460 
Journal o f Agricultural Research 
Vol, XXIX, No. 0 
The literature on the type of mycor- 
rhizal relation considered in this paper 
has been reviewed in recent papers by 
Magrou, Peyronel, and Demeter, cited 
elsewhere. It will suffice to note here 
that the first description of a mycorrhi- 
zal fungus in a plant belonging to the 
Leguminosae appears to have been 
made by Janse (2) in Pithecolobium 
montanum . Magrou (4) has given an 
excellent description of a similar or 
identical fungus in Orobus ( Lathyrus ) 
tuber osus. Doctor Magrou has ex¬ 
changed stained preparations with the 
writer, and he states in personal corre¬ 
spondence that the fungus in the roots of 
the four legumes sent him appears 
identical with that which he found in 
Orobus. The writer concurs in this 
opinion. Peyronel (5) has recently 
published an abstract of a forthcoming 
paper recording this type of mycorrhi- 
zal invasion in a number of plants, and 
he describes what he believes to be the 
method of germination of the so-called 
vesicles. Demeter (1) has found the 
same type of mycorrhizal fungus in 
roots of Vinca minor and some related 
plants. His attempts to secure it in 
pure culture have yielded the first 
success that has been reported. From 
its cultural characters Demeter desig¬ 
nates the fungus Rhizoctonia apocyna- 
cearum, without giving a formal de¬ 
scription; but his attempts to infect 
plants with this culture did not satis¬ 
factorily reproduce the condition found 
in the field. In view of these unsatis¬ 
factory inoculation experiments and 
the obviously great differences be¬ 
tween the fungus described in culture 
and that found in the plant, Demeter’s 
conclusions need confirmation. 
DISTRIBUTION OF THE MYCORRHIZAL 
FUNGUS IN THE ROOTS OF LEGUMINOUS 
PLANTS 
The mycorrhizal fungus of legumes, 
like that of similar character in other 
plants, is found only in the cortex of 
roots. In annuals such as peas and 
sweet peas it does not enter the cortex 
of the base of the stem, which is very 
rootlike in structure. In fact, the 
cortex of the taproot seems to be quite 
resistant to it. It develops most 
abundantly in the small lateral roots 
which produce little or no secondary 
thickening. In biennials and perenni¬ 
als such as clover and alfalfa the cortex 
of the taproot is sloughed off as soon 
as secondary growth increases the 
diameter of the root. Therefore the 
fungus is not commonly found in tap¬ 
roots or even in the large lateral roots. 
The small laterals and terminal 
branches of all roots in older plants 
produce little or no secondary thick¬ 
ening, and the primary cortex persists 
during the life of the roots. This cor¬ 
tex is readily penetrated and in it the 
fungus develops extensively. All the 
above-named plants have a compara¬ 
tively thick cortex which occupies 
from two-thirds to three-fourths of 
the entire diameter, and in these 
hosts (Pis. 1, 2) the fungus develops far 
more abundantly than in the roots of 
beans and soybeans, which have com¬ 
paratively thin cortical layers. Thus 
the thickness and character of the 
tissue outside the endodermis seem to 
determine the extent to which the 
plant may harbor the fungus. Per¬ 
haps the large intercellular spaces in 
the cortical tissue, which permit easy 
passage of the hyphae along the root, 
is the character to which the fungus has 
become adapted. 
DISCOLORATION OF ROOTS BY THE 
MYCORRHIZAL FUNGUS 
When the cortex of the root has 
become thoroughly invaded, the condi¬ 
tion is usually indicated by a yellow or 
greenish-yellow discoloration, some¬ 
times accompanied by a slightly water- 
soaked, translucent appearance. This 
discoloration is due, as will be shown 
later, to the color of the contents of the 
deeper cells, and to this source is due 
the characteristic appearance distin¬ 
guishing it from those discolorations 
which arise from changes in the walls 
of the outer cells. In advanced stages 
the walls of the outer cells may turn 
yellow as the roots begin to disintegrate 
or from some other cause, whereupon 
the characteristic appearance is lost. 
The larger part of the discoloration in 
the roots of legumes which have been 
examined seems to be due to this 
fungus. The color does not indicate 
the extent to which invasion has pro¬ 
gressed; at times it will be found to 
have advanced far beyond the dis¬ 
colored region. In all doubtful cases, 
the presence of the fungus can readily 
be determined by microscopic examin¬ 
ation of sections of the root. 
PATHOLOGICAL HISTOLOGY 
An examination of free-hand longi¬ 
tudinal sections from fresh roots which 
are yellow and water-soaked will show 
at once where the color is located. It 
is usually limited to the cell sap of the 
cortical cells just outside the endodermis 
where its greenish-yellow or olive-green 
character is very striking in thick sec¬ 
tions. This color soon diffuses from 
the cut cells of razor sections mounted 
in water, and is more slowly lost from 
