THE PRIMARY MERISTEM AND THE APICAL CELL. 
tissue of the root \ but other segments also which build up the Rooi-cap. These 
latter are cut off from the apical cell by septa in such a manner that they cover 
them like a cap ; and every such segment belonging to a root-cap is hence termed 
simply a Cap-cell. According to the investigations of Nageli and Leitgeb, it 
appears to be the rule that whenever three segments have been formed (from the 
substance of the root), a new cap-cell arises ; but this rule is not always stricdy 
adhered to. 
The cap-cell increases quickly in breadth ; and its transverse section, originally 
spherically triangular, becomes circular. It is simultaneously divided into two equal 
halves by a vertical wall (parallel to the axis of the root) ; in each of these halves 
a wall again arises at right angles to the former ; and thus four quadrant-cells are 
formed. Each quadrant again breaks up into two cells (octants), the further divisions 
varying in different species. In the successive layers of the cap the quadrants are 
not superimposed but alternate ; i. e. the quadrant-walls of one layer deviate from 
those of the preceding and following ones by about 45°. 
The growth in length of the substance of the root, in so far as it is occasioned 
by divisions of the apical cell, proceeds, as has already been indicated, in such a 
manner that the septa which arise in spiral succession are parallel to the sides 
of the apical cell. Each segment-cell is bounded by five walls, as at the apex 
of the stem of Equisetiim, — two principal triangular walls, two oblong side-walls, 
and one somewhat convex outer wall, in contact with a root-cap. The first wall 
which arises in each segment-cell stands at right angles to the principal walls, and 
is, with reference to the whole root, a radial longitudinal wall. Two cells arise 
in this manner side by side, unequal in form and size, the septum meeting internally 
a side-wall, but externally the middle of the outer wall. In this manner the trans- 
verse section of the root, composed at first of three segment-cells, breaks up into six 
cells or sextants (compare the processes described above in the stem of Eqidsetuni) ; 
three of these sextants reach to the centre of the section; but the three which 
alternate with them do not. The sextant-walls are seen in Fig. 112, B, in the 
segments IV, V, VI, VII, as lines dividing the outer wall in half; in a deeper 
transverse section they would form, together with the three side-walls of the three 
segments, a six-rayed star, similar to that in Fig. iii, E'^. Each sextant-cell is next 
divided again by a wall parallel to the surface of the root into an inner and an outer 
cell ; in the transverse section of the root (/. e. in the corresponding transverse section 
beneath the apex), twelve cells can therefore be recognised at this stage, of which 
the six outer ones form a peripheral layer, and the six inner ones a central nucleus. 
The longitudinal section. Fig. 112, A, shows this wall at c c, and it may thus be seen 
how the mass of the substance of the root is broken up by it into an outer layer 0 c 
and an inner thick bundle c c c c. Out of the former arises by further division a 
tissue which becomes differentiated further backwards into epidermis 0 and cortex 
(between 0 and c) ; the axial bundle c c c c, oxi the other hand, which is the result 
of further longitudinal divisions of the inner sections of the sextants, forms the 
procambium-cylinder of the root, in which the vascular bundles arise. In this 
^ They are bounded by thicker lines in the longitudinal section A. 
^ Compare Book II, EquisetaccK, diagram of root, Fig. 284. 
I. 
