1 6 IXTR OD UC TION. 



and of the two branches of the first bhurcation, has, by reason of corresponding 

 divisions, the form of a four-sided pyramid, which however soon passes back to the 

 two-edged form ^. 



Each segment is separated from the apical cell as a tabular cell by a division 

 wall, which is approximately parallel to one of the sides of the apical cell. This wall is 

 called the principal ivall of the segment ^. Each segment has two principal walls, 

 one (acroscopic) by which it was separated from the apical cell, the other (basiscopic) 

 tliat which adjoins an older segment. Its outer ivall is that part of the free wall 

 of the apical cell which is cut oft' by the line of junction of the acroscopic principal 

 wall ; its lateral tvalls are the parts cut off by the lines of junction of the same 

 principal wall from the principal walls of the segments, which border it laterally. 



The principal walls, which cut off the successive segments from the apical cell, 

 are parallel in regular succession to the sides or principal walls of the latter. Thus in 

 the case of a two-sided apical cell they oppose the one and the other side of it alter- 

 nately, each fronting the older principal wall ; in the case of a three-edged apical 

 cell, they oppose the three sides successively in spiral sequence, each facing the third 

 oldest principal wall, and being attached laterally to the two younger principal walls. 

 All the segments therefore of a meristematic apex are arranged (if we ignore sub- 

 sequent displacement) in as many straight rows parallel to the axis as the apical 

 cell has sides. 



The principal walls of a segment, which has recently been cut off, are inclined 

 to the theoretically straight perpendicular axis of the meristematic apex at an acute 

 angle, which varies according to the form of the apical cell. As growth proceeds, 

 the form of the segment alters, and with it the direction of the principal walls (or 

 rather the planes in which these lie), so that, with reference to a perpendicular axis, 

 they assume a horizontal position. For a thorough discussion of these phenomena, 

 and of the growth of the apical cell itself, cf. Niigeli and Leitgeb, I. c. p. 91. 

 The figure 7 A, which is borrowed from these authors, may present the process 

 to the eye. 



The segments cut off from the apical cell become gradually divided up into 

 masses of meristem consisting of several or many cells. As the result of the changes 

 of form and position already mentioned as accompanying the joint growth, each 

 of these masses represents part of a more and more horizontal disc, and meets 

 the similarly shaped segments next in age to it at the central line. A transverse 

 section cut at some distance from the extreme apex includes so many united seg- 

 ments as there are straight series of these, i.e. where the apical cell is two-sided, 2 ; 

 where it is three-sided, 3. The divisions proceed rapidly, and if, as must be done 

 in this case, one disregards lateral outgrowths, they proceed in the same direction and' 

 in the same succession in the successive segments. One thus finds the segments of 

 each transverse section in about the same stage of division. 



In those cases where the successive divisions have been successfully and 

 accurately followed— apex of the stem of Equisetum, Azolla, Selaginella jMartensii, 

 partially also in Salvinia, and especially in the roots of Equisetum, Azolla, and many 



> Pfeffer, Entw. d. Keims v. Selaginella.— Hanstein, Bot. Abhandl. Bd. I. 

 * Cramer, Ueber Equisetum in Kageli iind Cramer, Pflanzenphysiol. Untersuchungen, 3 Heft, 

 P- 21 (1855)' 



