60 HOFMEISTER, ON 



undivided half of the leaf is very limited, often almost sup- 

 pressed in J. divaricata. The first rudiments of the two 

 points of the leaf in /. bicuspidata are of a very plump 

 form (PL VIII, figs. 8 10). In Ptilidium ciliare an active 

 multiplication of the cells of the superior leaves commences 

 at a late period, and is more clearly defined than even in 

 Lophocolea and /. bicuspidata. With it the formation of 

 the leaf commences by the protrusion outwards in the form 

 of an arch, of one of the stem-cells of the second order, 

 close underneath its apex; the protruding cell assumes 

 the form of a swollen seam, embracing nearly half the cir- 

 cumference of the stem (PI. VII, fig. 9, a). The cell divides 

 by a longitudinal septum radial to the axis of the stem ; 

 both halves of the seam are separated from the original 

 cavity of the cell by septa parallel to the outer surfaces of 

 the stem. Each of the two cells of the young leaf there- 

 upon developes itself independently in length. Each arches 

 itself outwards to some extent, so that the fore-edge of the 

 leaf exhibits two very blunt points ; thereupon each of the 

 cells divides by a transverse septum, which separates the 

 protruding portion from the original cell-cavity (PI. VIII, 

 tig. 9, b, where only half of the leaf is shown). This division 

 is repeated continually in each of the two apical cells. Each 

 interstitial cell (cells of the second order) is bisected by 

 a longitudinal septum. The cells of the third order divide 

 by septa, either parallel to the latter septum or converging 

 to it (PI. VII, fig. 51, c) ; thereupon the cells of the edge 

 of the leaf grow out into the long cilia which give the 

 specific name to the plant, extending themselves in the 

 form of papillae, and then repeatedly dividing in their apical 

 cell by transverse septa (PI. VII, figs. 7, 8). Ultimately, 

 longitudinal septa are formed in the lowest of the cells of 

 the second order of these excrescences of the edge of the 

 leaf. In highly developed leaves new cilia spring from the 

 marginal cells of these pointed appendages of the edge of the 

 leaf, originating in precisely the same manner as the parts 

 upon which they are borne. The leaf now consists of 

 two symmetrical halves, which have only a single row of 

 cells for their common basis, and are connected together 

 at the bottom only to the extent of a single cell (PI. VII, 

 fig. 8). It is this transverse row of cells which, by repeated 



