THE PRIMARY MERISTEM AND THE APICAL CELL. 1 19 



apical cell, here very large {s), growing straight out from its base, is divided by septa 

 (/", P), and thus forms the segments which lie in a row one over another ; but 

 each of these last is again immediately broken up by a septum (//'', IP) into 

 two disc-shaped cells, and in each of these there arise numerous small cells by 

 the formation of vertical and afterwards horizontal walls (as may be seen in the 

 figure), further back from the apex; and it is easily seen how the whole branch 

 is built up of portions of tissue, each of which is composed of a single seg- 

 ment. The same takes place on the lateral branchlets {x, y), which in this case 

 arise originally from lateral protuberances from the apical cell. These processes 

 are remarkably clearly seen in Stypocaulon, in the first place because only one 

 row of segments is formed lying one over another, and in the second place be- 

 cause the segments thems-elves are transformed into portions of tissue without at 

 the same time growing, as is usually the case ; distortions often occur from the 

 growth of the segments, which render difficult an investigation of the processes 

 of division. 



Figs. 99 and 100 show us a case in which the apical cell is divided alternately 

 right and left by oblique walls so as to produce two rows of segments attached 

 to one another in a zigzag manner by their inner and lower sides, but separated 

 to some distance in front; in the angle which the two youngest segments enclose 

 lies the apical cell {s). Fig. 99 shows the end of a shoot of Metzgeria furcata 

 in the act of bifurcation ; each fork ends in an apical cell {s) ; the segments and 

 the masses of tissue which are formed from them are drawn just as they appear 

 to the eye under the microscope in the superficial view of the flat strap-shaped 

 shoot. But from the course of the cell-walls and the resulting grouping of cells 

 around the apical cell the diagram represented in Fig. 100, A, is deduced, in 

 which the distortions of the cell-walls occasioned by growth are omitted, and 

 hence genetic relationships are represented more clearly. For further information 

 Fig. 100, B, is added, which also represents diagrammatically the longitudinal 

 section of the apical region, at right angles to the broad surface of the strap- 

 shaped shoot. This longitudinal section bisects, behind the apical cell, the central 

 nerve (Fig. 99, n, n), which consists of several layers of cells, while the lateral 

 expansions of the shoot are only one layer in thickness. The origin of the tissue 

 is now clear from the diagrammatic Fig. 100, A and B, if it is observed in the 

 first place that the portions of the surface indicated by m, n, 0, p, and q are the 

 segments of the apical cell (s) which were formed successively in the same order, 

 so that 7)1 represents the oldest, q the youngest segment. From each segment a 

 small piece is at first cut off behind by a wall oblique to the axis of the shoot; 

 from the zigzag row of these inner divisions arises the mid-rib of the shoot, 

 which attains a thickness of several layers of cells, each division first of all 

 splitting up by a wall parallel to the surface of the shoot into two cells lying one 

 over another ; each of these cells on its side again divides in the same manner. 

 Divisions at right angles to the surface of the shoot (Fig. 100, B) are then also 

 formed in the uppermost and undermost of the cells produced in this way; an 

 outer small-celled layer (covering the upper and under side) becomes formed on 

 the mid-rib, surrounding an inner bundle which consists of longer cells. While 

 the posterior sections of the segment produce the tissue of the nerve, the tissue 



