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 fiircata 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 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. no, ^ 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), 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. 110, A and B, if it is observed in the 
first place that the portions of the surface indicated by 7n, 7t, 0, p, and q are the 
segments of the apical cell which were formed successively in the same order, 
so that 7?i 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 splitdng 
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-muldplication 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 Fucaceae, Ferns, and Phanerogams. 
The differentiation of the tissue of the shoot of Melzgeria furcaia 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 
