45* 



LECTURE XXVn. 



it consists of a large number of superposed concavo-convex layers, the curvature 

 of which increases upwards; these are the layers the vertical sections of which 

 are represented by the anticlinal lines Aa oi our scheme. Since, however, in the 

 postulated revolution about the longitudinal axis XX, each of the periclinal lines 

 Pp has also described a parabolic surface, these concavo-convex layers are divided 

 up into corresponding rings which run concentrically around the longitudinal axis 

 X X. Now, in order that these concentric rings may fall into approximately cubical 

 cells, in agreement with the problem, we must suppose still another system of cell- 

 walls to be interpolated, which radiate out from the longitudinal axis X X'm radial 

 directions to the surface EE of the growing-point. These are the radial longitudinal 

 walls, of which, however, only four can cross one another in the longitudinal axis 

 XX : the others only commence further outwards, and will, according to our 

 construction, exhibit approximately the same course as the medullary rays on the 

 transverse section of a woody body. Or, in other words, the transverse section 



FIG. 289. 



of a paraboloidal growing-point shows concentric layers of cells which at the same 

 time exhibit radial rows, splitting up more and more as we proceed outwards. 



Growing-points of this description, as a matter of fact, are very common both 

 among roots and shoots. They may, however, also be formed quite otherwise, and 

 especially so that two successive longitudinal sections standing at right angles have 

 different diameters or parameters — i.e. the growing-point appears compressed on 

 the one side and dilated on the other, and accordingly the network constituted of the 

 anticlinal and periclinal walls will then also present different forms. However, here 

 again this indication must suflSce. 



In a large number of Algse and most Hepaticae, all true Mosses, Equisetums, 

 Ferns, and Selaginellas, there is found at the apex of the growing-point of the shoots 

 and roots a cell, which is usually characterised both by its size and shape, and 

 which occupies the actual apical region of the growing-point : by the repeated divisions 

 of this cell into two, as it itself goes on growing, daughter-cells become cut off in 

 definite sequence, from the further growth and repeated cell-divisions of which the 

 whole of the tissue of the growing-point in question arises. These daughter-cells cut 

 off from the apical cell are termed segments, and Naegeli, who first described this 



