Caryophyllaceæ. 



267 



isolaterality is, for the rest, common in the Caryophyllaceæ, 

 which must be connected with the fact that the leaves in 

 many of the species are narrow, and can therefore be illu- 

 minated equally on all sides. 



With regard to the mesophyll in general, the following 

 may moreover be remarked. Vesque had already recorded in 

 1883 that the mesophyll in the Caryophyllaceæ consists of 

 large cells, which are loosely connected so that it is rich is 

 intercellular spaces, and "spongy" 

 ("spongieux"). This is no doubt espe- 

 cially applicable to the Arctic species. 

 This feature is not only found in the 

 species figured above, but in as high 

 a degree in Cerastium nigrescens and 

 Arenaria ciliata (Figs. 22 and 23). In 

 the latter figure it is even highly iso- 

 lateral, with almost uniformly roun- 

 ded cells. That it may be more com- 

 pact is evident from what Th. Holm 

 says regarding it. 



"It has already been emphasized 

 by others (Bonnier and Börgesen) 

 that it is a feature common to all 

 the Arctic species, for the palisade- 

 tissue to be most frequently imperfectly with P alisade - Cells > and 



a lacuna (la) in the 

 differentiated, the layers few in num- sp0 ngy-tissue. B, Sto- 



bèr, the height of the cells inconsider- ma of ventral surface, 

 able, and the intercellular spaces large 



- — at any rate, not so strong and compact as in species from 

 more southern — ■ for instance Mediterranean — countries. 

 Börgesen even found no palisade-tissue at all in specimens 

 of Silène acaulis (1. c. Fig. 12) and Minuartia verna f. hirta; 

 see also Arenaria ciliata in Th. Holm (PI. XII, 2). 



Fig. 22. 



Cerastium nigrescens. 

 i From Dovre; 15th 

 June; E. W.) 

 . I . The transverse sec- 

 tion of leaf shows a 

 very lacunose tissue, 



