THE AMERICAN GREY SQUIRREL IN THE BRITISH ISLES. 189
that opinions upon the relations between the fungus and the
higher plant, in the case of ectotropic mycorrhiza, are conflicting.
The questions at issue cannot finally be settled in the laboratory.
More observations in the field must accompany the indoor work.
In conclusion, I wish to express my indebtedness to Mr.
J. H. Pledge for the help he has rendered in making photo-
micrographs from my preparations to illustrate this Address.
EXPLANATION OF PLATES.
PLATE XL
Fig. 1.—Portion of the root system of a birch tree showing two forms of
mycorrhiza ; (a) white in colour; (b) brown; (c) rootlet bearing
mycorrhiza. X I.
Fig. 2.—Transverse section of brown mycorrhiza of birch: (a) mantle;
(b) radially elongated cells of the root cortex; (c) inner cells of
cortex; (d) endodermis; (e) central cylinder of xylem and phloem.
X 20.
PLATE XII.
Fig. 1.—Inner portion of mantle, highly magnified; (a) Hyphae between
the walls of the cortical cells, two to four cells deep. X 410.
Fig. 2.—Portion of winged fruit of birch; (a) the styles, which remain
until the fruit falls in autumn, forced apart as they would be on
germination of the seed; (b) hyphae around and between the
styles; (c) portions of the membraneous wings of the fruit. X 25.
PLATE XIII.
Fig. 1.—Germinating seed of birch, the seed-coat having been shed; (a)
cotyledons; (b) hypocotyl with hooked processes on which hyphae
have been caught. X 25.
Fig. 2.—Two portions of hypocotyl from same seedling, that on the left
showing (a) hyphal threads being carried down to the surface
of the soil; that on the right showing (b) fully developed hooked
processes (modified hairs) in profile. X to.
ON THE AMERICAN GREY SQUIRREL (SCIURUS
CAROLINENSIS) IN THE BRITISH ISLES.
BY HUGH BOYD WATT, F.Z.S.
Read 27th January 1923.
"The quadrupeds cf Britain are so few that every new species is a
great acquisition."—Gilbert White, Nat. Hist. of Selborne, Letter
xxviii., 8 Dec, 1769.
THE decline and diminution in the mammalian fauna of
the British Isles, both in species and in individuals, has
for a long period been continuous and steady, and seems to be
inevitable under the non-natural conditions of life which we
have brought about. Under such conditions, the successful