86 Knud Jessen. 



has thin walls, and the protective layer described for the 

 roots is altogether wanting. 



The pith is heterogenous as in the two other species; 

 Fig. 33, F shows a somewhat tangential longitudinal section 

 through the central part of the pith of a runner. The active 

 colls, which have somewhat stronger walls in the aerial stem 

 than in the runners, form in addition to a continuous mar- 

 ginal part (of which mp in the figure is the innermost layer) 

 also slightly anastomosing rows in the central part (mc) of 

 the pith where the majority of the cells are dead. But not- 

 withstanding this structure the pith is the chief depository 

 of food-material, and during autumn is found crowded with 

 starch; in addition, the medullary rays and the pericycle in 

 the runners serve as reservoirs of starch. 



Though Rub. chamæmorus is not an evergreen yet its 

 leaves have in one respect a xeromorphous st ructure : the upper 

 epidermis has mucilaginous inner walls (Fig. 33, A), a fact 

 which has already been mentioned by Kruch (1. c). Kihl- 

 man (I. c, p. 113) was probably ignorant of this since he 

 mentions, among others, Rub. chamæmorus and Potentilla palu- 

 stris (which has also mucilaginous walls in the upper epidermis 

 of the leaf) as striking examples of bog-plants without special 

 protective devices against excessive transpiration (see p. 119). 

 Otherwise the leaf has the usual mesophyllous structure: it 

 is rather thin (170 — 190//) and the proportion between the 

 thickness of the leaf and the thickness of the palisade tissue 

 varied from about \ (Godthaab in Greenland) to ^ (Bosekop 

 in Arctic Norway). The outer wall of the upper epidermis 

 is 2 — 2.5// thick and that of the lower somewhat thinner. 

 Stomata occur only upon the lower surface; they are not 

 definitely arranged. The radial walls in the epidermis are 

 somewhat wavy, usually somewhat more so upon the lower 

 than upon the upper surface. The palisade parenchyma is 

 present in 2 — 3 layers and the spongy parenchyma is loose 

 in structure. (Fig. 33, A, R, C, D). 



