EQUISETINEM. 
397 
The end of the stem, enveloped by a large number of younger leaf-sheaths, 
terminates in a large apical cell, the upper wall of which is arched in a spherical 
manner, while the three infero-lateral walls are almost plane. The apical cell 
has therefore the form of an inverted triangular pyramid, the upturned basal 
surface of which is a nearly equilateral spherical triangle. The segments are cut 
off by walls which are parallel to the oblique sides of the apical cell, that is, to 
the youngest primary walls of the segment; the segments, disposed in a spiral J 
arrangement, lie in three vertical rows. Each segment has the form of a triangular 
plate with triangular upper and under walls, rectangular lateral walls lying right 
and left, and an outer rectangular wall which is curved. Each segment is first 
divided — as was shown by Cramer and Reess and confirmed by myself — by a wall 
parallel to the upper and under surfaces into two equal plates lying one above 
Fig. 278. — Eq2nsetiim Tebnateia ; A piece of an uprig-ht stem (natural size), i i' internodes, h its central cavity, 
/lacunae of the cortex, S leaf-sheatb, z its apex, a a' a" the lower internodes of young shoots; i>' longitudinal section 
of a rhizome (X about 2), A septum (diaphragm) between the cavities k h, g- fibro-vascular bundle, / lacunte of the cortex, 
^leaf-sheath; C transverse section of a rhizome (X about 2), ^- and / as before ; D union of the fibro-vascular bundle of 
an upper and lower internode i i', A' the node. 
another, and consequently each half the height of the undivided segment. Each 
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 He one above the other 
and reach as far as the centre, but the other two do not because the vertical wall is 
not accurately radial but intersects one of the lateral walls of the segment (the 
anodal wall), (Fig. 279, 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 
