PTERIS-RHIZOME 167 



segmental cells, and it may readily be seen that these again 

 undergo subdivision. If the section be not sufficiently trans- 

 parent, it may be treated with very dilute potash and weak gly- 

 cerine, or, better, with " eau de javelle," which will clarify the 

 tissues, and make the cell-walls more distinct. 



The form of the apical cell, and of the segmental cells which 

 surround it, will be readily appreciated on comparison of Fig. n. 



FIG. ii. View of a model of a three-sided pyramidal (01 tetrahedral) apical cell, as 

 seen from above ; the walls d e,f g, h k, denote successive walls by which seg- 

 ments have been cut off from the growing apical cell : i is the apex of the 

 pyramidal cell, at which point the three youngest segmental walls cut one another. 

 (After Sachs.) 



XI. From the apex of another plant cut median longitudinal 

 sections : mount in weak glycerine : a very little dilute potash 

 may be added if the sections are not transparent enough, or 

 they may be treated first with " eau de javelle," and then be 

 mounted in glycerine. 



If any one of the sections has passed through the apical cone, 

 in a median plane, the apical cell will be seen presenting a 

 wedge-like appearance, and the cells around it will show, in the 

 regularity of their arrangement, that they have been derived 

 from segments successively cut off from the apical cell. 

 (Compare Fig. 12.) It may be concluded from the observation 

 of transverse and median longitudinal sections that the form of 

 the apical cell is that of a three-sided pyramid. 



