THE FUNGUS-ROOT (MYCORRHIZA). 183
A useful method is to place small portions of the material bearing
mycorrhiza in a chromo-acetic fixing solution for two to three
days, and then to proceed by the usual method to cut them in
paraffin wax.
Sections from birch mycorrhiza may be taken as typical
of the ectotropic form. They are comparable with diagrams
of transverse sections of the fungus-root from other trees,
but actual diagrams of the birch are not available so far
as I am aware.
A transverse section of typical mycorrhiza has a diameter
of .35 mm., it exhibits a series of three concentric circles, the
outer of which, the mantle, consists of hyphae so closely com-
pacted together that they appear as a pseudo-parenchyma, the
outer surface of which is smooth. Very few hyphae of our type
project into the soil; the thickness of the mantle is approximately
.04 mm. Immediately under the mantle is a complete circle
of thin-walled cells that are in section oblong radially, followed
by others that are hexagonal. Next within these is the endoder-
mis of the root structure with walls thickened tangentially and
radially, a small amount of transfusion tissue being present.
The central cylinder consists of plates of xylem in the form of a
cross, tetrarch arrangement, with intervening phloem. The
pericycle is not clearly defined. Sections taken from other my-
corrhiza differ greatly in respect of the mantle, and in the
circle of cells immediately under it. The endogenous origin
of the branch of a fungus-root is seen in the transverse and
longitudinal sections. Longitudinal sections often show more
distinctly than the transverse the hyphae of the mantle that
have forced their way between the cortical cells. Root hairs
are not present after the mantle is complete.
It is stated that mycorrhiza are not always formed on the
rootlets of the young seedling, the oak not bearing them until
the second or third year of growth. The birch and hornbeam
bear mycorrhiza almost as soon as the lateral rootlets are
formed.
Birch roots have mycorrhiza of four kinds, judging from size,
kind of branching, colour, and the appearance of the outer surface
of the mantle, they are:—
(1) Brown, coralloid but somewhat lax in the branching;
diameter .35 mm.; fungoid mantle smooth, with