262 



THIRD GROUP. — VASCULAR CRYPTOGAMS. 



cells of the segments, which form an annular cushion ; one of the upper cell-layers of 

 the segment projects most strongly outwards, forming the apex (the circular apical line) 

 of the cushion (Figs. 214 A ds, 215 d'), and its cells which lie most outside divide by 



FIG. 215. Eqnisetum Tebnatfja. Left half of a radial longitu- 

 dinal section beneath the apex of an underground bud in September ; 

 ■vK lower part of the vegetative cone, *', b", V" leaves, bs bs their 

 apical cells, r' r" r"> cortical tissue of the corresponding intemodes, 

 »i m pith, z'W thickening-ring, ff g cell-layer from which the vascu- 

 lar bundle of the tip of the leaf is formed, z!j' apical cell of a branch. 



Fig. 216. The same as the preceding figure, but at a greater 

 distance beneath the apex, and showing the more advanced 

 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, ff £r the Umb of the vascular 

 bundle belonging to the leaf, ^ s' g" the descending limb be. 

 longing to the internode ; the first annular vessel is formed 

 n here the two limbs meet. 



walls inclined alternately to and from the axis of the stem, while the circular apical 

 line is constantly raised, and thus the annular cushion developes into a sheath which 

 envelopes the end of the stem. This layer of cells, the outermost cells of which form 



the apical line of the annular cushion, forms 

 a meristem in the interior of the sheath, 

 in which the vascular bundles of the sheath 

 originate. The lower layers of cells of the 

 whorl of segments do not grow much out- 

 wards and upwards ; they divide by vertical, 

 and subsequently and more vigorously by 

 transverse walls, and so produce the tissue 

 of the internodes, which passes continuously into the tissue of the leaf; an inner layer 

 of this tissue forming a hollow cylinder (Fig. 215 77, z^) is distinguished by its many 



Exterior view of three teeth of . 

 sheath of Equisetttm Tehnateja. 



