THE HIGHER CRYPTOGAMIA. 143 



twenty in Sphagnum acutifolium), which is afterwards only 

 doubled by transverse division of the marginal cells. The 

 above-mentioned phenomenon, on the other hand, is very 

 distinctly marked in Polytrichum and Fissidens. It is well 

 known that the leaves of the latter genus are arranged in 

 two rows. The terminal bud is surrounded by the peculiar 

 pocket-shaped duplication of the base of the last-formed 

 leaf; each older leaf of the bud also encloses the younger 

 leaves and the summit of the shoot in the already perfected 

 duplication of its base. The very young leaves resemble 

 the first rudiments of the leaves of Sphagnum. But when 

 the leaf is only five cells in height, the method of cell- 

 multiplication changes. As in Sphagnum, the cell of the 

 second degree divides by a septum at right angles to the 

 side walls. The septum which thereupon divides the outer 

 of the newly formed cells into two, stands at right angles 

 to the above septum, like the similar septum in Sphagnum ; 

 on the other hand, the membrane which originates in the 

 inner cell is at right angles to the median line of the young 

 leaf (PI. XVIII, fig. 17, a). The two cells of the fourth 

 degree belonging to the marginal cells of the leaf divide, 

 at first, by a septum parallel to the margin of the leaf. The 

 next septum, however, in both cells is at right angles to 

 the margin of the leaf. The transverse division corre- 

 sponding to this division is suppressed in the cells of the 

 two rows adjoining the longitudinal axis of the leaf, in con- 

 sequence of which these cells are double the length of the 

 neighbouring cells (PI. XVIII, fig. 17). The leaf continues 

 to widen by the further division of the cells of its margin, 

 caused by septa parallel to the edge. Sometimes indi- 

 vidual marginal cells are divided also by longitudinal 

 septa. 



The formation of the pocket at the base of the leaf com- 

 mences when the base of the leaf has attained a width of 

 eight cells. At this time from five to eight of the lowest 

 cells of that margin of the surface of the leaf which is turned 

 towards the terminal bud become arched upwards to a con- 

 siderable extent ; the protruded portions are then separated 

 from the primary cell-cavities by means of septa parallel to 

 the surface of the leaf. By this means a raised line origi- 



