51 



than the loss protected j^lant org-ans; they can be more 

 easily maintained in the subcooled state, the ice-inocula- 

 tion being prevented by the epidermal coverings. Sub- 

 cooling to 5 degrees below the freezing point was found 

 not uncommon in these less exposed plants. 



The cold resistance of growing rye seedlings was investi- 

 gated by Zacharowa (1926). She exposed them, when 

 the rootlets were 1.5 to 2 cm. long, to various degrees of 

 cold in a freezing chamber, and occasionally determined 

 the internal temperature with a thermocouple. As a cri- 

 terion of death, the change in coloration and the facility 

 of the subsequent drying were used. The results were 

 that: 1. An air temperature of -2.9° for several hours 

 did not injure the roots ; 2. A similar exposure to - 2.9° 

 to - 3.9° resulted in the death of the cortex and the root 

 hairs ; 3. An exposure to - 5.75 for 1 hour killed the entire 

 root except 1 to 1.5 mm. at the tip; 4. The meristem of the 

 tip died after an exposure of 1 hour to - 7.8°, after which 

 time the interior of the root had reached that tempera- 

 ture ; 5. Without ice formation, exposures to - 11.1° were 

 harmless. Similar experiments on seedlings of wheat, 

 pea, corn and buckwheat showed a decreasing resistance 

 in the order given, the meristematic tip of the buckwheat 

 rootlets being killed at - 2.9°. 



We have mentioned above Mez' theory (1905) accord- 

 ing to which death occurs at a minimal temperature spe- 

 cific for each plant or plant organ. Mez contended also 

 that congelation protects the tissue against death by re- 

 leasing heat and delaying the further drop of tempera- 

 ture. His disciple, Voigtlander (1909), claims to have 

 confirmed the theory in a variety of plant tissues. The 

 specific minimal temperatures that he obtained are in 

 general low, some of them being about 10 degrees below 

 the freezing point. The extreme cooling rapidity that he 

 used in his experiments (often 10 to 15 degrees per minute 

 at 0°) makes his results the most doubtful, since it has 

 subsequently been shown that the time the material takes 

 to freeze or the time that it stays at a given sub-freezing 

 temperature is to be considered in the damage caused. 



