and
the inference is that its leaves and young twigs were thus ren-
dered more liable to attack by fungi. The deeper rooted trees
growing on the same ground did not suffer.
The roots of the wood-sage, Teucrium Scorodonia L., so
closely associated with the birch, is covered with fungus
hyphae, one of which predominates. This fungus runs in some-
what parallel lines, with side branches, over the surface of the
root, resembling incipient mycorrhiza, which it may be, not
having passed the initial stage of development. Another member
of the oak-birch wood association, the bluebell, has been stated
to have endotropic mycorrhiza. It is probable that an investiga-
tion of the ground flora of the oak-birch association will add to the
number of plants known to carry mycorrhiza.
It is plainly evident, from the quotations given above,