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. 
Bower? (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. 
le 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 
' 4, Bower, F.O., Botany of the Living Plant, 1919. 
