•M\2 



Tlio tliickiH'ss (»f Ulf leal' iii Daiiisli atul (Ircciilaiid speci- 

 mens was ;U)Ü— 340/<, ill lliuse from Icoiaiid IGO— 225/>«. 



Two kinds ot" luiirs occur upon liie leaves; tlip more 

 common {V\'^. 16, O) are like those of the stem, and some of 

 Ihem contain crystals of a suhstance, of which the base is 

 caiciiim-liydratc ; the others (Kii.'. 15,6') only occur alonj; the 

 veins of the iip|)er side and are club-shaped, tliin-wallcd, and 

 rich in coiiteiils (compare INksti.eu, p. 291, Fif,^ 18). 



The epidermis of the 

 leaves from northern 

 localities, but Denmark 

 can scarcely be inclu- 

 ded, contains chloro- 

 phyll; otherwise it is 

 nearly alike in the 

 Danish and Icelandic 

 specimens. In the 

 latter the radial walls 

 of the cells of the 

 upper surface are less 

 Fig. 16. R. acer. undulating than those 



A, Epidermis of the upper surface of the leaf (Ice- Qf \^[^q loWCr SUrface, 

 land). B, Epidermis of the lower surface of the leaf 



(Iceland). C, Epidermis of the lower surface of the which haS StrOUgly 

 leaf (Donmark). D. Surface section of spongy paren- ^ i »• || 



chyma of leaf from Greenland, (A, B, C and D '«"li). UndUlatmg WallS espe- 



cially in the Danish 

 specimens. In these latter incomplete walls often proceed from 

 the radial walls, especially from the guard-cells of the stomata 

 {d. R.affinis; and see Fig. 16, C) ; there are found only mere in- 

 dications of such a condition in the leaves from other localities. 

 In the Greenland specimens the radial walls of the epidermis of 

 the lower surface are almost similar to those of the upper 

 surface in the Iceland specimens, while those of the upper 

 surface are slightly curved. The stomata are situated on a 

 level with the surface, are fairly evenly distributed and their 



