THE PRIMARY MERISTEM AND THE APICAL CELL. 121 



of which the shoot of Metzgeria furcata is built up does not attain a high 

 degree; the perfectly developed cells of the margin and of the mid-rib are only 

 slightly different from one another ; but it should be mentioned that this differen- 

 tiation is brought about very early, even in the first division of the segment, 

 so that the marginal tissue and the latest continuation of the mid-rib can be 

 followed close up to the apical cell. Fig. loo, C, finally, affords an oppor- 

 tunity 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 Mosses and higher Cryptogams ; 

 while the thallome of Stypocaulon (Fig. 98) shows how the apical cell of the 

 lateral shoot grows immediately from the apical cell of the principal process as 

 a lateral protuberance, which is then cut off by a wall. In Metzgeria furcata, 

 as is shown by the statements of Hofmeister, Kny, and Miiller, it appears that the 

 origin of a new apical cell may be brought about in a different manner; Fig. 100, 

 C, shows the case described by Kny. In the third-youngest segment ((?), which is 

 formed from the apical cell {s), the customary separation into a nerve-mother-cell 

 and a division belonging to the margin of the tissue has first taken place ; the 

 latter then breaks up, as is usually the case, into two cells lying close to one 

 another ; but the new apical cell is constituted by the appearance of a curved 

 wall in one of these marginal cells of the second rank; and this wall comes 

 into contact behind with the previous one, thus cutting out a wedge-shaped 

 piece (2), which assumes at once the function of the apical cell of a new shoot. 

 (We shall recur, in Chap. Ill, to this case of spurious dichotomy.) 



In the Equisetacex 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, spherically triangular, and free, and three converging obliquely 

 below and within, which form at the same time the upper principal walls of the 

 youngest segment (Fig. loi, J, D); the apical cell has hence the form of a seg- 

 ment of a sphere, or of a three-sided pyramid with spherical base turned upwards. 

 The three plane principal walls of the apical cell are of different age; one is 

 always the oldest, one younger, and the third the youngest. The next division- 

 wall arises in the apical cell, and is parallel to the oldest wall ; a segment is formed 

 bounded by tw-o triangular principal walls, an arched outer wall, and two nearly 

 oblong side-walls^; 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 out 

 of which the stem is bulk up all lie in three straight rows parallel to the axis, each 

 embracing a third of the diameter of the stem. In Fig. 101, B and C, the segments 

 are numbered /, //, ///, &c., according to the order of their formation, and are 

 represented as they appear when the apex of the stem is seen from above and 



* These side-walls are pieces of the principal walls of the previously existing adjoining 

 segments, as is seen in B and C. 



