363 



longitudinal axes have no fixed direction. Nf.stler found 15 

 stomata per sq. mm. upon the upper surface and 98 per sq. 

 mm. upon the lower surface (I.e. p. 294); he does not give 

 localities for his specimens. The specimens from Denmark, 

 Iceland and Greenland which have been investigated have per 

 sq. mm. upon the upper and lower surface, from to about 

 85, from to about 90, and from about 30 to about 150, respec- 

 tively. There is great variability in the number of the stomata 

 in the leaves from Iceland and also in those from Greenland. 



The Danish and Icelandic specimens have one palisade- 

 layer those from Greenland have generally two layers (Fig. 

 15,^); everywhere the palisade-layers constitute from ^in to 

 ^/2 of the thickness of the mesophyll, and the quota of the 

 individual cell is generally from ^Is to ^1-2. The spongy paren- 

 chyma in the upper part is distinctly more compact than in 

 the lower, and in the Danish and Icelandic specimens it consists 

 of longer and more abundantly branching cells than in those 

 from Greenland (Fig. Iß, Z>). 



Upon the upper surface the lamina is depressed like a 

 gutter over the larger veins, and the latter are accompanied 

 upon the upper and lower surface by stereom, which reaches 

 to the epidermis. The smaller veins are surrounded only by 

 a sheath which contains chlorophyll. 



At tlie leaf-apices there is an epithema the pores of which 

 open upon an oblique plane. The cells are often branched in 

 several planes and the walls are undulating, as in R. nivalis 

 (Fig. 15,5). 



The leaf- s talk is cordate in transverse section. The bundles 

 number 5—15, alternately large and small; in structure they 

 are similar to those of the stem from the same place, but no 

 inner arch of stereom occurs. Interfascicular stereom is absent. 

 The structure of the cortex and epidermis is similar to that of 

 those of the stem. The epidermis contains chlorophyll. The pith 

 is broken down gradually. 



