28 DOUGLAS HOUGHTON CAMPBELL ON THE 



Although the apical cell of the first leaf is at first like that of the other quadrants, 

 tetrahedral, after about two sets of segments have been cut off, no more — or at any 

 rate, no more, as a rule — are cut off from the side parallel to the transverse Avall. The 

 succeeding segments ai-e formed alternately from the sides originally parallel to the basal 

 and median walls. By the obliteration of one of the original cells of the leaf-quadrant, 

 and by subsequent growth and consequent displacement of cells, the apical cell of the 

 leaf, as seen from above, occupies nearly the center of the leaf-quadrant (PI. 6, fig. 

 1). As a result of the formation of segments in two directions only, the apical cell 

 loses its original tetrahedral form, and approximates the wedge-shaped apical cell of the 

 later leaves. Some time before the embryo is ready to break throngh the enveloping tis- 

 sue, a further change takes place in the apical cell of the leaf. It niidergoes division, 

 each half assuming the form of the original, and becoming the apical cell of one divis- 

 ion of the strongly two-lobed leaf (PI. 6, fig. 3). The division takes place as fol- 

 lows. The apical cell is divided into two equal parts by a vertical wall. Eacli of the 

 resulting cells is next divided by a cni-ved wall running from the outer margin of the cell 

 to the wall that belonged to the original cell. There are thus formed four cells, the two 

 central ones being elongated, the others nearly triangular in outline. The former be- 

 come divided by a transverse wall into an inner and outer cell; the two latter by walls 

 alternately i-ight and left: being, in fact, the apical cells of the lobes of the leaf. 



The young leaf grows much more rapidly than the stem, and soon projects beyond it. 

 The multiplication of the cells being greater on the lower side, the leaf begins to cnrve 

 upward. The cells of the leaf-stalk now elongate rapidly, and after the embryo has 

 broken through the ovei-lying tissue of the prothallium, the leaf pushes up between the 

 lobes of the prothallium, appearing above its surface (PI. 6, fig. 5). 



The apical cell of the stem divides much more slowly, and follows nearly the same plan 

 at first. The segments later divide somewhat diflerently, the first division wall of the 

 segment being a radial one, and dividing the segment into two nearly equal cells. The 

 stem is very short at the time the embryo breaks through the prothallium, but the apex 

 is more decidedly conical than is the case in the mature plant. 



The apical cell of the foot soon becomes obliterated by a process entirely similar to 

 that already described, but the cells continue to multiply rapidly with no i-egular order. 

 By means of the foot, which when complete is lai-ge and rounded in shape, the embryo 

 is kept in close connection with the prothallium, and by means of it the embryo is nour- 

 ished until the first leaf and root are developed. 



After the first segments are cut off in the apical cell of the root, it usually continues 

 to act for some time like that of the stem, the first wall of the segment being radial instead 

 of tangential. Of the subsequent divisions, the tangential are much more numerous than 

 the radial. After a varying number of segments have been cut ofi" as described, a wall 

 is formed in the apical cell parallel to its outer surf\ice; this is the first cell of the root- 

 cap. This cell becomes divided by a vertical Avail into two, and next by a wall at right 

 angles to the fii-st, into four. Others are formed later, but only in two planes, there be- 

 ing no tangential walls formed (PL 5, fig. 16, PI. 6, figs. 2, 4). From this time on, 

 each cycle of segments in the apical cell consists of four cells. The development of 

 the root varies greatly at the time the embryo breaks through the prothallium. In the 

 mnjority of cases examined (fig. 2) the root was much shorter than is usually figured 



