Rosaceæ. 4;) 



The outer wall of the epidermis of both leaf-surfaces was 

 3 — 4 p> thick. In the epidermis of the upper surface a muci- 

 laginous inner wall was found in a great many of the cells. 

 and in specimens from a single locality in Greenland (Kang- 

 erdlugsuatsiak) a few cells in the lower epidermis had also 

 mucilaginous inner walls. Such cells were often larger than 

 the other cortical cells (Fig. 18, A, B, C; cf. p. 57). The 

 stomata are far more numerous upon the lower than upon 

 the upper surface; they are situated on a level with the sur- 

 face. 



The palisade parenchyma consist of 2 — 3 layers of cells 

 and contains large intercellular spaces. The spongy paren- 

 chyma is often composed of short, stellately branched cells 

 and is more or less loose in structure. In the subepidermal 

 layer of the present species more branched cells occur than 

 in the other Potentillas mentioned in this work. 



As may be seen in the two figures of the transverse 

 section of the leaf, this varies considerably in thickness, from 

 160 /i to 240/;. In the thick leaf from Spitzbergen Fig. 18, A, 

 which was somewhat fleshy and almost smooth there were 

 three layers of well-marked palisade-cells, and the propor- 

 tion between the leaf-thickness and the thickness of the pali- 

 sade parenchyma was about -j-. The spongy parenchyma was 

 specially loose in structure owing to its large intercellular 

 spaces. In Fig. 18, B are shown 2 layers of less well-marked 

 palisade-cells and the proportion mentioned above was in 

 this leaf about -j-; here the spongy parenchyma was somewhat 

 more compact. Unfortunately nothing is known with regard 

 to the conditions in which the different plants were living. 

 It is however of some interest that this difference in 

 the leaf-structure in a single species is also found, though 

 in a more marked degree, in other Arctic plants, viz. among 

 the land-species of Ranunculus. The thin leaf of Pot. maculata 

 (Fig. 18, B) resembles in the structure of the mesophyll the type 



