507 



varieties, which had remained out of doors during this time, wqre found 

 absolutely uninjured by rapid thawing. The killed plants had been made 

 more tender by the retention in the greenhouse. Kornicke^ also comes to 

 the same conclusion in his observations that French varieties of grain, on 

 an average, more often fall victim to frost than the varieties which originate 

 from the provinces of Prussia and Silesia. The longer cultivation in a 

 country with a mild winter has made the varieties less resistant. 



Under otherwise equal conditions, Haberlandt- found that the seed- 

 lings of field beans, field vetches, carrots, barley, peas, rape, poppy, red 

 clover, alfalfa and flax, grown in a greenhouse at 20 to 24 degrees C, were 

 frozen to death even at 6 degrees C. below zero; rye and wheat at 10 to 12 

 degrees below zero, while plants of the same variety, grown at the same time 

 in a cold frame, died only at 9 to 12 degrees below zero, and rye and wheat 

 only at 20 to 24 degrees C. below zero. 



The plants and parts of plants whose growth has entered upon a dor- 

 mant period, on an average, suffer less and it is well known that dried seeds 

 survive uninjured many degrees below freezing, while they go to pieces in 

 a germinating stage with much slighter frost. 



During the vegetative development the susceptibility to frost changes 

 with the different phases of the cell life. 



In unfolding apple blossom buds, which had suffered from a spring 

 frost, I found the youngest cells, richest in protoplasm, were not injured, 

 but those somewhat older, in an energetic stage of elongation, had turned 

 brown, while the still older parenchyma cells in turn seemed healthy. 



The cases, cited up to the present, show clearly the difficulty in giving 

 definite thermometer degrees as fixed minimum and maximum boundaries 

 for the developmental capacity of any species. Each plant is certainly con- 

 nected with a definite scale of heat, but the boundary and optimum values 

 may change, to a certain extent, according to the combination of the remain- 

 ing vegetative factors, momentarily present, which earlier contributed to 

 the construction of the individual. 



On the other hand, it must be maintained that in spite of all the vege^ 

 tative conditions, which increase susceptibility to frost, many plants (espe- 

 cially numerous algae, mosses and Alpine plants) never show any damage 

 from frost. We will have to explain this phenomenon by the fact that the 

 need of heat of such plants is so small that the greatest reduction in tem- 

 perature is generally insufficient to produce those molecular changes in 

 the tissues which would prevent a reassumption of the normal life functions. 



Theories as to the Nature of Frost Action. 

 After discussing the circumstances which modify the freezing of plant 

 parts, we will consider the theories which have been formed as to the nature 

 of frost action. 



1 Annalen d. Landw.; cit. in Neue landw. Zeitung v. Fiihling 1871, Part 8, 

 p. 586 ff. 



- Haberlandt, tJber die Widerstandsfahigkeit verschiedener Saaten. Wissensch. 

 praktisch. Untersucliungen, Vol. I. 



