270 Eue Warming. 



A. peculiar lacuna is found in the leaves of many of 

 the species. It is caused by the sub -epidermal cell-layer of 

 the dorsal surface separating, for a shorter or longer dis- 

 tance, from the mesophyll lying internal to these cell-layers. 

 The cells bordering on the lacuna are rounded off inwardly 

 towards it. I have already in 1884 pointed them out in 

 Silène acaulis, Arabis petræa, Dryas octopetala, Draba incana 

 and Saxifraga oppositifolia, and they are found figured by 

 Th. Holm in Stellaria humifusa (PI. X, Figs. 2 and 8), Salix 

 reticulata (PI. XII, Fig. 12) and species of Saxifraga (PI. X, 

 Figs. 3 and 5). See also Börgesen 

 1. c. In Helgi Jonsson (1895, p. 291) 

 we find the accompanying figure (Fig. 

 24), which shows a section through a 

 bud of Silène acaulis. In the figures 

 given above these lacunæ are shown 



in Fig. 14, A and B (Viscaria alpina); Fi S- 24. Silène acaulis. 

 _,. _ , _ .,., . 7 . (From Iceland; Helgi 



Fig. 17, A, B and G {Cerastium alpi- Jônssoh>) Section 



num); Fig. 22 {Cerastium nigrescens). through a bud. In each 



I have also observed them in Dian- leaf three vascular 



bundles are seen; k, cells 

 thus superbus. I cannot imagine that w jt n crystals. On the 



they are of any other use than as a dorsal side of the leaves 



, , . , , air-containing lacunæ 



protection against cold, since they ,,, 



r & J (I) are seen. 



must be bad heat-conductors. 



Crystals. With regard to the mesophyll, it may 

 further be remarked, that in many species cells are found 

 with crystals of calcium oxalate. They are situated in that 

 layer of the leaves, in which the vascular strands are found, 

 or at any rate, in close proximity to them. They are no 

 doubt found in all the species. They are figured in Fig. 14, 

 A and B; Fig. 15, A ; Fig. 16, E; Fig. 17, E; Fig. 20, B\ 

 Fig. 23, A, C and Z), and also in B where they are indicated 

 by dots; their position is similarly indicated in Fig. 21, C 



