THE PRIMARY MERISTEM AND THE APICAL CELL. 
segments are to be seen from without. The formation of tissue begins by each 
segment dividing into two, soon after its production, by a partition parallel to the 
principal walls; the new half-segments are indicated in B, C, and D by i, i. Since 
in each of these two half-segments the further processes are almost exactly the same, it 
is necessary to keep in view only one of them. Each half-segment becomes divided 
first of all by a curved vertical wall, which meets internally a side-wall, externally the 
outer wall of the segment at its middle (Fig. iir, E). Since three segments com- 
pose a transverse section of the stem, and each half-segment divides into two cells, the 
Fig. HI.— Apical reg-ion of the stem of an Eqnisetian; A longitudinal section of an underground very strong bud of 
] '. maximum, in September (x SSo) ; B view of the apex from above (both from nature) ; C, Z>, E the same of E. arvetts-e (after 
Cramer). C diagrammatic ground-plan of the apical cell and of the youngest segment; D external view of the apex of 
a slender stem; E transverse section through this at D, /; 5 is in all cas»s the apical cell, /, //, ///, &c. the segments; 
I, 2, 3, &c. the division-walls iu the segments in the order of their formation ; x, y, b, bs in A the first rudiments of leaves. 
section of the stem now appears as if composed of six cells or sextants, whose walls 
are placed nearly radially, forming a six-rayed star, as is shown in the transverse 
section Fig. iii, E, Hence the walls by which this division is brought about are 
called sextant-walls; in C and D they are indicated by the figure 2. Each of the 
sextant-cells is still further broken up by vertical walls into an outer larger and 
an inner smaller cell (Fig. iii, ^) ; and thus the foundation is laid of the two 
layers of tissue into which the primary meristem separates, viz. into an outer and 
an inner layer, as is clearly shown in Fig. iii, A. In the outer layer divisions 
