GROUP III. PTERIDOPHYTA : FILICIN2E ; HYDROPTERIDE2E. 407 



and the two lowest the ventral rows. In Salvinia the foliage-leaves 

 are alternately developed from the dorsal and the dorso-lateral 

 cells; hence they lie in four alternating longitudinal rows; the 

 submerged leaves are developed from the ventro-lateral cells in 

 two rows. In Azolla the leaves are developed only from the dorso- 

 lateral cells, and the roots from the ventral cells. In both genera 

 the lateral branches are developed from the ventro-lateral cells. 



In the Marsileacese the apical cell is tetrahedral : of the three 

 sides from which segments are cut off, two are dorso-lateral and 

 one is ventral : the leaves are developed from the upper portions, 

 the branches from the lower portions, of the dorso-lateral 

 segments : the roots are developed from the ventral segments. 



The foliage-leaf presents a considerable variety of form. In 

 Salvinia it is broad and flat, sessile and entire, with a well-marked 

 midrib : in Azolla the leaf is small and two-lobed, the lower lobe 

 being submerged whilst the upper floats on the surface of the 

 water: in Marsilea the leaf has a long erect petiole bearing a 

 paripinnate bijugate compound lamina of four leaflets: in Pilularia 

 the leaf is cylindrical and erect. 



The growth in length of the leaf is effected, in Salvinia, Marsilea, 

 and Pilularia, by an apical growing-point having a wedge-shaped 

 apical cell: the growth in breadth of the lamina is effected, 

 in the two former, by marginal growth. In Azolla the develop- 

 ment of the leaf is effected altogether by marginal growth. 



Circinate vernation obtains in the Marsileacese, but not in the 

 Salviniaceee : in Salvinia the vernation is conduplicate, and in 

 Azolla the lamina is expanded from the first. 



In Salvinia the leaves are borne in a whorl of three at a node, 

 two being a pair of opposite foliage-leaves, and the third a 

 submerged leaf : in the other genera the phyllotaxis is alternate. 



The submerged leaf of Salvinia consists of a number of long 

 filamentous branches, springing from a short petiole, and densely 

 covered with multicellular hairs. 



The sporangia are of two kinds, microsporangia and 

 macrosporangia : they are borne in sori enclosed in structures 

 termed sporocarps. The morphology of the sporocarp is, however, 

 altogether different in the two orders, and the same term ought 

 not to be applied to both : it would be well to restrict the term 

 " sporocarp " to the more complex fructifications of the 

 Marsileaceae. 



In the Salviniaceae the so-called sporocarp is simply a sorus of 



