Nov. 1, 1924 
A Mycorrhizal Fungus in the Roots of Legumes 
465 
The vesicles are terminal, and usually 
intercellular in position in the outer 
cortex, where the mycelium produces 
but few haustoria. The number pro¬ 
duced varies greatly in different roots 
of the same plant and also in different 
species. They are usually few in al¬ 
falfa and very abundant in sweet 
clover. In shape they are typically 
ovate with a slight papilliform projec¬ 
tion at the distal end, but pressure 
from the surrounding cells forces them 
into a great variety of distorted forms 
PENETRATION OF THE ROOT BY 
THE FUNGUS 
When small newly infected rootlets 
are crushed under a cover glass and 
examined under the microscope, fungus 
hyphae are often seen passing along 
the epidermis and entering the root, 
where the characteristic haustoria de¬ 
velop. The course of penetration has 
also been traced in stained sections 
from material imbedded in paraffin. 
One of these is shown in Plate 1. Fig- 
Fig. 3—Penetration of a root of Lathyrus odoratus by the mycorrhizal fungus. 
(fig. 1). They vary in length from 75 
to 150 ju and in width from 25 to 65 ju. 
They appear to be separated from the 
hyphae by merely a thickehed por¬ 
tion of the membrane which surrounds 
the contents of the vesicle. The first 
vesicles formed in a root usually con¬ 
tain a few large oil globules, and appear 
imperfectly developed. Occasionally, 
when the cortical tissue begins to dis¬ 
integrate, great numbers, which appear 
filled with small oil globules of uniform 
size, are formed. 
ure 3 was drawn from a similar pene¬ 
tration found in a razor section of a 
sweet-pea root, and shows the character¬ 
istic appearance of the fungus in its 
advance to the deeper cells of the cor¬ 
tex. The hyphal strands outside the 
root are always empty at this stage. 
In this instance one of the two enter¬ 
ing hyphal branches is greatly swollen 
in the epidermal cell, producing a struc¬ 
ture like an appressorium. The myce¬ 
lium in the outer cells is empty, and is 
contorted in a way suggesting that its 
passage from cell to cell is difficult. 
