THE HIGHER CRYPTOGAMIA. 345 



by septa parallel to the chords of the arched free outer' 

 surfaces — into an inner and an outer cell. In the latter a 

 septum is produced at right angles to the one immediately 

 preceding it, and radial to the longitudinal axis of the leaf ; 

 close under the growing tip. of the leaf eight peripheral 

 cells enclose four axile ones. The form of the leaf, which 

 at first is flattened above and below, is gradually changed 

 by this cell-multiplication into a conical one (Pl.-XLVI, 

 fig. 24). The leaf increases in thickness by repeated divi- 

 sion of the cells of its circumference, produced by radial 

 longitudinal septa alternating with septa parallel to the 

 tangents of the free outer walls. When it has attained a 

 certain stage of development its longitudinal growth is 

 much accelerated by the occurrence of transverse division 

 in most of its cells. This multiplication commences close 

 underneath the apex of the leaf, and progresses from 

 thence, on the one side towards the base, and on the other 

 side towards the apex, so far at least as it extends into those 

 cells of the apex of the leaf which have been formed since 

 its commencement. The cells of the circumference divide 

 first ; from the latter the multiplication proceeds towards 

 the longitudinal axis, without reaching the four rows of 

 cells adjoining the latter. The latter remain twice as long 

 as the cells of the peripheral layer ; they are destined at a 

 later period, by repeated longitudinal divisions, to become 

 transformed into vascular bundles (PL XLVII, fig. 1). 



The first leaf either shoots out at right angles from 

 the longitudinal axis of the archegonium and embryo, or 

 it trends upwards, often at such an acute angle that its 

 apex penetrates into the upper arch of the central cell of 

 the archegonium. The latter case is the most frequent ; it 

 very rarely happens on the other hand that the leaf takes a 

 downward direction towards the centre of the prothal- 

 lium. 



As early as the time when the number of the cells of the 

 leaf, counted in a longitudinal direction, ariiounts to from four 

 to six only, the free outer wall of the cell which occupies 

 the middle of the base of the upper surface of the leaf — that 

 surface which is turned towards the apex of the prothal- 

 lium — begins to swell in a vesicular manner (PL XLVI, 



