142 
MORPHOLOGY OF TISSUES, 
very early, even in the first division of the segment, so that the marginal tissue 
and the youngest extremity of the mid-rib can be traced close up to the apical 
cell. Fig. no, C, finally, afi"ords an opportunity of learning the mode of formation 
of a new apical cell out of a cell of the meristem, a case which occurs often enough 
in Muscineae and the higher Cryptogams. While the thallome of Stypocaulon 
(Fig. 1 08) shows how the apical cell of the lateral shoot grows immediately from 
that of the principal shoot as a lateral protuberance, which is then cut off by a 
wall, in Metzgeria /urcaia, as is shown by the researches of Hofmeister, Kny, 
and Müller, it appears that the origin of a new apical cell may be brought about 
in a different manner. Fig. no, C shows the case described by Kny; in the 
third-youngest segment 0, which is formed from the apical cell s, the customary 
separation into a mid-rib-cell and a margin-cell has first taken place ; the latter 
then divides, as usual, into two cells lying side by side ; but the new apical cell is 
constituted by the appearance of a curved wall in one of these margin-cells of 
the second order ; and this wall intersects the dividing wall of the margin-cell 
previously formed, thus cutting out a wedge-shaped piece z, which assumes at 
once the function of the apical cell of a new shoot ^. 
In Equisetacese and many Ferns, the axis of the shoot terminates in a 
comparatively very large apical cell, which is bounded by four walls — an outer one, 
overarching the apex and spherically triangular, and three converging obliquely 
below and within, which form at the same time the upper principal walls of the 
youngest segments (Fig. in, Ä, D); the apical cell has hence the form of a 
segment of a sphere, or of a three-sided pyramid with its spherical base upper- 
most. The three plane principal walls of the apical cell are of different ages. 
The next division-wall arises in the apical cell, and is parallel to the oldest wall ; 
a segment is formed bounded by two triangular principal walls, an arched outer 
wall, and two nearly oblong side-walls 2; after the apical cell has again grown to 
its original size, a second division follows parallel to the next-younger principal 
wall, which is followed again, after fresh renewal of the apical cell, by a division 
parallel to the youngest principal wall. Three segments are now formed, placed 
somewhat like the steps of a winding staircase ; each is in contact with a principal 
wall of the apical cell ; and in this manner the divisions are repeated ; and since 
each segment takes in a third of a circuit of the winding staircase, the segments 
Oüt of which the stem is built up all lie in three rows parallel to the axis, 
each embracing a third of the diameter of the stem. In Fig. iii, B and C, the 
segments are numbered /, II, III, &c., according to the order of their formation, 
and are represented as they appear when the apex of the stem is seen from above 
(not in transverse section), or as if the arched surface of the apex were spread 
out flat. If the segments are followed according to the order of their numbering, 
the path described is an ascending spiral, because each segment lies higher than 
the older ones, as is shown in Fig. in,. D, where, however, only two rows of 
^ We shall recur, in Chap. Ill, to this case of spurious dichotomy. 
2 These side-walls are pieces of the principal walls of the adjoining segments, as is seen 
in B and C. 
