262 



Eue Warming. 



The leaves of Silène acaulis are often somewhat oblique 

 in transverse section (Fig. 16, B). 



Cerastium alpinum also may have distinctly dorsiventral 

 structure (Fig. 17); this is probably connected with the posi- 

 tion of the fairly broad leaves which are found also in this 

 species. The difference between the epidermis of the dorsal 

 and the ventral surface is not so great as in the species 

 already mentioned: there are stomata on both surfaces; but 



Fig. 17. Cerastium alpinum. 

 (From Greenland and Iceland.) 

 A, Transverse section of leaf, the two kinds of hair are indicated. 

 There are two lacunae (la). B, A similar section of a leaf from a bud. 

 C, The transverse section shows that there are stomata on both sur- 

 faces, that palisade-cells are indicated in the thin-walled, lacunose 

 mesophyll, and that there is a sheath around the vascular bundle. 

 The epidermal cells are nearly of equal height on both surfaces, per- 

 haps if anything, a little higher on the ventral surface. D, From ventral 

 side. The base of a hair is seen. E, Cell with crystal of calcium oxalate. 

 F, Three portions of one and the same hair. G, Transverse section 

 of a leaf, recently cleared of snow; la, lacuna on the lower side. (Uper- 

 nivik; 7. 5. 1887; C. Ryder.) H, A glandular hair (dorsal surface). 

 J, Both leaf-surfaces have highly undulating epidermal cells and 

 stomata. K, Epidermis and stoma of ventral surface. L, Same of 

 dorsal surface. (E. W.) 



the epidermis of the ventral surface is somewhat thicker 

 than that of the dorsal (Fig. 17, K and L). Whether the 



