206 Fr. J. Mathiesen. 



and the elements of the external thickened layers are finely 

 pitted, both in the end and the lateral walls. The central 

 cylinder has a narrow medulla, surrounded by a continuous 

 ring of vessels (fig. 21, A and C). A circular, continuous 

 cambium commences its functions at a very early stage of 

 the development, forming new vessels towards the centre. 

 Outwards it forms sieve-tissue, the walls of which soon be- 

 come somewhat thickened collenchyma-like, thus resemb- 

 ling, in a transverse section, the thick-walled parenchyma of 

 the many-layered pericycle; in this, thin radial walls were 

 noticed in a few of the cells (fig. 21, C). 



The medulla becomes larger in the stem of the rosettes, 

 and the continuous ring of vessels and sieve-tissue, present 

 in the long internodes, is here interrupted by the leaf-traces 

 inserted amongst them. The sclerenchyma in the periphery 

 is here composed of much shorter elements. 



The peduncle differs from that of the previous species, 

 by having, most frequently, a somewhat less-developed 

 stereome. It also has, like the calyx, long cover-hairs 

 consisting of up to 7 thin-walled cells; in appearance they 

 are minutely warted by cutinization. Glandular hairs with 

 a 1 — 2-celled head on a 1 — 2-celled stalk, are also present. 



The leaf. The epidermis has thin, undulating lateral 

 walls ; those of the upper surface are somewhat less undulated 

 than those on the lower surface, though there is no great 

 difference (fig. 22, A and B). Glandular hairs, with a structure 

 similar to those on the peduncle, are evenly distributed all 

 over the surface. The cover-hairs are most numerous near 

 the margin of the leaf; they are hyaline, 2 — 5 cells long, 

 more or less thick-walled, and their surface is closely and 

 minutely warted by cutinization ; on the lamina of the leaf 

 these hairs are few, and the short (2-celled) form is most 

 frequent. The variety arctica Knuth (Herschell Island) is, 

 however, distinguished by being closely covered with long 





