62 HOFMEISTER, ON 



secting the last-formed septa, and parallel to those lateral 

 cell-surfaces which are turned towards the median line of 

 the leaf. In the cells of the interior of the leaf this latter 

 division occurs once only ; in those of the margin it is 

 repeated sometimes as many as eight times in the outer- 

 most cells. The lower part of the leaf thus increases 

 considerably in breadth, in proportion to the increase in 

 circumference of the stem. As the longitudinal growth 

 of the leaf draws to a close, the two apical cells do not 

 keep pace with one another in their multiplication ; one 

 of them is usually a generation in advance of the other 

 (PL XI, fig. 14). 



By the time that the leaf is of the height of four 

 cells, one of the marginal cells of its base begins to 

 protrude laterally in the form of an arch. The protu- 

 berance is soon separated from the original ceU- cavity 

 by a transverse septum. By repeated transverse divi- 

 sion of the apical cell the latter is transformed into 

 a row of flattened cells, into a ribbon-shaped appendage 

 of the young leaf, embracing the stem (PL XI, fig. 8 c ) ; 

 this is the first rudiment of the lower lobe (which 

 presses against the upper one) of the superior leaves. 

 It increases in breadth by the division of its cells by 

 means of septa parallel to the longitudinal axis. This 

 cell-division is repeated much oftener in the marginal 

 cells of that side of the lower leaf-lobe which is turned 

 away from the upper lobe, than in those of the opposite 

 side. It frequently does not continue in the cells nearest 

 to the apical cell. This latter cell grows regularly into 

 a club-shaped hair (PL XI, fig. 2). 



After the form of the leaf is thus prepared the cells 

 of its base divide by frequently alternating longitudinal 

 and transverse septa. The outline of the leaf is not 

 thereby changed, but the number of its cells is veiy 

 much increased, and the space over which the upper 

 leave-lobe coheres to the lower is considerably extended. 



The first stages of development of the inferior leaves 

 of Frullania dilatata entirely resemble those of the bi- 

 dentate superior leaves of Lojjhocolea bidentata, two teeth 

 being formed on the fore-edge in the same maimer as 



