THE PRIMARY MERISTEM AND THE APICAL CELL. 



123 



Fig. 10 r, I^. 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. The sextant-cells 

 are still further broken up by vertical walls into an outer larger and an inner smaller 

 cell (Fig. 1 01, ^); 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. loi, A. In the outer layer the divisions parallel to the 

 })rincipal walls and in 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 its 

 hollowness ; from the outer layer of tissue of the primary meristem are also formed 

 downwards the cortex, the system of the fibro-vascular bundles, and later the 

 epidermis \ The external conformation also of Equisetum is brought about 

 by the outermost layer of the primary meristem, as has already been shown in 

 Fig. 10 1, A, where the protuberances x, j>, 6, bs represent the rudiments of the 

 leaves; processes to which I shall recur at length hereafter. Here it need only 

 be mentioned that each three consecutive segments undergo at an early period 

 a small vertical displacement, of such a nature that they form, at least with their 

 outer surfaces, a diagonal belt, which becomes arched 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 

 Fern-root, with which the greater number of roots of Cryptogams agree in the main. 



riG. 102 —Apical region of .i Fern-root ; A longitudinal section through the end of the root of Pteris hastata; 

 Ji transverse section through the apical cell and adjacent segments of the root of Asplenium FUix-fceniina (after 

 Nageli and Lcitgeb). 



Fig. 102, A, shows the axial longitudinal section through a Fern-root, with the point 

 turned upwards. From the apical cell v arises not merely the tissue of the substance 

 of the root (<?, <:), but also the root-cap k, /, m, n, a mass of tissue which covers like a 

 helmet the puficlum vegetationis of every root. The apical cell in this case resembles 

 those of the stem of Equisetacece and of many other Cryptogams, in so far as it 

 presents a three-sided pyramidal segment of a sphere; this form is sufficiently 



Compare Book II, Class of Equisetacere, and the formation of their tissue. 



