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 hyphz so closely com- 
pacted together that they appear as a pseudo-parenchyma, the 
outer surface of which is smooth. Very few hyphe 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 hyphe 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 :— 
(rt) Brown, coralloid but somewhat lax in the branching ; 
diameter .35 mm.; fungoid mantle smooth, with 
