403 



44, Б, С) and stomata occur in great numbers; Resvoll found 

 136 per sq. mm. The stomata are on a level with the surface. 

 The epidermal cells above the larger nerves are elongated and 

 have less undulating walls. The palisade-cells usually occur in 

 two layers or else in only one, and are at most twice as long as 

 they are thick; the layers, taken as a whole, constitute scarcely 

 one-half of the thickness of the mesophyil. The individual cells 

 are often irregular. The spongy parenchyma is loose in structure, 



Fig. 44. B. lapponicus. 



A, Transverse section of leaf (Sassenbay in Spitzbergen ; ""Л) ; t, brown-coloured cells 



probably containing tannin. B, The epidermis of the upper surface (Greenland; ^^li)\ 



t, as in A- C, The epidermis of the lower surface (Greenland; '"Vi); t, as in A. D, Sur- 

 face section of palisade-cells (Greenland; ^'U). 



its cells are slightly branched or polygonal (Fig 45, Л), but the 

 subepidermal layer consists of abundantly branching cells. There 

 is a gradual transition from the form of the tissue of the upper 

 to that of the tissue of the lower surface. The bundles are 

 without stereom, but the larger ones among them upon the 

 upper and lower surface are accompanied by elongated cells. 

 Around each bundle there is a sheath containing chlorophyll. 



