148 



a palisade-like transition-zone, a spongy parenchyma with large 

 lacunae, and lastly, just above the epidermis of the lower sur- 

 face, a zone of from one to a few layers of elongated un- 

 branched parenchymatous cells. The cells of the spongy paren- 

 chyma are much branched — but their length is greatest in 

 the direction of the longitudinal axis of the leaf — and they 

 are exceedingly thick-walled. The intercellular spaces in the 

 spongy parenchyma take up considerable room. As gradually 

 their number and size diminish towards the palisade-tissue, 

 the walls become less thick, until in the palisadt-tissue the 

 cells are not characterized by any special thickness of wall 

 although always proportionably thick when compared with those 

 of the deciduous leaves. The elongated, unbranched cells 

 above the epidermis of the lower surface are somewhat less 

 thick-walled than those of the spongy parenchyma, neither have 

 they among them the large intercellular spaces which occur 

 in the spongy parenchyma, and they doubtless form a kind of 

 protective tissue for the spongy parenchyma. The cells of the 

 spongy parenchyma have well-marked pores. None of the 

 Ericaceae I have investigated, not even forms with highly 

 developed intercellular spaces such as Ledum and Arctostaphylos 

 JJva-ursi, have such thickened walls in the cells of their 

 spongy parenchyma as has Diapensia lapponica. 



Stereom is not developed around the vascular bundles. 



In the palisade-cells of specimens from Hekla Havn (East 

 Greenland, Nov. 1891, N. Hartz) I have observed the Kraus 

 winter-condition of the plasma. I also found in the leaves of 

 these specimens, and especially in their palisade-cells, an 

 abundance of tannin; on the other hand specimens from Godt- 

 haab in the month of June did not give particularly good reac- 

 tion for tannin. Oily substances, so far as they could distinctly 

 be demonstrated to occur besides tannin (Sudan Hi, Osmic 

 Acid [brown colour]), appear to be present in somewhat varying 

 quantities at the diflerent seasons of the year. Some summer- 



