XII 



LEPTOSPORANGIA T^ HETEROSPORE^ 



393 



both outer and inner surfaces. The bulk of the leaf is 

 composed of a sort of paHsade parenchyma, and the cavity is 

 partly encircled by an extremely rudimentary vascular bundle. 

 The ventral lobe of the leaf is but one cell thick, except in the 

 middle, where there is a line of lacunar mcsophyll, traversed by 

 a simple vascular bundle. 



In Salviiiia the leaves are of two kinds. The dorsal ones 

 are undivided, and traversed by a single vascular bundle. The 



Fig. 203. — Salvinia natans (L.). A, Horizontal section of the stem apex, X450 ; L, young leaf; B, 

 a young leaf, showing the apical cell (.r), X450 ; C, longitudinal section of a segment of a 

 ventral leaf, X430 ; D, section of a dorsal leaf; i, lacunee ; h, hair, X 225 ; E, cross-section of the 

 stem, X 50 ; F, the vascular bundle, X225. 



mature leaf shows two layers of large air-chambers, separated 

 only by a single layer of cells, whose walls are like those of the 

 epidermis. From both upper and lower surfaces, but especially 

 the former, numerous hairs develop. The ventral leaves are re- 

 peatedly divided, and each segment grows by a definite apical cell ; 

 the segments are long and root-like, and covered with numerous 

 long delicate hairs, looking like rhizoids. These submersed 



