70 Knud Jessen. 



The epidermis of the lower surface together with the sub- 

 epidermal layer is as a rule separated from the rest of the 

 leaf-tissue by large slit-like lacunæ occurring between the 

 larger bundles (see Fig. 27, A, B). Holm (1. c), who has des- 

 cribed and figured the anatomy of leaves from Nova Zembla, 

 mentions similar lacunæ. 



According to Hooker (Fl. Bor. Amer.) the veins in the 

 leaves of D. octopetala are distinctly visible on both sides of 

 the leaf, and the upper surface acquires thereby a somewhat 

 wrinkled appearance, while in the leaves of D. integrifolia they 

 are only slightly or not at all visible, and therefore the upper 

 surface is smooth. Also as regards the anatomy there is 

 a difference in the leaves of the two forms in connection with 

 the bundles, there being in the leaf of Z). integrifolialess chloro- 

 phyll above and below the bundles than in the leaf of D. octo- 

 petala, so that the transverse section of the former shows as 

 many bright transverse bands as there are larger bundles, 

 while this appearance is far less distinct in D. octopetala. In 

 other words, the vein-parenchyma is more strongly developed 

 in D. integrifolia than in D. octopetala. 



The stomata are situated on a level with the surface 

 and occur especially in the spaces between the bundles. — 

 The majority of the hairs are unicellular, they are erect and 

 intertwining and form thereby the dense, thick layer of felt 

 (Fig. 27, A). Upon the larger bundles which project as ribs 

 the felt-hairs are partially wanting, but here large, several- 

 celled, branched hairs and several-celled glandular hairs occur 

 (Fig. 27, E). 



During winter the chlorophyll takes up in a marked 

 degree the winter-position described by Krause. In the 

 palisade-cells it gathers in the inner part, and in the cells of 

 the spongy parenchyma it retires from the parts of the walls 

 which border on the intercellular spaces. 



The two species of Dryas flower generally very abundantly; 



