CiiAr. HI 



HEAT 



39 



want of water, and not of tlic low icinpcratnrc. H. Mayr ' states quite 

 correctly : ' One is surprised to find what low temperatures a wood}^ plant 

 sheltered from the wind can endure, provided the air is fairly moist or 

 transpiration natural!)- restricted, as is the case under dense forest, in an 

 insular climate, in narrow mountain- or river-valleys ; on the other hand, 

 most plants are more sen- 

 sitive to winter frost the 

 drier the air is ; nine-tenths 

 ofthe cases reported of dam- 

 age by frost during winter 

 are really phenomena due 

 to desiccation, owing to the 

 interference with or stop- 

 page of the movement of 

 water b\- frost. Thus also 

 perhaps the apparent con- 

 tradiction may be ex- 

 plained, that many plants 

 have been termed ' hardy ' 

 in a notoriously colder 

 climate, yet are considered 

 ' tender ' in a notoriously 

 milder one ; probably the 

 plants in the former lo- 

 calities were growing in 

 moister air or were shel- 

 tered from evaporation, 

 while the tender plants of 

 the warmer climate had 

 to contend against both 

 drought and frost.' 



Irrespective of seeds and 

 spores, it is not yet known 

 which species of plants are 

 least sensitive to frost and 

 what degrees of cold they can support without injury. But certain observa- 

 tions relating to arctic plants prove that the degree of cold may be 

 extraordinarily low. Thus Kjellman, who as botanist accompanied the 

 'Vega' expedition, makes the following .statement regarding Cochlearia 

 fenestrata (Fig. 37): 



'There are few places on earth where tlie winters are so severe as the spot on 

 which the "Vega" expedition passed the winter. The cold was very persistent, and 



* H. Mayr, op. cit. p. 36S. 



Fig, 37. Cochlearia fenestrata from Pittlekaj. A plant 

 that hibernated in llower and continued its development 

 after winter was over. Natural size. After^ Kjellman. 



