120 Knud Jessen. 



external conditions as are evident* in the structure and 

 hairy covering of the epidermis of at least several 

 species. The palisade-tissue in almost all the species 

 consists of 2 — 3 layers of cells; in the Alchimilla- 

 species only, there is usually a single layer of very long 

 palisade-cells. In these the relation between length and 

 breadth is as 4 or 5 to 1; the same relation is also found 

 in the upper layer of palisade-cells in Sorbiis; but in the 

 remaining species the pahsade-cells are only 2 — 3 times 

 as long as they are broad ; they are often somewhat barrel- 

 shaped. In all the species the relation between the thick- 

 ness of the leaves and the thickness of the palisade-tissue 

 is about ^; as regards this point some variations have 

 been found to occur in various species, but such special 

 records of the nature of the habitats as are necessary 

 in order to be able to understand such variations have 

 unfortunately not been at my disposal. In the majority 

 of the species there are large intercellular spaces in the 

 palisade-tissue. In Pot. tridentata these are however small, 

 and as regards the mesophyll, this species has the most 

 xeromorphic leaf, while the least xeromorphic is that 

 found in Rubus saxatilis and R. arcticus. 



The transpiration-parenchyma is throughout of loose 

 structure and consists of short, branched cells. Large 

 slit-like lacunae occur commonly within the subepidermal 

 layer on the lower surface of the leaf (e. g. in Dryas. 

 Pot. pulchella, Alch. alpina). 



The species may be divided into two groups accord- 

 ing to the anatomy of the leaves; viz. into those species 

 which have a mesomorphic leaf-structure and those which 

 have more or less well-marked xeromorphic leaves. To 

 the first group belong Rubus saxatilis and Rub. arcticus, 

 Alch. vulgaris and Alch. f ærøens is, and perhaps also Pot. 

 emarginata and Sorbus americana. The remaining species 



