THE HIGHER CRYPTOGAMIA. 61 



bisection, commencing at a late period, forms the cellular 

 surface, often ^'" in length, which carries the two-pointed 

 heads of the leaf. 



In Frullania dilatata, which is furnished with such wide 

 leaves, the leaf, at its first appearance above the surface of 

 the stem (the periphery of which at this period exhibits only 

 four cells) consists of a single wide-stretched cell (PI. XI, 

 fig. 9). It divides at first, once or twice, by a transverse 

 septum ; the newly-formed cells then divide by longitudinal 

 septa (PL VIII, fig. 3). Each cell of the lower pair is now 

 divided by a septum coinciding with the longitudinal axis 

 of the leaf, and parallel to the first septum (PI. XI, fig. 

 8 a ' h ). The two apical cells of the rudiment of the leaf then 

 divide by septa almost at right angles to its longitudinal 

 axis, thus forming three-sided superior cells of the first 

 degree of the second order, and four-sided inferior trans- 

 versely extended cells of the second degree. The latter are 

 divided immediately after their formation into inner and 

 outer cells by means of a septum parallel to the longitudi- 

 nal axis of the leaf (PI. XI, fig. 8 c ). The like processes 

 are repeated several times in the two apical cells of the 

 young leaf, so that the latter soon assumes the form of an oval 

 cellular surface, consisting of two inner rows of cells bounded 

 right and left by a row of marginal cells. The leaf soon 

 begins to increase in breadth by repeated division of the 

 marginal cells by means of septa parallel to the margin of 

 the leaf. At the same time the mode of multiplication of 

 the apical cells changes ; the division by means of a septum 

 almost at right angles to the longitudinal axis of the leaf, 

 is followed by a division by means of a septum, mounted 

 upon the latter septum, and only slightly diverging from 

 the median line of the leaf. The now four-sided elongated 

 apical cells continue to divide by alternate longitudinal and 

 transverse septa (PL XI, fig. 14). In the later stages of 

 development of the leaf, the cells produced by the division 

 of the two apical cells, both the marginal cells and the double 

 row of inner cells adjoining the median line of the leaf, are 

 divided soon after their formation by septa at right angles 

 to the longitudinal axis of the leaf (PL XI, fig. 13). This 

 division is followed by a division by means of septa inter-. 



