i8 



INTRODUCTION. 



In the roots of the Equisctums, Polypodiaceae, and Marsileaceae (Fig. 7, 8) the division 

 of each segment (/', 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, however, does not extend 



A 



•Fig. 7. — Scheme of the succession of cells in the '>pe:c of the roit of Equisetum hiemale, after Nageli and Leit- 

 geb. y1 longitudinal section ; /^transverse section at the lower end of-^ ; h principal walls, jsextant walls, c(*cambial 

 ivall ') 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 i, 2, 3. 



In j4 the figures I — XVl denote the successive segments ; the letters i, I, m, fi, />, the successively older 

 portions of the root cap ; o epidermis (dermatogen). From Sachs' Textbook. 



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 rule, 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 this 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. — (250) A longitudinal section through the apex of the root of Pteris hastata. M transverse section through ■' 



the apical cell of the root and the neighbouring segments of Athyrium filix feniina both after Nageli and Leitgeb ; 

 V apical cell; the other letters and figures as in Fig. 7. From Sachs' Textbook. 



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

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

 between the two is greater the thinner the root is, but, as stated, always so that the outer 

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



