161 



into the lateral walls. Beneath the epidermis a hypoderm of 

 1 — 2 layers of cells occurs, the walls of which are somewhat 

 thickened; the inner-walls m*ay also be cuticularized. 



The primary bark consists principally of large, transparent 

 cells, which are very thin-walled and devoid of chlorophyll, 

 but contain numerous crystals. The bark, on the whole, has 

 no important assimilatory tissue ; its function is rather to retain 

 water. The bark is limited internally by a distinct endodermis. 



Even during the first year, in the lowermost part of the 

 shoot, cork-formation may begin from the pericycle. With age, 

 the thin-walled bark-tissue curls up and is shed together with 

 the hypoderm and the epidermis. The formation of the cork 

 has been described by Segerstedt (14, p. 64). 



In the wood, the limits of the different annual rings can 

 be distinctly seen. The vessels are very narrow, even narrower 

 than those of the first year. The major portion of the wood 

 consists of tracheids (see 0. G. Petersen, Vedanatomi, p. 47). 

 In relation to their age, the stems attain only a slight thickness 

 in the Arctic regions; the width of the annual rings is naturally 

 inconsiderable. Kihlman (10, p. 226) has examined some stems 

 which were, on an average, 7 — 8 mm. in diameter, a few even 

 10 — 12 mm., the average width of the annual rings at the point 

 of their maximum growth-radius was 0'09 mm. Schroeter (13, 

 p. 178) cites some other measurements. Two instances from 

 Greenland should be mentioned here. A stem from East Green- 

 land, gathered by N. Hartz in 1900 (Liverpool coast, about 71° N. 

 lat.), had a maximum radius of 4 mm., of which about half, 

 the inner half, was darker coloured. The stem was about 

 35 years old, and the average width of the annual rings was 

 0-08 mm. From. Ilona in South Greenland Mrs. Lundholm has 

 sent stems of which the largest, a strongly twisted stem, had 

 a maximum radius of about 8 mm.; the average width of the 

 annual rings was 0-18 mm. The age of this stem was 35 years 

 and more; but it cannot be given exactly, as the innermost 



