260 



Eue Warming. 



a large lacuna (la Fig. 14, A, B and D); then again in 

 the height of the epidermis of both surfaces (B, C and Z>); 

 in the form of the epidermal cells (E and F); in the form 

 of the cells of the mesophyll (G and H)\ and lastly in the 

 height of the cells which form the sheaths of the vascular 

 bundles, since these cells are higher on the dorsal than on 

 the ventral side (B). 



Similar, but not quite so decided dorsiventrality is 



m 



Fig. 15. Melandrium involucratum. 

 (From West Greenland; 30. 7. 1887; C. Ryder.) 

 A, Transverse section of leaf; the ventral side (v) has higher epi- 

 dermis than has the dorsal side; compare the somewhat more highly 

 magnified B and C. D, Hair. E, Epidermis of ventral surface and F, 

 of dorsal surface. (E. W.) 



found in Melandrium involucratum (Fig. 15, A, B and C, as 

 also E and F). 



Also in the much more narrow-leaved species Silène 

 acaulis decided dorsiventrality is found; this is probably 

 connected with the fact that the leaves are more or less 

 horizontally placed (Fig. 3). Here also the dorsiventrality is 

 expressed in the structure of the mesophyll (Fig. 16, ^4); in 



