904 PROFESSOR W. C. WILLIAMSON AND DR. D. U. SCOTT ON THE 
types of stele are present in different specimens. ‘The one, as seen in transverse 
section, has a bluntly triangular form, with the vascular bundles situated at the 
prominent corners (see Plate 80, figs. 23 and 24). In the other type, the stele is 
approximately quadrangular, with a bundle at each angle (see Plate 74, photograph 14 ; 
and Plate 80, fig. 25). In the specimen photographed the angles are especially 
prominent, and the sides concave, so that the whole section of the stele is somewhat 
ernciform. The triangular and quadrangular types are quite distinct. The difference 
between them does not depend on the level at which tbe transverse section is taken ; 
for we find the two structures indiscriminately in sections passing through the 
sterile node, the fertile node, or an intermediate internode. On the whole the 
triangular form is the more frequent of the two. We found the quadrangular form 
of stele only in cases where the number of sporangiophores in a verticil is 8; the 
converse, however, does not hold good, for in one preparation showing eight 
sporangiophores, the stele is distinctly of the bluntly triangular type.* 
We will describe this type first. The whole middle portion of the cylinder, 
forming much the greater part of its mass, so far as primary structure is concerned, 
consists of the pith. The whole of the pith appears to have been persistent ; some- 
times a few cells near the centre have disappeared (Plate 80, fig. 23), but this is 
obviously due to imperfect preservation. In many cases the outer zone of medullary 
cells has thickened walls, while those nearer the middle are more delicate (Plate 80, 
figs. 23 and 24). In other specimens the entire pith is thick-walled throughout.t 
It is often difficult or even impossible to distinguish the thick-walled medullary 
cells from tracheides, in a transverse section. From this difficulty the mistake arose 
of regarding the stele of Calamostachys as a solid vascular axis, a mistake which 
was corrected in a previous memoir published in 1889.t 
Longitudinal sections show that the cells of the pith are elongated and often 
prosenchymatous, especially those towards the periphery (see Plate 81, fig. 27). 
The vascular bundles are placed at the projecting angles of the triquetrous stele. 
It is often difficult to say whether one bundle or two are present at each corner. In 
some of the clearest sections it is evident that there are two, so that the. total 
number of bundles in the cylinder is, in these cases, six (see Plate 80, figs. 23 and 24).§ 
At the inner margin of each bundle is an empty space, less definite and regular 
than an internodal canal in Calamites, but constantly present, except at the sterile 
nodes. These spaces or canals always contain the disorganized remains of spiral or 
annular tracheides, and evidently mark the position of the protoxylem-group of the 
pericycle (see vAN Tincnem, “Sur la Polystélie,” ‘Aun. des Sci. Nat., Bot.,’ sér. 7, vol. 3, 1886; and 
‘Traité de Botanique,’ 2nd edition, p. 765.) 
* O.N., 1898. 
t Sec Wirtiamson, “ Organization,” Part V., Plate 6, fig. 38. (C.N. 989.) 
~ Witu1aMson, “ Organization,” Part XV., p. 160; Plate 2, fig. 7. 
§ See also the figure in Winntamson, “ Organization,” Part X., Plate 15, fig. 15, 
