276 



5. Uuaizoma. 



Tlie two species of tliis i,toii[) wliicli liave been investigated 

 show their close relationship in |1) the form of the leaves 

 (both are spathulate, and serrate, with a hydathode at each 

 tooth), (2) the distribution and structure of the hairs, (3) the 

 transverse elongation of the epidermal cells of the upper sur- 

 face and the longitudinal elongation of those of the lower, (4) 

 the pits in the radial walls of the epidermal cells, (5) the 

 structure of the palisade-tissue and of the spongy parenchyma, 

 (6) the structure of the veins, (7) the hydalhodes with a cavity, 

 and with secretion of lime, (8| and the stomata, surrounded by 

 4 — 6 smaller cells. The differences between the two species are 

 so slight, that on the basis of the anonymous section at hand it 

 would be difficult, if not impossible, to separate them from 

 each other with any certainty; presumably they differ more 

 particularly as regards the epidermis of the upper surface of 

 the leaf, which in S. Aizoon has more decidedly transversely 

 elongated cells than in S. Cotyledon. As far as is known, wax 

 is absent from the epidermis of the latter species, while it is 

 found in S. Aizoon. A key to their determination would there- 

 fore be as follows : — 



Epidermal cells of the upper surface 



very distinctly transversely elongated: S. Aizoon, 

 somewhat indistinctly, or more rarely not at all trans- 

 versely elongated: S. Cotyledon. 



Saxifraga Cotyledon L. (Figs. 23 and 24.) 

 Saxifraga Cotyledon L. This species is a Sub-alpine 

 lowland plant which here and there extends almost down to 

 the sea-level and scarcely ever extends higher into the moun- 

 tains than about 500 feet; found most commonly at elevations 

 of 200 — 300 feet above the sea. It grows partly upon level, 

 partly (and most commonly) upon sloping ground, where it is 



