Caryophyllaceæ. 271 



and D. I have seen them also in Silène venosa^ Honckenya 

 and Minuartia hirta (accompanying the three vascular bund- 

 les), and several authors mention them for instance Vesque 

 (1883, pp. 130 and 133), Rüssel and Heinricher (see 

 PI, 30, Silène inflata). There may be a difference in indivi- 

 duals of the same species; in specimens of Silène acaulis 

 from Finse in Norway there were many crystals of calcium 

 oxalate, more than in the Icelandic individuals. Whether 

 this difference be due to the season of the year, or to the 

 habitat, 1 am not prepared to say. 



Mechanical tissue I have not found except in certain 

 cases, and then within the sheath around the vascular 

 bundle mentioned below, for instance, on the ventral side 

 in Silène acaulis (Fig. 16, A and D), on the dorsal side of 

 them in Minuartia rerna (Fig. 18, B, D, E and F), M. groen- 

 landica and M. biflora (Fig. 21), and on both the ventral 

 and the dorsal side, or only on the dorsal side (of the erect 

 leaves) of Melandrium involucratum (from Upernivik). Re- 

 cords of this are also to be found in Borgesen, Vesque, 

 and others. 



The epidermis always consists of one layer only, and 

 the cell-walls are rather thin on all sides, but are naturally 

 thickest on the outer side. The cuticle is thin. This must 

 be an adaptation to the nature of the habitat, and can vary 

 with it. W. Rüssel has demonstrated, that the leaves of 

 Dianthus prolifer have much thicker epidermal walls in the 

 Mediterranean countries, than near Paris, and therefore 

 become much stiffer. 



Only in Viscaria alpina did I find the walls of the epi- 

 dermal cells to be somewhat thicker, and the lateral walls 

 poröse (Fig. 14, C, Z), E and F). The outer walls of the epi- 

 dermis were found to be thick in Sagina nodosa from dunes 

 in Denmark. 



18* 



