INTRODUCTION. 31. 



'to form the body of the root. In, longitudinal section therefore in the younger 

 transverse zones, each successive segment is laterally overlapped by a new cap (or, 

 as the case may be, by a pair of them). There are, however, exceptions to this 

 rule. Further, each cap-cell abuts on the principal walls of the cycle of segments 

 cut off immediately before it, and this condition remains often for a long time 

 recognisable by this character, that each cap rests with its margin upon the step-like 

 outer walls of two successive segments. This arrangement is obliterated sooner or 

 ■later by the smoothing down of the steps resulting from growth. 



We must now return to the phenomena in many Selaginellas, and in the 

 Marattiaceae, which were before left unexplained. 



As before stated, a number of species of the first-named genus have on the 

 stem a two-sided apical cell, which forms segments from its two sides. RuSsow ' 

 first drew attention to the fact that in many species — e.g. S. arbor escens, Pervillei, 

 Walltchii, Lydllii — there is not a single apical cell, but an apical group of common 

 initial cells. Strasburger "^ has carefully investigated S. Wallichii, and found that here, 

 in place of one apical cell, two are present, which form segments in conjunction with 

 one another. Each of them has the form of a wedge with narrow rectangular 

 section, and is bounded lay five planes ; i. e. two nearly equal lateral planes, in form 

 of isosceles triangles, the .bases of which are the long sides of the rectangle presented 

 by the cell in transverse section ; two narrow-rectangular lateral planes, and a fifth 

 also narrow-rectangular, which, is the free apical plane : the lateral planes, like those 

 of simple apical cells, are sunk in the meristem. Both cells are joined by one of 

 their broad triangular lateral faces into a double wedge of correspwuding form, and 

 the relative position of this is such that the two triangular lateral faces are 

 perpendicular to the dorsal and ventral faces of the (bilateral) stem, while the 

 joint wall of the pair of apical cells is in a median position. One may therefore 

 shortly name the broad triangular faces lateral, and the narrow rectangular ones the 

 upper and lower. Segments are formed similarly in each of the two apical cells in 

 the following succession. First a principal wall parallel to the lateral faces cuts off a 

 segment almost similar to the apical cell, then two narrow segments of rectangular 

 section are cut off by principal walls parallel to the uppver and lower sides. After 

 these follow again lateral segments, &c. Thus four straight rows of segments arise, 

 as from a single four-sided apical cell, the series right and left being wedge-shaped, 

 while the upper and lower segments are rectangular, and are arranged in each case 

 in a double series. The latter form the ventral and dorsal portions of the stem, 

 the former the lateral portions. 



In the Marattiaceae the apex of the stem is as yet but little investigated. 

 Hofmeister (Beitr, II) ascribes to Marattia ' cicutcBfoUa a three-sided apical cell. 

 The meristematic apex of the root of these plants, as shown by Harting ', and more 

 carefully described by Russow (I.e., p. 107), consists of a numerous group of large, 

 polygonal, pyramidal, common initial cells ; cap-cells are cut off from these by trans- 

 verse walls near their broader, outer, or apical face : from these the root-cap is formed : 

 . near their inner face, cells are cut off which as initial cells form the plerome cylinder. 



' Vergl. Untersuchungen, p. 176. [Cf. Schwendener, iiber Scheitelwachsthutn mit mehreren 

 "Scheitelzellen, Botan. Zeitg. 1880, p. 716.J _ ^ Botan. Zeitg. 1873, p. 115. 



' DeVriese et Harting, Monogr. des Marattiacees, p. 41, Taf. 4. 



