CONFOCAL AND CO-AXIAL STRUCTURES. 



451 



Fig. 287. 



Moreover, such a separation between the growing-point of the root and its cap 

 does not exist in all true roots ; there may occur quite other distributions of growth 

 at the apex of the root, and accordingly also very different net-works of cell-walls. 



That distribution of growth by 

 which the intercalation of mass in- 

 creases from the apex downwards, and 

 at the same time from the longitudinal 

 axis outwards, and by means of which, 

 as we have seen, the anticlines are 

 caused to radiate like a fan, occurs, 

 however, not only in root-caps, but 

 also in many other cases, especially, 

 for example, in the growth of very 

 young ovules. Fig. 288 may finally give 

 another simple scheme for the cell- 

 wall net-work, and the course of the 

 anticlines and periclines in such a case. 

 It must be left for the reader, however, 

 to picture how the structure B has 

 arisen from the two cell-series in A 

 by means of the corresponding pro- 

 cesses of growth and subsequent cell-divisions, it being simply observed that the 

 walls marked a and 6, as well as those marked i, 2, 3, 4, are in both figures the 

 same. 



Finally, the remark may be added that the first described arrangement of 

 the cells at the growing-point may be 

 termed confocal, while the last one may 

 be regarded as co-axial, or fan-shaped, 

 and that young organs of similar out- 

 ward form present sometimes the one, 

 sometimes the other structure : in other 

 words, the intercalation of mass in the 

 interior of an organ may accord with 

 the one or the other type, though the 

 external form of the organ is the same 

 in both cases. 



If we now return once more to 

 the parabolic, dome-like growing-point, 

 from the consideration of which we 

 started, and the median longitudinal 

 section of which is here once more re- 

 presented, we may now assume further ^^<^- '^■ 

 that the figure included by the line 



£U has made a complete revolution round the longitudinal axis XX. It 

 is intelligible that then also each of the periclinal and anticlinal lines must have 

 described a parabolic surface, and if we represent to ourselves the resulting figure, 



G g 2 



