872 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
the study of accurately longitudinal sections is necessary in order to complete the 
argument. 
Tangential sections passing through the primary wood of the bundles are of special 
value, as in them the position of the canals is clearly shown, and there can be no 
doubt that we are really examining the fascicular passages, and not any chance 
lacunze. Such a section is represented in Plate 80, fig. 21. The part figured touches 
on two canals, and shows the partly disorganized protoxylem and primary xylem of two 
bundles.* In all such preparations, when sufficiently well preserved, remains of the 
primitive trachez are found in the canals and can be recognized at once, though more 
or less broken up, by their annular, spiral, or reticulated thickenings. By careful 
comparison of sections in somewhat different planes it is easy to prove that the inner- 
most trachez of the solid xylem-strand are themselves somewhat disorganized, the 
disruption becoming much greater towards the interior, where the canal itself is 
reached. 
Having become familiar with the appearances presented in oblique and tangential 
sections, the canals can easily be recognized with certainty in radial view.t (See 
Plate 77, fig. 5; Plate 78, fig. 10.) Where remains of trachez are found quite at the 
jnner margin of the canal, they are usually of the annular type, with remote rings. 
Many of the elements are broken up in such a way that a series of several consecutive 
rings is left in position ; then there is a gap, then another short series of rings, and 
soon. The nearer the trachez are to the inner side of the canal, the longer are the 
gaps, and the shorter the series of persistent rings. (See Plate 77, fig. 5.) Evidently 
here, as in Equisetum, the development of the primary xylem is centrifugal; the 
innermost elements have been differentiated earliest, and have consequently under- 
gone the greatest amount of disruption during the extension of the internode. In 
many of the tracheee the rings are connected here and there to form short spirals or 
reticulations. 
In some cases, a radial section passes through a canal so as to show its termination 
at anode. Here it is clear that the protoxylem in the internodal canal is continuous 
with the innermost elements of the nodal xylem, ‘This point is illustrated in Plate 78, 
fig. 10. 
In no case, except where the preservation is bad, has a canal been examined in 
longitudinal section, without traces, at least, of the protoxylem being discovered. 
From these observations we conclude: that the internodal canals of Calamites 
represent the earliest-formed xylem of the primary vascular bundles; that this 
primitive wood became disorganized and ruptured, owing to the longitudinal and 
transverse extension of the growing internode; and that the formation of the canal 
was due to a tearing of the tissue caused by the presence of a strand of inextensible 
lignified elements among the actively-growing parenchymatous cells. 
* Other tangential sections showing the same facts are O.N. 24, 37, 38, 49, 91, 130*, and 1937, 
+ Kspecially good radial sections for this purpose will be found in C.N, 20, 20a, 21, 22, 48, and 1937, 
