379 



respectively per unit of surface, Resvoll (1. с p, 357) 25 per 

 sq. mm. upon the upper surface as against 58 per sq. mm. 

 upon the lower surface; according to my computation there 

 are three times as many upon the lower, as upon the upper 

 surface. The stomata are distributed fairly evenly over the 

 whole surface, but they are 

 less numerous above the 

 veins and just at the margin 

 of the leaf; their direction 

 on the leaf-lobes is almost 

 parallel with that of the 

 main vein, but less regu- 

 larly so upon the rest of 

 the surface. The bundles 

 are without stereom, only 

 the larger ones upon tlie 

 upper and lower surface are 

 accompanied by a few elon- 

 gated, cylindrical cells with- 

 out thickened walls ; each 

 bundle is accompanied by 

 a sheath in which the chlo- 

 rophyll appears especially 



Fig. 27. R. nivalis. 



,. • 1 il L II Л, Loiieitudinal section of leaf-ape.\ wiUi epi- 



to he agamSt the outer wall, .^^ша (Spitzbergen; «ОД); ,,, openings in the 



From one to two mli- epidermis probably answering to the collapsed 



guard- cells of the water-stomata. B, Portion 

 Sade-layerS occur which are of the area with water-stomata (Disco; -'^"/i); 



w, the apertures of the water-stomata. 



slightly inclined towards the 



leaf-apex; the individual cells are about twice as long as they 

 are thick and are often barrel-shaped and somewhat irregular; 

 the palisade-layers constitute about one-half of the mesophyll. 

 There is a gradual transition from the palisade-to the spongy 

 tissue with numerous large intercellular spaces (Fig. 26, A)\ 

 the cells are branched, which is best seen in surface section 

 (Fig, 26 E). There often occurs a slit-like lacuna between the 



