Chap. I] CHARACTERS OF THE ARCTIC CLIMATE 



677 



its base counted 544 annual rings, each averaging 0-19 mm. in thickness. 

 The counting the rings in shrubs had to be done under the microscope ; 

 the annual ring in a radial direction being often (Empetrum, Vaccinium 

 uliginosum) composed solely of a vessel and a wood-parenchyma cell. 

 These details refer to the sub-arctic belt. Data are not available in 

 regard to the age and the growth in thickness of the dwarf-shrubs of the 

 tundras. 



The problem as to what cause is responsible for the diminution of the 



Fig. 402. Left hand : Vaccinium uliginosum. Right hand: Vaccinium Vitis-Idaea from the 

 island of Vaigach. Natural size. After Kjellman. 



intensity of growth cannot be solved out of hand, for, as has been shown 

 before, arctic vegetation is exposed to three factors which are hostile to 

 growth, the low summer-temperature, the coldness of the soil acting 

 physiologically like drought, and continuous illumination. 



The last of these three factors, the length of the polar day, as Kjellman's 

 experiments show, is not of marked importance in the shade, owing probably 



