XI LEPTOSPORANGIAT^ HETEROSPORE^ 411 



papillate hairs, which in some species, c. g., A. CaroUniana, are 

 two-celled. Stomata of peculiar form (Fig. 239, B) occur on 

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

 posed of a sort of palisade 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 mesophyll, traversed by a 

 simple vascular bundle. 



In Salvinia the leaves are of two kinds. THe dorsal ones 

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

 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 sub- 

 mersed leaves doubtless replace the roots. The leaves in Sal- 

 vinia are arranged in alternating wdiorls of three, correspond- 

 ing to the nodes, and this arrangement accounts for the six rows 

 of leaves previously referred to. 



The mature stem shows a central concentric vascular bundle 

 (Fig. 238, E, F), whose tracheary tissue is somewhat more 

 compact and the tracheae in Azolla than in Salvinia. This is 

 surrrounded by a definite endodermis and one or two layers of 

 larger parenchyma cells, and radiating from the latter are plates 

 of cells separated by large air-spaces, and connecting the central 

 tissue with the epidermis (Fig. 238, E). 



The lateral branches arise in acropetal order, but apparently 

 not always at equal intervals. Their development is a repetition 

 of that of the main axis. Like the branches, the roots in Azolla 

 arise acropetally, and their number is very much less than the 

 leaves. They arise from superficial cells and follow exactly ia 

 their development the primary root of the embryo. The inner 

 layer of cells of the sheath, however, in these later roots be- 

 comes disorganised, and there is a space between this and the 

 root itself. A single root-cap segment only is formed subse- 

 quent to the primary one from which the sheath forms, and this 

 secondary cap segment undergoes division but once by periclinal 

 walls (Fig. 239, C). 



Leavitt (i) found in the older roots of both A. iiliciiloides 



