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 XI. 
Fic. 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. 
Fic. 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 xylemand phloem. 
xX 20. 
PLALIE: -XIT: 
Fic. 1.—Inner portion of mantle, highly magnified; (a) Hyphe between 
the walls of the cortical cells, two to four cells deep. X 410. 
Fic. 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) hyphe around and between the 
styles ; (c) portions of the membraneous wings of the fruit. x 25. 
PLATE, 1 
Fic. 1—Germinating seed of birch, the seed-coat having been shed; (a) 
cotyledons ; (b) hypocotyl with hooked processes on which hyphez 
have been caught. X 25. 
Fic. 2.—Two portions of hypocotyl from same seedling, that on the left 
showing (a) hyphal threads being carried dcwn to the surface 
of the soil; that on the right showing (b) fully developed hooked 
processes (modified hairs) in prcfile. x co. 
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. 
HE decline and diminuticn in the mammalian fauna of 
the British Isles, both in species and in individuals, has. 
for a long period Seen 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 
