Killing of Plant Tissue by Low Temperature 223 



EFFECT UPON HARDINESS OF PREVIOUS EXPOSURE TO 



TEMPERATURE SLIGHTLY ABOVE THE 



KILLING TEMPERATURE. 



Closely related to the questions of the relation of maturity 

 to hardiness, and the relation of the rate of growth to hardiness, 

 is the relation of exposure to low temperature, above that at which 

 the plants may kill, to hardiness. In fact these problems are so 

 intertwined that it is difhcult, if not impossible, to separate them. 

 Thus in case of the greater hardiness of roots kept in cold storage as 

 compared with those kept in warmer places, unquestionably maturity 

 plays a large part but it is not impossible that exposure to cold also 

 had its effect. However, by referring to Table 14 it will be seen that 

 there was little difference among the killing of roots kept in cold 

 storage at a temperature of 31 F., those kept frozen up in the soil, and 

 those kept at a higher temperature in our basement storage room. 

 The relation of exposure to cold to hardiness of winter buds and 

 wood may also be confused with the rate of temperature fall. This 

 problem will be discussed for peach buds in a later part of this paper. 

 In case of some succulent plants, however, the temperature at which 

 they grew must exert an influence on their hardiness. Thus when 

 cabbage, kale and lettuce were grown out of doors in late autumn 

 or early winter, their hardiness was increased over those grown in 

 the greenhouse more than can be explained by the increased sap 

 density. At least their hardiness was increased more than the same 

 increase in sap density brought about by any other means would in- 

 crease it. When these plants were grown out of doors in early spring 

 or late autumn, it required a much lower temperature to kill them 

 than was required in June or July. On the other hand, plants like 

 tomatoes or cowpeas are influenced in hardiness but slightly by the 

 temperature at which they grow. 



Goeppert^ found little increase in hardiness due to continuous 

 exposure to low temperature with tender tropical plants, but there 

 was such adaptation with more resistant plants. Apelt- found that 

 potatoes kept at a temperature of 22.5 C. four to seven weeks were 

 killed at -2.14 C. while potatoes kept at C. for the same length 

 of time killed at -3.08 C. Rein' found that a rather large list of 

 very tender plants kept at a temperature of 8 (' were not apprccia- 



'Uebor die WlirmeentwIckcIuriR In dom ITlanzoii. otc, book. 18.10. (Ulhl. No. J J). 

 f 'ohn's ntltraj?o z. Hlol. d. ITl. Vol. 0, 1007, p. 215. (BIbl. No. 3). 

 Zolts. f. Naturw. Vol. HO. 1908. p. 1. (lUI)l. No. 02^. 



