SIGNIFICANCE OF THE APICAL CELL. 



457 



z being the corner in which the three youngest segment-walls always cut one another. 

 The preceding figure (Fig. 293) shows now in the main for this case also, how 

 the whole of the tissues of the organ in question arise by successive cell-divisions 

 from the segments of such an apical cell. 



The segment-walls of apical cells with two or three sides are in fact 

 simply anticlines, and the subsequent division-walls in the segments are some- 

 times anticlinal and sometimes periclinal, so that the segments, just as we have 

 seen to be the case before, break up into a network of walls consisting of orthogonal 

 trajectories. A simple alteration of the scheme given in Fig. 289 here comes in 

 only in so far that the anticlines under consideration (Fig. 295 A a) do not at once 

 cut through across the whole apex, but meet one another from two or three sides : 

 Fig. 2 95-5 shows, however, how each of the anticlinal segment-walls is subsequently 

 completed by means of a further piece into a complete anticline A a. The 

 difference, which is of importance, will be evident at once on comparing the 

 figures A and B. IVIoreover, since tetrahedral apical cells only occur in solid 



V X r 



growing-points which are circular in transverse sections, and not in flat ones, radial 

 longitudinal walls also appear in the segments in addition to the anticlines and 

 periclines, and alternating with them. 



Since, as Naegeli showed so long ago as 1845, ^^^ ^^he tissues of a root or shoot 

 may be derived progressively and in genetic sequence from the apical cell, wall for 

 wall, the opinion arose gradually that the whole process of growth in the growing 

 point is ruled by the apical cell itself, which, by means of its segments, adds stone 

 upon stone to the structure like a builder. Nay, matters even went so far that the 

 belief arose, without any foundation, that the apical cell is in fact the position of most 

 rapid and vigorous growth in the embryonic tissue of the growing-point. I have 

 already referred in my previously quoted treatises to the erroneous character of both 

 views : the apical cell, as also the apical region corresponding to it in growing- 

 points not provided with such a cell, is, on the contrary, the place of slowest growth, 

 and in no part of the growing-point do cell-divisions occur so rarely as the segmenta- 

 tions in the apical cell. This can be concluded witli certainty from the increase in 



