45« 



LECTURE XXVII. 



sides. This can of course only be recognised with certainty when the apical cell is 

 observed from above, or in transverse section ; since such a tetrahedral apical cell 

 seen in longitudinal section presents a very similar appearance to the biseriate 

 ones described above. This state of affairs will be clear on regarding Fig. 293. 

 A represents the longitudinal section, and B the view from above of the apical region 



Fig. 293. — Apical region of the roots of Ferns. ^ longitudinal section through tile end of the root 

 of Pteris hasiaia. B transverse section through the apical cell and the surrounding segments of the 

 root oi Asfieniumjttix/emtna (afi^r Naegeli and Leitgeb). 



of the growing-point of the root of a Fern, » being the apical cell in each case. In B 

 the numerals I- VIII maxk the transverse sections of the concentric segments of the 

 apical cell v. in A these are to be recognised in the longitudinal section by their walls 

 being drawn with thicker contours. In this example there is added a further com-: 

 plication, however, since we are concerned with a root, and accordingly also with a 

 root-cap. This root-cap is indicated in Fig. 293 A by means of the letters klmn, 

 and its cap-shaped superposed layers are in fact also segments of the apical-cell v, 

 which have been produced by successive transverse divisions which have then grown 



further.. In the growing-point of a shoot 

 ^ with a tetrahedral apical cell these caps 



klmn would be absent. It is not quite 

 so easy to see that even such tetrahedral 

 apical cells obey the law of rectangular 

 intersection of the division-walls ; and even 

 the most distinguished observers in this 

 diflScult province have for years erroneously 

 supposed that the segment-walls of a te- 

 trahedral apical cell cut one another at 

 angles of 60°, because such appears to 

 be the case from the view in transverse 

 section, which represents ah equilateral triangle. I have shown, however, that 

 such an apical cell is correctly apprehended by supposing a corner so cut off from 

 a cube that the triangular surfaces which meet in the corner are equal to one another, 

 and, in addition, that the four bounding surfaces are arched outwards, as shown 

 by the lines abc in Fig. 294. This represents the upper view of a tetrahedral apical 

 cell, and the walls de,/g, hk are the successive segmentations which appear in it: 



Fig. ^94. 



