XIII EQUISETINE.-E 439 



The next walls are like the sextant walls 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, 

 apparently not always in the same order, parallel with the 

 primary and lateral walls, and very soon there are pcriclinal 

 divisions by which an inner cell is cut off from each segment 

 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 Equisetuni. 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 by a row of marginal 

 cells, which are the initial cells, and from which segments are cut 

 off alternately upon the inner and outer sides (Fig. 233, A). 

 The growth is stronger at certain points, which, according to 

 Rees,^ have a definite relation to the early divisions. Thus in 

 E. scirpoides the teeth are always three, and correspond to the 

 primary nodal cells ; in E. arvense there are six or seven, in 

 the first case corresponding to the sextant cells, in the latter to 

 the sextant cells plus the first division in one of them. In the 

 large species, like E. telmateia, it is difficult to trace any such 

 relation. In most forms, by subsequent dichotomy of some or 

 all of the primary teeth, others are formed, so that the number 

 in the fully-developed sheath exceeds that first formed. As 

 soon as the young sheath begins to project, a section through 

 one of the teeth shows that it is divided into an upper and 

 lower tier of cells, the apical cell terminating the upper one. 

 This division no doubt corresponds to the first horizontal 

 division in the outer nodal cell from which the leaf- tooth 

 originally comes. In one a little older (Fig. 233, B), in this 

 upper tier of cells a line of cells occupying the axis is evident 

 {fb), extending from the base of the leaf nearly to the summit, 

 and growing at its outer end by the addition of cells derived 

 from the inner part of the youngest upper segments of the 

 terminal cell of the leaf.- This is the beginning of the single 

 vascular bundle found in each leaf 



^ Rees (2), p. 228. 

 - Each tooth is here regarded as a leaf, the sheath as a circle of confluent leaves. 



