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 may 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 Sesersteor (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 O. 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. Kırıman (10, p. 226) has examined some stems 
which were, on an average, 7—8 mm. in diameter, a few even 
10—12mm., the average width of the annual rings at the point 
of their maximum growth-radius was 0°09 mm. Scuroerer (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 
55 years old, and the average width of the annual rings was 
0°08 mm. From Ilona in South Greenland Mrs. Lusp#orm 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 
XXXVI. 11 
