16 'bulletin 1120, U. S. DEPARTMENT OF AGRICULTURE. 
susceptibility to injury before the excessive heat occurs. Investi- 
gations along these lines are being continued by the Office of Cereal 
Investigations in cooperation with the North Dakota Agricultural 
Experiment Station. Tentative suggestions for lessening this injury 
are (1) higher rates of seeding, (2) earlier dates of seeding, and (3) drill- 
ing north and south rather than east and west. In this connection it 
is well to remember that the beginning of these intermittent periods 
of overheating in this section usually occurs in the first or second 
week of June. 
SUMMARY. 
Investigations have shown that the cause of the heat-canker type of 
flax injury is nonparasitic in its nature. 
It occurs somewhat uniformly each year in the northern Great 
Plains area and causes a marked loss in flax production. 
The cortex of the stem is killed at the surface of the ground. Sooner 
or later the cankered plants topple over. Young cankered plants die 
at once, while those that are a little older may remain alive for days 
or weeks, as long as the vascular systems function. Stems of the older 
cankered plants usually enlarge just above the injury, and sometimes 
also just below it. The result is a girdling of the plants at the soil 
line. 
In these experiments and observations flax was cankered only 
during and immediately following very hot days. Flax plants when 
more than 4 inches high are only slightly susceptible. Flax plants 
which have developed under hot, dry conditions are less susceptible to 
injury from high soil-surface temperatures than more succulent 
plants. 
Flax plants which are grown in a soil having a shallow surface 
mulch over a firm seed bed are less readily injured than those grown 
in a soil in which the surface layer has been compacted into a crust 
by rains. Plants shaded by a vertical strip of canvas 10 inches high 
were not cankered, while many unshaded plants in the same row were 
cankered. Thinly sown flax was cankered more than thickly sown 
flax. 
Flax sown with cereals as nurse crops was cankered comparatively 
little, and flax with weeds was cankered less than flax free from 
weeds. 
Evidence indicates that heat canker of flax results from a combina- 
tion of succulence in the young flax plants and high temperatures of 
the surface soil in immediate contact with the succulent stem tissues. 
Killing the cortex of young flax plants by artificial heat produced 
typical heat canker. 
Promising control measures seem to be thicker seeding and early 
sowing, and possibly drilling rows north and south instead of east and 
west may prove helpful in lessening the severity of canker injury. 
