353 



walls of the guard-cells of the stomata often have horn-shaped 

 thickenings which project into the lumen of the neighbouring cell 

 like incomplete walls, (see R.acer, Fig. 16, C). The stomata are'on 



Fig. 11. R. affinis* Wilanderi. 



A, Epidermis of the upper surface of the leaf. B. Epidermis of the lower surface of the 

 leaf. C. Surface section of spongy parenchyma (A, B and C, ^"/i)- 



a level with the surface; according to Resvoll their number aver- 

 ages 29 per sq. mm. upon the upper surface, and 76 per sq. mm. 

 upon the lower surface; my computation gave a similar numerical 

 result. Nestler (1. c, p. 294) found 38 per sq. mm. upon the 



Fig. 12. R. affinis. 



A, Transverse section of leaf (Cape Thordsen ; ""/i). B, Longitudinal section of leaf-apex 



(Gåselandet; -^i); tra cheids are scattered in the epithema; o. upper surface; u, lower 



surface. C, Surface section of pahsade-tissue (-w/i). 



upper, and 69 per sq. mm. upon the lower surface. They are 

 fairly evenly distributed, but they are absent from the larger 

 veins where the cells of the epidermis are elongated and more 

 or less straight-walled. The above-mentioned differences appeared 

 in the palisade-tissue; some of the investigated specimens 

 from Cape Thordsen (Fig. 12, A) had two palisade- layers con- 



