SEGMENTATION OF THE APICAL CELL. 



455 



FIG. 291. — End of a ribband-shaped flat shoot of the Alga Dictyota. 



wall has arisen in the apical cell itself, wherewith is given the commencement of 

 a dichotomous branching of this growing-point which itself proceeded from a 

 dichotomy. As for the rest, how- 

 ever, the same relations as before 

 prevail, and it is at once noticed 

 in the figure how the two growing- 

 points a and b show by the anti- 

 clines further backwards their origin 

 from a previous dichotomy. 



In the two cases examined the 

 segments are cut off from the apical 

 cell by simple transverse walls. Fig. 

 292 may serve as an example of the 

 case where from an apical cell on 

 a flat organ (leaf of the Fern Cera- 

 topteris) the segments are formed 

 by successive division-walls situated 

 obliquely right and left (.S^, whereby 

 two rows of segments arise, from the 

 further growth and corresponding 

 cell-divisions of which the whole of 

 the tissue of the leaf is produced. 

 The thick hnes in the figure are the 



older segment- walls of the apical cell.; and it is noticed, proceeding backwards from 

 this, how the superficies of the segments have become larger and divided by anticlinal 

 and periclinal cell-walls in accord- 

 ance with their age. At Z a lateral 

 lobe of the leaf is protruding ; this, 

 as is easily seen, belongs to two dif- 

 ferent segments, and since numerous 

 anticlinal walls already exist in it, 

 which radiate in a fan-like manner 

 into the lobe and participate in its 

 whole growth, no apical cell is 

 formed here. 



Such apical cells, segmented in 

 two rows, are common in flat organs, 

 as in the leaves and flat shoot-axes 

 of many Algae, Liverworts, and in 

 some dorsiventral shoot-axes of Vas- 

 cular plants — e.g. in many Ferns 

 and all Selaginellas. In growing- 

 points the transverse section of which 

 is circular and the growth upright, 

 and especially in the true roots of the Vascular Cryptograms, we find, on the contrary, 

 at the tip of the growing-point an apical cell from which segments are cut off on three 



FIG, agz.— Young leaf of Ceratopteris (after Kny), 



