144 
MORPHOLOGY OF TISSUES. 
parallel to the principal walls and in the vertical radial direction at first preponderate ; 
in the inner layer the divisions are less numerous, so that the cells become more 
uniform in diameter. This inner mass of tissue, arising from the inner sections 
of the sextants, is the pith which splits as the stem developes, dries up, and thus 
causes the hollowness of the stem ; from the outer layer of the primary meristem 
are also formed further from the apex the cortex, the fibro-vascular bundles, and 
later the epidermis \ The external organs of Equisetum are also derived from the 
outermost layer of the primary meristem, as has already been shown in Fig. iii, ^, 
where the protuberances x, b, bs represent the rudimxcnts of leaves. To these 
processes I shall recur hereafter ; here it need only be mentioned that each set of 
three consecutive segments undergoes at an early period a small vertical displace- 
ment, so that at least their outer surfaces form a horizontal zone which then 
bulges out and is the origin of a leaf-sheath. 
As a final example of the formation of the primary meristem from an apical 
cell, we may now consider the processes that take place at the growing end of a 
Fig. 112.— Apical region of a Fern-root; A longitudinal section through the end of the root oi Pteris hasta/a; 
/.'transverse section through the apical cell and adjacent segments \.\\&xoo^ oi Asplenium Filix-fcemina (after 
Nägeli and Leitgeb). 
Fern-root, with which the greater number of roots of Cryptogams agree in the main. 
Fig. 112,^ represents an axial longitudinal section through a Fern-root, with the 
apex uppermost. From the apical cell v arises not merely the tissue of the sub- 
stance of the root o, c, but also the root-cap k, /, 7?i, n, a mass of tissue which 
covers like a helmet the growing point of every root. The apical cell in this case 
resembles that of the stem of Equisetaceae and of many other Cryptogams, in so 
far as it presents a three-sided pyramidal segment of a sphere; this form is 
sufficiently seen by comparing the longitudinal section A with the transverse 
section B. Here also three straight rows of segments are formed by successive 
divisions of the apical cell, which are numbered according to their order in age, 
/, //, ///, &c., in Fig. B ; and here also a spiral is described by the line con- 
necting the centres of the consecutive segments. The great diff"erence between 
the apices of roots and the growing stem of Cryptogams is however, that in the 
former the apical cell not only produces these segments which build up the 
^ Compare Book II, Class Equisetaceae, under the formation of their tissue. 
