EQUISETACE.E. 



3^9 



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 wall ; one of the upper transverse cell-layers of 

 the whorl of segments projects outwardly, forms the apex (the circular apical line) 

 of the wall (Figs. 273, 274, ds), and those of its cells which lie most on the outside 

 (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 

 wall becomes a sheath enveloping the end of the stem. This same layer, of which 

 the outermost cells form the apical line of the annular wall, produces in the interior 

 of the sheath a tissue in which the fibro-vascular bundles of the leaf-sheaths arise. 

 The lower transverse cell-layers of the whorl of segments grow only slightly outwards 



Fig. 276. — External view of three teeth of a young 

 leaf-sheatli of Eq%usetnni Telmateia. 



Fig. 275. — The same as Fig. 274, but at a greater distance from the apex, showing a further advance of the differentiation of leaf- 

 sheath and internode ; r r cortex of the upper, r' r' r' cortex of the lower internode, e e the inner, e' e' the outer epidermis of the 

 leaf-sheath, ^^ the foliar portion of the fibro-vascular bundle, g" g" g^ '^ts descending portion belonging to the internode ; the first 

 annular vessel is formed at their point of meeting. 



and upwards, become divided by vertical and afterwards rapidly by transverse walls, 

 to produce the tissue of the internode, which passes gradually into that of the 

 leaf. A vertical layer of this tissue forming a hollow cylinder (Fig. 274, v v) is 

 distinguished by numerous vertical divisions ; it forms a ring of meristem (or 

 thickening-ring in Sanio's sense), in which the vertically descending fibro-vascular 

 bundles of the internode are formed. These bundles form the prolongations of 

 those of the leaf- teeth, which they meet, as shown in Fig. 275, g g\ at an obtuse 

 angle, and coalesce to form curved ' common ' bundles. The layers of cells which 

 lie outside this 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 



B b 



