Frost Resistance in Flax. 



always resulted in pronounced injury. A temperature of 19° F. 

 killed all except 7 per cent of the most resistant variet3\ A tem- 

 perature of 25° F. may not kill more than 10 to 25 per cent of the 

 stand. For these tests a temperature range of 21° to 24° F. appears 

 to be best. 



THE EXTERNAL NATURE OF FROST INJURY. 



Frost first attacks the flax seedling about 2 centimeters above the 

 ground line, where a slight ring of purple develops in the hypocotyl 

 (see Fig. 1.) When the injury is more pronounced a slight constric- 



c^:: 



r 



Fig. 1. — Frost injury to flax seedlings: 1, Entire plant normal except for slight purple 

 dis col orations on tlie hypocotyl; 2, purple band deepens; the hypocotyl is slightly con- 

 stricted ; 3, the band of purple is still darker ; a deep constriction of the hypocotyl is 

 accompanied by a swelling directly beneath ; 4, the tissue of the hypocotyl has been 

 destroyed ; the cotyledons and the root remain apparently normal one week after the 

 frost injury. 



tion occurs at this point. If the injury is severe the hypocotyl will 

 swell below the point of constriction and the purple band will be- 

 come quite dark. Unless the temperature falls below 19° F. or 

 below the flax plant's power of resistance the frost only injures 

 the roots or cotyledons indirectly. The extreme condition (see fig. 

 1) occurs when the entire hypocotyl is shriveled by the frost. The 

 cotyledons and roots may remain healthy and apparently in good 

 growing condition for a week to 10 days after this occurs, provided 

 they are protected from additional freezing. The root system is 

 at last apparently starved as a result of the girdling of the hypo- 



