347 



The leafis somewhat succulent; the upper surface is glabrous 

 and the lower somewhat hairy with long (as much as 2 mm.), flaccid 

 hairs. The structure of the leaf is decidedly dorsiventral; in 

 the majority of the leaves three distinct palisade-layers (Fig. 

 7, A) were found, the height of the cells of which was about 

 three times the breadth : the palisade-cells were slightly inclined 

 towards the leaf-apices. There is an abrupt transition from 

 the palisade to the spongy parenchyma which is extremely 

 loose in structure and consists of abundantly branching cells. 



^ - C 



Fig. 7. R. glaciaUs. 



A. Transverse section of leaf (Jan Mayen). B. Surface section of the uppermost layer of 



the palisade-tissue (Tromsö). C, Surface section of spongy tissue (Tromsö). ep. Epidermis 



pal, palisade-cells, i, intercellular spaces {A, about ^k; B, C, ""/i). 



The vascular bundles are surrounded by a sheath containing 

 chlorophyll, and are entirely enclosed by the mesophyll; even 

 above the principal vein three palisade-layers occur. The 

 epidermis of both the upper and lower surface contains chlo- 

 rophyll; the outer walls are slightly thickened. The stomata 

 are situated on a level with the epidermis (Fig. 8, I)). The 

 cells of the upper epidermis have almost straight walls while 

 those of the lower epidermis are somewhat undulating (Fig. 8, 

 A, B). There are about three times as many stomata upon 

 the upper as upon the lower surface. Th. Resvoll found 78 



