Rosaceæ. 119 



cially in Dryas, Pot. nivea, Alch. alpina and others; (b) by 

 a revolute leaf-edge, which is found especially in several 

 Dryas-îorms and in Pot. pulchella f. humilis; (c) by sunken 

 stomata, which are found in Alchimilla, Pot. palustris, 

 Pot. tridentata, and Sibbaldia, while in several of the spe- 

 cies, which have a dense hairy covering upon the under 

 side of the leaf, the stomata there project slightly above 

 the leaf-surface. For the rest, stomata occur on both 

 surfaces of the leaf in almost equal number in Sibbaldia 

 only; of the other species, stomata are entirely wanting 

 upon the upper side in the following: Alch. alpina, Alch. 

 færøensis, Dryas, Pot. Vahliana, Rubus and S orb us. Lastly, 

 transpiration must also be lessened by (d) a thick outer wall 

 in the epidermis, and here Dryas and especially Pot. tri- 

 dentata may be mentioned; and (e) by the mucilaginous 

 covering of the inner wall in the upper epidermis (pectin- 

 mucilage). This is found in Dryas, Pot. maculata, Pot. 

 nivea, Pot. palustris, Pot. pulchella and Pot. tridentata, Sib- 

 baldia and S orb us. Very often not all the cells are muci- 

 laginous; in Pot. tridentata besides the inner wall the outer 

 wall is also somewhat mucilaginous and in Pot. maculata 

 a few mucus-cells were also found in the lower epidermis. 

 The importance which should at any rate be attached to 

 a cellulose-mucilaginous covering of the inner walls of a 

 leaf-epidermis is among others mentioned by Volkens 1 , 

 who is of the opinion that such mucilage in the cells does 

 not function as a water-reservoir, but checks transpira- 

 tion in the same manner as does a gelatinous layer which 

 is spread over a slightly evaporating surface of water 

 (compare also Warming, 1887). 



The mesophyll in the leaves of the species which have 

 been investigated has a very ordinary structure, and it 

 shows only to a slight degree such adaptations to 

 1 Die Flora der Aegypt.-Arab. Wüste. Berlin, 1887, pp. 43 — 45. 



