76 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
while the fungus grows with equal rapidity at 24 0 and 31 0 . Again, the 
growth of the fungus at 33 0 to 34 0 , the upper limit for the disease, was 
much less vigorous than the growth of the fungus at 19 0 and 21 0 , the lowest 
temperature at which the disease occurs. 
Both host and parasite, taken individually, develop at wider ranges of 
temperature than does the disease occasioned by their interaction, the 
minimum and maximum temperatures being 9 0 and 37 0 C. for the parasite 
and approximately io° and 36° C. for the host, while for the disease the 
minimum is 20° and the maximum 33 0 C. The relative rate of development 
of host, parasite, and disease corresponds in a general way as the tempera¬ 
tures are raised or lowered. 
The Cause of Wilting and Death of the Host 
The cause of wilting and the subsequent death of Fusarium-infected 
plants is still undetermined. It has often been assumed that the fungus 
actually fills the xylem vessels of the stem and thus limits the passage of 
water through the plant. Recently, both Haskell (6), studying the Fu- 
sarium wilt of potatoes, and Brandes (1), in his observations of the Fusarium 
wilt of bananas, have concluded that death is due to toxic substances 
produced by the fungus. There is some evidence that a toxic substance 
may be produced in Fusarium-infected tomato plants. 
If it is true that the wilting results from plugging of the xylem tubes, 
then it should be possible t,o reproduce the symptoms by cutting away 
tissue, including xylem elements, in the corresponding parts of a healthy 
plant. As a matter of fact, although it is true that when one side of the 
leaf is diseased the infected vascular bundles are found only on the diseased 
side, yet, if the petiole of a healthy leaf is cut half-way through, the injured 
side does not wilt. There seems to be sufficient lateral diffusion of water 
to keep the entire leaf turgid. Similarly, leaves on one side of a stem 
can not be made to wilt by cutting through the vascular bundles on that 
side of the stem. 
In Plate IX, A , is shown a plant which has thrown out vigorous suckers 
from an old diseased stem. This plant was transferred from a temperature 
of about 27 0 to one between 15 0 and 20° C. when the disease was in an 
advanced stage. The fungus was recovered from the very tip of the plant 
at the time of the transfer. All the leaves and the growing tip were killed, 
but in the cool temperature the old roots and stem were able to supply the 
moisture and other nutrient materials for a new top, which grew rapidly 
and appeared healthy. If the old stem had wilted because of plugging of 
the xylem tubes, it is difficult to believe that the change in temperature 
could have relieved this condition so rapidly. But while the temperature 
change would not affect the structural relations within the old stem, it 
would immediately influence such physiological reactions as the formation 
and activity of an injurious substance and the ability of the host tissue to 
neutralize it. 
