69 



their vitality by plasmolysis. It was found that : 1. A stay 

 of 18 hours at temperatures down to - 10° caused no es- 

 sential injury; 2. At -15°, a considerable number of cells 

 in most of the species, and almost all in some species, 

 were destroyed; 3. At -20°, all the leaf cells of most of 

 the species were killed; 4. At -30°, they were all killed. 

 The apical cells and the cells of the dormant buds were 

 more resistant than the rest of the plant. The setae and 

 the protonemata did not differ essentially from the other 

 organs in their cold resistance. Gametophytes, previously 

 dried for 3 days in air at 20°, then made fully turgescent 

 again by immersion in water, and thereafter frozen, 

 showed a higher frost resistance than specimens not pre- 

 viously so treated. Repeated freezing at -10° to -15° 

 was found to increase the number of dead cells so as to 

 become equivalent to a single freezing at -15° to -20°. 

 Between the plants frozen in water and those frozen in 

 air there was no signiticant difference. 



Becquerel (1932d) obtained some traces of growth in 

 the moss Hypnum, desiccated in the air and immersed for 

 18 days in liquid nitrogen. He thinks that cells of the 

 stem must be particularly resistant. 



5. Higher Plants. The isolated organs or tissues of 

 higher plants, (in contrast to those of higher animals), 

 are capable of independent life. Consequently, almost all 

 the studies on death temperatures in plants are concerned 

 with the death of organs or tissues (while in animals it is 

 the organismal death which is considered primarily). 

 Since we have treated the tissues and organs previously, 

 little remains to be added here. 



As stated above, most of the plant tissues are killed at 

 a temperature slightly below^ the freezing point of their 

 sap. But, since given tissues may have various freezing 

 points, depending on their water content, their structure, 

 their age, etc., they die at different temperatures; and 

 since their death influences the death of the whole plant, 

 the lethal temperature of the latter will vary to a great 

 extent and hardly any precise statement concerning it can 

 be made. 



