5o6 



groups and, as frequently observed, the cells of the epidermis can be raised 

 from the underlying parenchyma, while a rupturing of the individual cells, 

 due to the freezing of the water, has as yet been rarely observed. There- 

 fore, the theory formerly generally expressed and now frequently held by 

 practical growers, that the frost kills the plants by rupturing the cells, has 

 been given up as untenable. 



In the same plant the same degree of cold can be uninjurious at one 

 time and fatal at another, according to whether thawing takes place gradu- 

 ally or suddenly. This latter case may be observed if frozen leaves or 

 herbaceous stems of soft-leaved plants are held in the warm hand. The 

 places of contact frequently become black after thawing and die. We will 

 return to these phenomena in the following. 



Rapid and violent changes in temperature within a scale above zero 

 degrees C. also did not remain ineffective. Sachs^ has proved that each 

 rapidly appearing rise or fall of temperature is followed by an increase or 

 decrease of the rate of growth. While de Vries could observe no disad- 

 vantageous results from such fluctuations, I found a dropping of the leaves 

 in the most extreme cases, especially if the fluctuations took place in a scale 

 which began several degrees under zero and rose considerably above zero. 

 The same plants in fact die if a change of temperature is repeated several 

 times within a short period, as shown by Goppert's experiments"-. Milk- 

 weed (Euphorbia Lathyris) was taken from a temperature of 4 degrees C. 

 below zero into a room at 18 degrees C. The leaves, bent backward and 

 against the stem, because of frost, were raised at once and assumed their 

 normal horizontal position. The same process was found in a repetition of 

 the experiments, which took place five times within two days. On the third 

 day the raising of the leaves began to be less and after eight days the plants 

 were dead. Here, therefore, the cause of death was the re,peated action of 

 slighter degrees of frost, while out of doors, and uncovered, they could 

 qndure 10 to 12 degrees below zero for some time without bad effects. The 

 same experiments gave similar results with many other plants. This ex- 

 plains the observation in general practice that slighter degrees of cold in 

 many places kill plants which, at the same time, in a place with more con- 

 stant temperature, can endure much greater cold. 



Goppert also calls attention to another fact which may serve to explain 

 the frequent contradictions in regard to the fatal action of slighter degrees 

 of frost in those plants which usually defy greater cold. It depends espe- 

 cially upon the conditions under which the plant may find itself at the time, 

 as shown by the experiment with the common groundsel {Senecio vulgaris) 

 and meadow grass {Poa annua). Pots of these plants, which had already 

 withstood a temperature of 9 degrees below zero, v^ere placed for 15 days in 

 a greenhouse at 12 to 18 degrees C. above zero. After this time they froze at 

 a temperature of 7 degrees below zero, while other examples of the same 



Lehrbuch d. Bot., 3d ed., p. 638. 



tJber die Warmeentwicklung in den Pflanzen usw. 1830, p. 62. 



