368 



VASCULAR CRYPTOGAMS. 



half-segment is then again halved, in the most usual case, by a vertical nearly radial 

 wall. The segment now consists of four cells, two of which lie one above the other 

 and reach as far as the centre, but the other two do not because the vertical wall is 

 not radial but intersects one of the lateral walls of the segment (the anodal w-all) 

 (Fig. 273, E). Divisions now take place without any strict rule in the four cells of 

 each segment parallel to the primary and the lateral walls ; and tangential divisions 

 also soon make their appearance, by which the segment is split up into inner and 



outer cells, in which further divisions 

 afterwards take place. The former 

 produce the pith, which is soon 

 destroyed as far as the septum at 

 the base of each internode by the 

 expansion of the stem; the latter 

 produce the leaves and the entire 

 tissue of the hollow internodes. The 

 segments are, as has been men- 

 tioned, disposed originally in a 

 spiral with i arrangement ^ and 

 since each segment without excep- 

 tion (as in Mosses) produces a leaf 

 or what corresponds to a part of a 

 leaf-sheath, the leaves of Equise- 

 tum must also be inserted on a 

 spiral. This does, in fact, some- 

 times occur when the growth is 

 abnormal ; but when the growth 

 is normal, a small displacement 

 takes place at a very early period, 

 of such a nature that the three seg- 

 ments which form a cycle always 

 become arranged into a disc trans- 

 verse to the stem, their outer sur- 

 faces thus forming an annular zone 

 or girdle. According to Rees, to 

 whom this observation is due, the 

 three segments of each cycle are 

 formed in rapid succession, while 

 a longer time elapses between 

 the formation of the last segment 

 of the preceding and that of the first of the succeeding cycle. Thus by the un- 

 equal growth of the segments in longitudinal direction each cycle of segments or 

 turn of the spiral produces a whorl, which therefore, strictly speaking, is a pseudo- 

 whorl, because resulting from subsequent displacement. Each whorl of segments 

 now 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 



Fig. 274.— Left half of a radial longitudinal section beneath the apex 

 of an underground bud of Hqm'setitm Tehnateia in September : pK 

 lower part of the vegetative cone, // b" b'" leaves, bs their apical cells, 

 r" r'' r'" the cortical tissue of the corresponding internodes ; vi m 

 pith, vvv thickening ring, g g layer of cells from which the fibro- 

 vascular bundles of the leaf-tooth arises. 



