EQIJISETINEM. 



399 



forms a leaf-sheath, and the corresponding internode or joint of the stem. The 

 above-mentioned divisions take place in the three segments during their arrange- 

 ment into a transverse disc, each segment becoming converted into a mass of cells 

 consisting of from four to six layers. As soon as the transverse zone is formed, 

 the formation of the leaves commences by the growth of the outer cells of the 

 segments. They form an annular ridge ; one of the upper transverse cell-layers of 

 the whorl of segments projects out- 

 wardly, forms the apex (the circular 

 apical line) of the ridge (Figs, 279, 280, 

 bs), and those of its cells which lie 

 most externally (the apical cells) divide 

 by walls inclined alternately towards 

 and from the axis. The circular apical 

 line becomes more and more elevated, 

 and thus the annular ridge becomes a 

 sheath enveloping the end of the stem. 

 JThis same layer, of which the outer- 



lost cells form the apical line of the 

 mular ridge, produces in the interior 



>f the sheath a meristem in which the 



ibro-vascular bundles of the leaf-sheaths 



rise. The lower transverse cell-layers 



>f the whorl of segments grow only 



lightly outwards and upwards, become 



livided by vertical and afterwards rapidly 



^y transverse walls, to produce the 

 5sue of the internode, which passes 



radually into that of the leaf A ver- 

 tical layer of this tissue forming a hollow 

 cylinder (Fig. 280, v v) is distinguished 

 by numerous vertical divisions; it forms 

 a ring of meristem (procambium, thick- 

 ening-ring in Sanio's sense), in which 



the vertically descending fibrO-VaSCUlar fig. 280.— Left half of a radial longitudinal section beneath the 



J, r 1_ • J r 1 s^p&x of an vtvAergronnA hud oi Egutsetum Telmaieia m S&^ismher ; 



bundles 01 the mternOae are lOrmed. /viewer part of the vegetative cone. ^''iJ",^"' leaves, *j their apical 

 rp, 1 ji r 1 1 • cells, r* r" r'" the cortical tissue of the corresponding internodes ; 



1 heSe bundles lOrm the prolongations ^ ^ pith, v v v thickening ring, g g layer of cells from which the 



fibro-vascular bundles of the leaf-tooth arises. 



or those 01 the leaf-teeth, which they 



meet, as shown in Fig. 281,^/, at an obtuse angle, and coalesce to form curved 

 * common' bundles. The layers of cells which lie outside the ring of meristem 

 that gives rise to the bundles produce the cortex of the internode, in which air- 

 conducting canals soon arise. Even at an early period the first rudiments of the 

 sheath-teeth appear as protuberances at regularly distributed points, each of them 

 ending in one or two apical cells (Fig. 282) ^ 



^ On the original number and subsequent increase of the sheath-teeth, &c., compare Hofmeister 

 and Reess, /. c. 



