ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 871 
Equisetum. Is this resemblance accidental, or are the two structures really homo- 
logous? If the latter alternative be true, then the canal, in Calamites as in Equisetui, 
must represent a disorganized strand of protoxylem—the first-formed part of the 
wood of the collateral bundle. It is impossible to follow the development in the 
fossil; as already stated, the youngest stages preserved have their canals fully 
developed. All that we can expect is to find recognizable remains of the elements 
which originally filled the cavity; if these elements have the character of primitive 
trachez, then the homology with the carinal canals of Hquwisetum may be taken as 
established. 
Happily, there is a long series of sections in the Williamson collection which serve 
to.set this question completely at rest. 
In every well-preserved transverse section it is common, or in fact the rule, to find 
isolated rings of about the diameter of a small trachea, within the canal. The resem- 
blance of these annular fragments to the remains of tracheze always seen in the carinal 
canals. of Hquisetum has long attracted our attention. The same point has been 
observed and figured by Mr. Cormack,* who, we believe, was the first to publish the 
true interpretation. Occasionally, where the canal is small, these rings may collec- 
tively occupy the greater part of its area.t In some cases, also, we can trace a transi- 
tion from the isolated rings in the canal itself, to the loosely arranged and partly 
disorganized tracheze which abut on its outer margin.{ Even in transverse sections 
the elements in the canal sometimes lie obliquely ; in these cases we can see that the 
rings are placed one above another, as in an annular trachea, while sometimes we 
find portions of a spiral or a reticulated cell-wall. That we have really to do with the 
remains of trachez is evident. The tracheal remnants are most frequent towards the 
outer edge, or at the sides of the canal. For reasons which will appear immediately, 
they are less commonly met with at its inner edge. Transverse sections, however, 
can rarely afford by themselves conclusive evidence as to the nature of the elements 
contained in the canal. It is only when we examine a decidedly oblique section of a 
well-preserved specimen that all doubt is removed. Such a preparation is represented 
_ in Plate 77, fig. 4. The specimen is a corticated one, but the curtex has not been 
shown in the figure. Here the section is sufficiently transverse to show clearly the 
position of the canals, and at the same time sufliciently oblique to leave no doubt as 
to the character of the elements within them. Annular, spiral, or laxly reticulated 
tracheze are evident in every canal, and sometimes extend to its inner margin. The 
gradual transition from these primitive trachee to the more scalariform elements 
towards the exterior can be clearly traced. Such a section is convincing by itself, but 
* ‘Annals of Botany,’ vol. 7, loc. cit., 1893. 
+ As in C.N.19. We have thought it convenient, for the benefit of future investigators, occa- 
sionally to refer to the cabinet number of specimens in the Williamson collection other than those 
figured. 
{ As in C.N. 10, 1007, &e. 
MDCCCXCIV. —B, aT 
