THE PRIMARY MERISTEM AND THE APICAL CELL. 141 



distance in front; in the angle which the two youngest segments enclose lies 

 the apical cell s. Fig. 109 shows the end of a shoot of Metzgeria furcata in 

 the act of bifurcation; each fork ends in an apical cell j; the segments and 

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

 to the eye under the microscope on a superficial view of the flat ribbon-shaped 

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

 round the apical cell, the diagram Fig. wo, A is deduced, in which the dis- 

 tortions of the cell-walls occasioned by growth are neglected, and hence the 

 genetic relationships represented more clearly. For further elucidation Fig. no, B 

 is added, which also represents diagrammatically the longitudinal section of 

 the apical region, at right angles to the broad surface of the ribbon-shaped 

 shoot. This longitudinal section bisects, behind the apical cell, the mid-rib 

 (Fig. 109, n, n), which consists of several layers of cells, while the lateral ex- 

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

 is now clear from the diagrammatic Fig. no, A and B, if it is observed in the 

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

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

 so that ni represents the oldest, q the youngest segment. From each segment a 

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

 from the zigzag row of these posterior sections 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, and each of these cells on its part again dividing in the same manner. 

 Divisions at right angles to the surface of the shoot (Fig. no, B) are then also 

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

 outer small-celled layer is formed on the mid-rib covering its upper and under 

 side, and surrounding an inner bundle which consists of longer cells. While 

 the posterior sections of the segment produce the tissue of the mid-rib, the tissue 

 of the flat lateral portion (Fig. 10^,/,/') proceeds from the anterior sections which 

 face the margin of the shoot ; and this tissue is only one cell-layer in thickness, 

 no division taking place in it parallel to the surface of the shoot. All the divisions 

 in these marginal sections of the segment are, on the contrary, at right angles to 

 the surface, and are produced by the marginal section first of all breaking up 

 into two cells lying side by side (see Fig. no, A, 0), each of which then forms 

 several shorter cells by repeated bi-partition, and these may again undergo further 

 division according to the activity of the growth. In general the first divisions 

 only of the segment are constant; the further course of cell-multiplication is, 

 according to the minute investigations of Kny, subject to many deviations. Since 

 the tissue produced from the marginal sections becomes prominent during growth, 

 it results that the apical cell, with the youngest segments, lies in a depression of 

 the margin of the shoot; and we have here a simple example of the depression 

 of the growing point in the tissue which grows more luxuriantly around it, such 

 as often occurs to a much greater extent in Fucacese, Ferns, and Phanerogams. 

 The differentiation of the tissue of the shoot of Meizgeria furcata does not 

 attain a high degree ; the cells of the margin and of the mid-rib, when mature, 

 differ only slightly from one another; but this differentiation is brought about 



