186 THE ESSEX NATURALIST.
Weevers (1916) is of opinion that mycotropic plants are, with
the help of their fungus partner, able to utilise fully the organic
compounds of the soil.
Rexhausen (1920) considers that the fungus and the root
together form an osmotic unit for the absorption of nutrient salts.
At the conclusion of McDougall's account of his recent re-
search he writes, "It was formerly believed that the ectotropic
mycorrhizal fungi were of considerable benefit to the host plants,
in that they aided them in absorbing materials from the soil, and
this old idea is still retained in many, even of the latest, botanical
text books. There is no evidence in favour of such a hypothesis,
and it is the consensus of opinion of recent workers on these
structures that the fungi are merely parasites on the roots of the
higher plants and that the higher plants receive no benefit at all
from the association." The effect of the above quotation loses
much of its force when the writer adds immediately following it,
"It is probable that as a rule no great harm to the higher plants
results from this parasitism of its roots by mycorrhizal fungi."
Surely this admission of the comparative harmlessness of the
parasitism affords considerable ground for the hypothesis that the
so-called host-plant gets some benefit from the fungus. It is to
be regretted that in the paper from which the above quotation is
taken, no citations from the writings of the recent workers on
ectotropic mycorrhiza, with whose views the author agrees,
are given. The literature cited, not including the writer's own
publications, falls within the dates 1880-1906.
Bower7 (1919) is of opinion that "on the side of the fungus,
which is already leading a saprophytic existence in the soil, a
direct supply of carbohydrate will be obtained by contact with
the root. The initial advantage from the coalition would then
lie with the fungus. The advantage which the tree derives is,
in the first place, the more ready supply of salts, and of com-
bined nitrogen, extracted by the fungus from the soil."
From my own investigation of the birch tree I would recapitu-
late that:—
(1) The shallow root-system of the tree bears ectotropic
mycorrhiza for the greater part of the year, if there are
no long periods of drought and if the winter is not
severe. A really hard winter has not occurred in
7. Bower, F. O., Botany of the Living Plant, 1919.