Oct. 14, 1918 Hardening Process in Plants 93 



much more easily injured than those watered with N/io sodium chlorid 

 or N/io, sodium bicarbonate. The latter solutions, like exposure to low 

 temperature, retard growth. Cultures watered with all the foregoing 

 N/io solutions showed a lower freezing point than controls grown in poor 

 clay soil. An increase in the cell-sap concentration and lowering of the 

 freezing point can also be produced by growing plants in dry soil. The 

 increased resistance of cabbage watered with salt solutions indicates 

 that it is not the increase of concentration of just the salts of the cell 

 sap on freezing that causes injury. 



METHOD OF TESTING HARDINESS 



An electrically-controlled constant-temperature room which can be 

 maintained at any desired temperature withi^ a few tenths of a degree 

 was used in determinijig the hardiness of the plants under test. This 

 compartment was cooled by the direct expansion of ammonia. The 

 temperature was kept the same throughout by keeping the air stirred 

 vigorously with an electric fan. Plants were exposed to a temperature 

 just sufficient to freeze them, and they were then observed at intervals 

 for the appearance of injury. Comparative tests were run together and 

 for the same length of time. 



TIME AND TEMPERATURE FACTORS IN ACQUIRING OR LOSING 



HARDINESS 



The time necessary for plants to become hardened was determined 

 by placing cabbage in dark chambers at constant temperatures of 3°C., 

 and 5°, using 18° and 25° as controls. Exposure for 24 hours to 3° 

 was found to produce a slightly jncreased hardiness as judged by the 

 extent of the injury. After 5 days' exposure to 3° the cabbages were 

 not injured by 30 minutes' exposure to —3°, although frozen stiff. The 

 control plants were killed throughout. On placing such hardened plants 

 in the greenhouse at room temperature and at a constant temperature 

 of 18° in the dark, it was found that the hardiness was lost in the green- 

 house in about the time taken to acquire it, while the hardiness lasted 

 a few days longer at 18°. By alternating cabbage plants between 3° 

 and 25° it was shown that the hardening process is an accommodation 

 brought about not by changes of temperature such as occur in the natural 

 hardening of plants, but by low temperature. From the above state- 

 ments it is seen that plants acquire and lose hardiness rather rapidly. 

 Under natural conditions the hardiness acquired in one night of low 

 temperature may be lost during the succeeding warm day, and there 

 is accumulative effect only when the average temperature is low. 



The maturity of the tissue is a factor of great importance in frost 

 resistance. Young leaves of cabbage are more easily injured than old 

 leaves. During hardeping these young leaves become resistant, indi- 



