i8 



INTRODUCTION. 



In the roots of the Equisetums, Polypodiaceae, and Marsileacese (Fig. 7, 8) the d'visiott 

 of each segment {h, h. Fig. 7) begins with the appearance of the sextant wall s This 

 stands vertically, and, as before stated, is nearly, but not exactly radial; it is attached to 

 the middle of the outer wall of each segment, its inner edge, Jiowever, does not extend 



Fig. 7.— Scheme of the snccession of cells in'the *pex of the root of "Equisetum Iiiemale, after Nageli anfl Leit-' 

 geb. ..4 long^itudinal section ; i^transverse section at the lower end of^; ^principal walls, f sextant walls, f<'cainbial ' - 

 wall ') the first, e (epidermal wall) the second, r (cortical wall) the third tangential wall ; the successive further tangential 

 divisions between c and r are figured.!, 2, 3, . 



', In A the figares I— XVI denote the successive segments ; the letterr k, I, m, n, f, tlie successively older 

 portions of the root cap ; o epidermis (dennatogenj. From Sachs* Textboolc. 



to the central angle of the latter, but, -curving slightly, meets the central part of the 

 lateral wall further from the centre than the angle. The convexities of successive sextant ' 

 walls are as a rul^, but not always, homodromous, and turned toward the ascending side 

 of the segmental spiral. The sextants of one transverse section are therefore alternately 

 unequal in form and size, according to the distance of the point of junction of the sextant ; 

 walls from the angle of the segment ; among the cases observed tliis inequality is greatest 

 in Equisetum, least in the Marsileaceae. The inequality of the quadrants, octants, etc. of , 

 a transverse section from the above-named plants with two series of segments depends | 



FIG. 8.— (230) A longitudinal section tlirough the apex of the roof of Pteris hastata. S transverse Action through ■ 

 the apical cell of the root and the neighbouring segments of Athyrium filix femina, both after Nageli and Leitgeb ; ... 

 Zf apical ceil J the other letters and figures as in Fig. 7. From Sachs' Textbooic. 



upon similar conditions. Each sextant is in the second place divided by a tangential wall 

 (c) into an inner cell which is usually small, and a larger outer one : the difference in size i 

 between the two is greater the thinner the root is, but, as stated, always so that the outei':! 

 cell has the advantage. The inner cell is the initial cell of the plerome, the outer is in the 



