468 
Journal of Agricultural Research 
Vol. XXIX, No. 9> 
growth and extending through about 
a third of its length the fungus was 
usually found, apparently advancing 
from the old root. Plants in soil too 
dry for vigorous growth showed more 
invasion than those in more favorable 
locations. The dry weather, which 
was also retarding the growth of clover, 
continued and one week later roots 
from the most vigorous field were in¬ 
vaded to within an inch of the end, 
while those from fields a little less 
vigorous showed in half the roots in¬ 
vasion extending almost to the tip. 
Soon after this the fungus was found 
advanced nearly or quite to the root 
tip in almost all roots, and later in 
midsummer great numbers of rootlets 
in the upper foot of soil died. The 
factors responsible for the death of 
these roots can hardly be distinguished 
and evaluated from the evidence at 
hand. The dryness of the soil alone 
certainly must have contributed, but 
it appears also that the invaded rootlets 
were less resistant to drought, and it 
may be that the mycorrhizal fungus 
advanced so far into the growing point 
of many of them as to cause death. 
In any case, this loss of roots, as ob¬ 
served during two summers, has left 
the clover plant with a greatly reduced 
root system during midsummer. Al¬ 
falfa suffers quite as heavily, and during 
July and August has a surprisingly 
meager number of living root ends 
possessing root hairs which can absorb 
water in the upper 8 inches of soil. 
Deeper roots do not suffer so great a 
loss; and in fact the mycorrhizal fungus 
is less abundant at lower levels, being 
absent in one instance at a depth of 
3 feet. The roots of the plant exam¬ 
ined in the instance cited continued to 
an undetermined depth in very hard 
subsoil, but there were comparatively 
few rootlets below the region within 
which the fungus was abundant. 
When rainfall came in late summer 
and autumn and root development was 
resumed, longer and longer root ends 
were free from the fungus. On Sep¬ 
tember 24, the vigorous new roots of 
red clover near the surface of the soil 
had 1 or 2 inches of clean growth, 
while the deeper roots which had grown 
less rapidly were still invaded almost 
to their ends. From this time on 
root growth progressed more rapidly 
than the fungus invasion. In garden 
peas, which mature in early or mid¬ 
summer, there is a steady increase in 
the proportion of root system invaded 
until the plant is mature. In the 
sweet pea, an annual which requires a 
much longer period of time for its 
maturit}, the root system is found to 
be thoroughly invaded in mid-July. 
Little root growth seems to take place 
thereafter, and the root system slowly 
dies off during the latter part of the 
life of the plant. 
From this brief outline of the con¬ 
ditions observed in the field it appears 
that the extent to which roots are 
found invaded at any time depends 
largely upon the rapidity with which 
roots have been growing previously, 
and presumably also on a temperature 
favorable to growth of the fungus. 
Diminished growth of roots at high soil 
temperatures apparently leads to a 
complete invasion of the root system. 
Unfortunately, no study of the rate of 
root growth of perennial legumes 
during the season has been made. 
Therefore much work will be required 
to determine to what extent root inva¬ 
sion at any time is due to retarded root 
growth and to what extent it is due 
to fungus growth. 
Some experimental work has been 
done to determine the range of soil 
temperature within which the fungus 
will enter roots. In a preliminary trial 
in the winter of 1922, it was found that 
the fungus entered clover roots abun¬ 
dantly at soil temperatures ranging 
from 16° to 30° C. Another experi¬ 
ment was started in November, 1923, 
to determine how far beyond this range 
infection occurs. In this experiment 
soil was maintained at temperatures of 
33°, 15°, 12°, and 9° C. The soil used 
was loam from woodland, mixed with a 
small amount of sand containing 
mycorrhizal-infested sweet-clover roots. 
Peas and sweet peas were grown in the 
sand alone. The moisture content of 
the soil was not determined, but it was 
ample for good growth, and was main¬ 
tained uniformly during the experi¬ 
ment. Garden peas, sweet peas, and 
the taproots of young sweet-clover 
plants were planted on November 17. 
Growth of all of these plants was 
exceedingly slow at 9° C. On Decem¬ 
ber 29, roots of peas at 15° were found 
extensively invaded, the plants at that 
time having formed the fifth true node, 
but no infection was found at 12°. On 
January 11, peas and sweet peas at 33° 
had small portions of rootlets invaded, 
but none of the few branches produced 
by the Melilotus taproots showed 
invasion. On January 15 some inva¬ 
sion of sweet pea and Melilotus roots 
was found at 12°, and finally on Feb¬ 
ruary 9, one sweet-pea plant grown at 
9° was found abundantly infested. 
Thus root invasion can take place 
within the entire range of temperature 
at which these plants are able to make 
much growth. 
