HEAT CANKER OF FLAX. 6 
or light relations; but under the Michigan conditions few of the 
plants toppled over, and in the moist latter half of the season there 
was almost complete recovery. Anthracnose canker seems to be 
rather rare in the United States during some years, and when present 
its damage is confined almost entirely to young seedlings. 
HEAT CANKER OF FLAX, A NONPARASITIC TYPE. 
This type of flax canker, according to continuous studies since 
1916 reported by the writers in 1920 {18), causes severe losses and 
occurs to about the same extent each year in the semiarid flax- 
producing section of the United States. It is most evident during 
the latter half of June and the first half of July. The outside por- 
tion of the stem is killed at or near the surface of the ground when 
the plants are comparatively young (PL II, A and B ; PL III, A and B) . 
Generally speaking, if the injury occurs when the plants are less than 
3 inches in height the tissues collapse at the point of injury and the 
plants wither and die (PL III, A and B ; PL IV, A to D) . If the injury 
occurs somewhat later, when the plants are 3 to 5 inches in height, 
only the cortex is killed, allowing the plants to topple over, but 
usually to remain alive for days or weeks' because of the uninjured 
vascular systems within (PL II, A and B; PL V, A to C). Only in 
rare instances are plants more than 5 inches in height injured in this 
way. Numerous more mature specimens of heat-cankered flax can 
be found, but in such cases growth continues after the initial injury. 
Enlargement of the stem occurs just above and sometimes just 
below the iiijury (PL II, A and B; PL V, A to C). In most cankered 
plants the stem is severed, sooner or later, at the point of girdling 
by the winds or by the disintegration of the remaining tissues, due 
to the action of saprophytic organisms. Otherwise the plant dies 
when the starving roots can no longer support the increasing needs 
of the aerial portion. For this type of canker the name "heat canker 
of flax" is suggested. 
In order to avoid confusion, it may be well to state here that the 
nonparasitic flax canker discussed in this paper does not include a 
type which occurs late in the season in the driest districts of the 
northern Great Plains area. Following drought the base of the 
stem becomes very woody, dry, and brittle, with little tensile 
strength, and is snapped off by the wind. This trouble also seems 
to be nonparasitic in its nature. 
CAUSE OF HEAT CANKER. 
ISOLATIONS AND PATHOGENICITY EXPERIMENTS. 
Hundreds of isolations from cankered specimens of this type 
resulted sometimes in the growth of no organisms at all and never 
in the constant association of any one organism with the disease. 
