226 XVII. THE ROOT APEX. 



three side- walls, is sunk in the body of the root. The divisions, 

 as in the stem of Equisetum, take place successively parallel 

 to the side- walls, and, therefore, as it were, in spiral sequence ; 

 besides these, however, from time to time, usually after each 

 series of three lateral segmentations, a wall parallel to the eonvex 

 base is formed (compare the figure, at Jc). The apical cell retains 

 its form throughout its divisions ; the cell cut off from the base 

 has, however, approximately the form of a segment of a sphere. 

 This cell (k) is an initial cell of the root-cap, giving rise to one of 

 the layers constituting it. It divides first by a wall perpendicular 

 to its base into two halves ; each half repeats this division, so 

 that four approximately similar cells are formed. In these the 

 division is repeated, and always by walls at right angles to the 

 original base, so that an older layer of the cap (k n ) consists of a 

 large number of cells. The cells of the older cap layers are full 

 of starch grains. They are gradually disorganised, while the 

 apical cell cuts off continually new initial cells. The outer walls 

 of the, for the time being, outermost cells of the cap, are strongly 

 thickened. The cells which are cut off parallel to the side-walls 

 undergo quite regular segmentation, so .that the internal tissues 

 of the root originate from them in very definite fashion. Soon 

 a similar separation of histogens is effected to that present in the 

 root-apex of a phanerogamic root, and gives us the impression as 

 if the apical cell formed only a hiatus in the sheathing of the 

 histogens. The basal cell cut off from the apical cell, giving rise 

 gradually to the root-cap, may conceivably be looked upon as 

 equivalent to the dermatogen ; from which point of view r the root- 

 cap as a whole is all that represents the epidermis in the root of 

 the fern, and the outer layer, the ostensible epidermis, would be 

 the outermost layer of the cortex, just as the endodermis is its 

 innermost layer. If in the development of the root we leave out 

 of consideration the root-cap, and the processes which give rise 

 to it, we find processes of growth which in all essential points 

 resemble those existing in the growing apex of a stem. The 

 apical cell of roots, also, does not vary in form as that of a stem 

 does. 



If the student finds difficulty in cutting sections of such a thin 

 structure as a root-apex, he may have recourse to the assistance 

 of embedding, and cutting with a simple microtome such as is 

 described in Chapter XVI. In the last chapter of this book will 

 be described a more complex microtome, by means of which 



