A*MYCORRHIZAL FUNGUS IN THE ROOTS OF LEGUMES 
AND SOME OTHER PLANTS 1 
By Fred Reuel Jones 
Pathologist , Cotton , Truck , and Forage Crop Disease Investigations , Bureau of 
Plant Industry, United States Department of Agriculture 
INTRODUCTION 
During an investigation of a fungus 
parasite of the roots of peas, it became 
necessary in the spring of 1922 to 
examine under the microscope fresh 
razor sections of rootlets of a great 
number of plants in different stages of 
development. In the course of this 
work it was found that very many of 
the rootlets of plants which were con¬ 
sidered altogether normal were exten¬ 
sively invaded by a characteristic fun¬ 
gus which entered a few rootlets when 
the plant was small and had traversed 
at least half of the root system by the 
time the plant reached full bloom. The 
fungus was restricted to the primary 
cortex of the root, and usually indicated 
its presence by the straw-vellow color 
that the root assumed. When the pea 
plant was mature, all of the roots were 
usually yellow and thoroughly invaded 
by the fungus. This uniform and wide¬ 
spread infestation of pea roots indi¬ 
cated that the fungus must be very 
thoroughly distributed in the soil, and 
suggested the examination of roots of 
other legumes to learn whether they, 
too, might be hosts of this fungus. It 
was found that the roots of clover, 
alfalfa, sweet pea, and some other 
legumes were as thoroughly infested by 
this fungus growth as were those of 
peas. 
The root-inhabiting fungus thus 
found so abundant in the smaller roots 
of these common and much studied 
legumes is so coarse in character, has 
such distinctive structures, and pro¬ 
duces such conspicuous discoloration of 
a portion of the invaded tissue as seen 
under the microscope, that it is some¬ 
what surprising to find that its presence 
is not a matter of common knowledge 
among botanists. The fungus itself, or 
very similar fungi, have long been 
known in the roots of some other plants 
under the name of myeorrhizal fungi, 
and recently several studies of these 
root parasites by botanists and pathol¬ 
ogists have added many details to our 
knowledge of their development and 
have extended the list of plants in 
which they occur. But the list appears 
to contain only two species among the 
Leguminosae. 2 
Since the fungus is clearly parasitic 
in character, occurring abundantly in 
all or nearly all plants of the species 
infested, and producing in the invaded 
tissue unmistakable local pathological 
conditions, it became necessary to dis¬ 
tinguish between the results of its in¬ 
vasion and those following the attack 
of more aggressive parasites which were 
under investigation. This study, which 
began with the roots of peas, was soon 
extended to the roots of other legumes, 
and has produced evidence suggesting 
that this fungus infestation may in¬ 
fluence profoundly the development of 
some of our common legumes, at least 
when they are grown under certain en¬ 
vironmental conditions. It is the 
purpose of this paper to record more 
fully than in a previous note (8 ) 3 the 
list of plants in the roots of which this 
fungus is found, to describe the local 
effect it produces, and to indicate the 
modification in the development of 
some of the infested plants to which 
this parasitism may contribute. 
It may be unnecessary to add that 
the term myeorrhizal fungus, in its 
original coinage and as used in some 
recent papers dealing with this type 
of parasitism, does not carry with it 
any implication in advance of proof 
that the fungus is of any benefit to the 
plant invaded. In fact, it seems to be 
generally recognized that among the 
numerous observed examples of root 
invasions which have been designated 
by this term, many will undoubtedly 
be found to be wholly disadvantageous 
to the host and are properly within 
the plant pathologist’s field of study. 
1 Received for publication May 28, 1924—issued January, 1925. 
2 Since this was written Peyronel has published a host list that includes many of the common cultivated 
legumes. Peyronel, B., prime ricerche sulle micorize endotrofiche e sulla micoflora radicicola 
NORMALE DELI.E fanerogame. Riv. Biol. 5: 463-485, illus., 1923; 6: 17-53, illus., 1924. 
3 Reference is made by number (italic) to “Literature cited,” p. 470. 
Journal of Agricultural Research, 
Washington, D. C. 
(459) 
Vol. XXIX No. 9 
Nov. 1, 1924 
Key No. G—452 
