274 Eue. Warming. 



branch on each side, which may itself branch ogain (Fig. 21, 

 C, D and Fig. 23, B). In the most narrow-leaved species 

 this does not often happen, so that the entire number of 

 bundles in a transverse-section will be three only (Figs. 18 

 and 21) — these are especially prominent in older leaves 

 which have been emptied of their cell-contents, — - but more 

 frequently there are several, or even many, in the broadest- 

 leaved specimens (Figs. 14, 17 and 20). As regards the vascu- 

 lar tissue, I have no particulars to communicate. 



In some of the species mechanical tissue occurs on the 

 ventral or dorsal side of the strongest vascular bundles, 

 as mentioned above. 



It appears to be an exceedingly constant characteristic 

 in the Caryophyllaceæ to have around the vascular bundles 

 a thin-walled, sharply differentiated sheath consisting of 

 one layer of cells, which are almost square in transverse 

 section; this is illustrated in most of the figures given above. 

 In the majority of cases, the cells are of equal height, but 

 cases may occur in which those of the dorsal surface are 

 higher than those of the ventral (e. g. Silène acaulis, Fig. 

 16, A, D). 



For further particulars regarding the anatomy of the 

 Caryophyllaceæ, the reader is referred for instance to Regn- 

 ault (1860; Ann. se. nat., 4. Sér., 14); Seignette (Revue 

 gen. I, p. 564); O. G. Petersen (Botan. Tidsskr., Køben- 

 havn, 1888); Heinricher (1884); Joh. Erikson (1896); 

 Warming (1890, 1891, 1897) and Solereder. As regards 

 the structure of the leaves in the Alps, see Wagner, Laz- 

 niewski and C. Schroeter. 



III. Adaptions to the Environment. 



The morphological and anatomical structure of plants 

 is, in the first place, dependent upon their genetic relation- 



