PTERIDOPHYTES. 115 



All of the organs of the fern grow from a definite apical cell, but it is 

 difficult to study except in the root. 



Selecting a fresh, pretty large root, a series of thin longitudinal sections 

 should be made either holding the root directly in the fingers or placing 

 it between pieces of pith. In order to avoid drying of the sections, as is 

 indeed true in cutting any delicate tissue, it is a good plan to wet the 

 blade of the razor. If the section has passed through the apex, it will 

 show the structure shown in Figure 68, D. The apical cell (a) is large and 

 distinct, irregularly triangular in outline. It is really a triangular pyramid 

 (tetrahedron) with the base upward, which is shown by making a series 

 of cross-sections through the root tip, and comparing them with the longi- 

 tudinal sections. The cross-section of the apical cell (Fig. L) appears also 

 triangular, showing all its faces to be triangles. Regular series of seg- 

 ments are cut off in succession from each of the four faces of the apical 

 cell. These segments undergo regular divisions also, so that very early 

 a differentiation of the tissues is evident, and the three tissue systems 

 (epidermal, ground, and fibro-vascular) may be traced almost to the apex 

 of the root (68, D). From the outer series of segments is derived the 

 peculiar structure (root cap) covering the delicate growing point and pro- 

 tecting it from injury. 



The apices of the stem and leaves, being otherwise protected, develop 

 segments only from the sides of the apical cell, the outer face never 

 having segments cut off from it. 



