460 



AIOSSES AND FERNS 



CHAP. 



the teeth of the sheaths ahernate. Each cycle of three seg- 

 ments comes to he practically in the same plane, and consti- 

 tutes a disc which later forms a node and internode of the stem. 

 Each segment is first divided by a wall nearly parallel to the wall 

 by which it was cut off from the apical cell, into two overlying 

 cells. The upper cells or semi-segments give rise to the nodes, 

 the lower to the internodes. 



The next walls are like the sextant w^alls in the roots of 

 the Ferns, and a cross-section just below the apex presents 

 exactly the same appearance. Each cell now divides by walls, 



Fia 267. — A, Median section of a strong- subterranean (vegetative) bud, X30; k, 

 lateral bud; B, the apex of the same section, X200. 



apparently not always in the same order, parallel with the 

 primary and lateral walls, and very soon there are periclinal 

 divisions bv which an inner cell is cut off from each ses^ment 

 cell that extends to the centre. This primary group of central 

 cells is the pith, which later in the internodes is usually torn 

 apart and destroyed, leaving the large central hollow met with 

 in all the larger species of Eqiiisetiim. From the outer cells 

 are developed the leaves, the vascular bundles, and cortex. 



The annular leaf-sheaths begin as outgrowths of the super- 

 ficial nodal cells of each cycle of segments, and these form a 

 circular ridge or cushion running round the base of the apical 

 cone. The summit of this ridge is occupied l>y a row of mar- 

 ginal cells, which are the initial cells, and from these segments 

 are cut off alternately upon the inner and outer sides (Fig. 272, 



