424 



riulimeiitary. On the inner side of the vessels, which are ar- 

 ranged as a \ or in 2 lateral masses separated by tliin-walled 

 parenchyma, some small-ceiled parencliyma occurs, easily distin- 

 guishable from the large-celled pith ; the latter was broken down 

 in the older stems, leaving a pith-cavity. 



The leaf is less hydrophilous in striuture than are those 

 of the previous species. Hairs are absent. The epidermis of 

 the upper surface is about 28/^ thick, while tiiat of the lower 

 surface is about 17/^, and it is almost entirely devoid of 

 stomata; its radial walls are somewhat undulating, and the cells 

 are homogeneous. The cells of the lower surface, on the other 

 hand, vary more in form; most frequently the radial walls 

 are undulating, almost like those of the upper surface, but 

 they may also resemble those shown in Fig. 55. C, and 

 the difference does not appear to have any connection with 

 the number of the stomata; the number varies greatly, and 

 numbers between 280 and 42Ü per sq. mm. are the most 

 common. The stomata are on a level with the surface; their 

 direction is not fixed; they occur only between the larger veins 

 and not above them. The epidermis contains a little chloro- 

 phyll. The radiais walls of the lower surface are furnished 

 with pores. 



The bundles are situated nearer to the lower surface, 

 the larger ones, both upon the upper and lower surface, are 

 accompanied by stereom which extends to the epidermis of 

 both surfaces; the smaller bundles either have a little bast 

 upon their under sides or are only provided with a sheath in 

 which chlorophyll occurs, especially along the outer walls. 



The palisade-cells are in 2 — 3 compact layers which con- 

 stitute about V2 or -is of the mesophyll; the quota of the 

 palisade-cells is about ^/a; there is a somewhat abrupt transition 

 from the palisade to the spongy parenchyma, the rather small 

 cells of which were somewhat branched and loosely connected 

 (Fig. bbA.D). 



The specimens from Iceland differed somewhat from the 



