ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 891 
we really have the primary wood of the branch, is conclusively proved by sections 
which are transverse to the main stem, and pass tangentially through the base of the 
branch (see Plate 79, fig. 20).* Here we see the bundles of the branch in longitudinal 
section, with their annular and spiral trachez, and the bands of interfascicular 
parenchyma between them. In the tangential sections of the main stem (transverse 
to the branch) it is also easy to demonstrate the elements of the primary xylem 
of the branch, when they are cut at all obliquely. Our best example of this is 
the section shown in Plate 80, fig. 21, which passes close to the pith of the parent 
stem, and therefore shows the branch at its actual base. Here the groups of primary 
xylem around the pith of the branch are perfectly obvious, and we can distinguish 
their spiral tracheee. On the lower side of the branch we can directly trace the 
continuity of its primary xylem with the nodal wood of the main axis. 
The characteristic internodal canals of the Calamitean stem are not present at the 
actual base of the branch (see photograph 9 on Plate 73, and fig. 21 on Plate 80). 
Neither are they found, in their typical form, in any part of the branch, so far as it is 
embedded in the wood of the main axis. In sections which pass transversely through 
the branch, at any point beyond its actual inner extremity, we frequently find a ring 
of gaps in. its tissue (not to be confused with accidental lesions), situated immediately 
within the xylem-bundles (see Plate 72, photograph 5, and more especially photo- 
graph 6). These gaps are less regular in form than the normal canals, and also differ 
from them in a more important point, for, as a rule, they are not empty, but are 
occupied by a lax tissue, consisting of rather large and thin-walled cells. Within the 
gap, among the thin-walled cells, we can often detect the protoxylem elements of the 
primary bundle.t Hence we regard these gaps as corresponding to the typical canals 
of an ordinary stem, in so far as they mark the position of the disorganized proto- 
xylem-groups. As to the lax tissue which usually fills these spaces, two views are 
possible. It may have been primary, consisting of parenchymatous cells which were 
present among the primitive tracheze from their first origin; or it may have been a 
new formation, analogous to a thylosis, such as has been observed in the carinal canals 
of Equisetum.t We regard the latter view as the more probable. All our specimens 
showing branching are comparatively advanced stems, with a considerable thickness 
of secondary wood. Hence it is almost certain that the primary tissue at the base of 
the branch would have already become functionless, especially since many of these 
branches were evidently abortive, as will be shown below. The formation of thyloses 
under such conditions is quite probable. We are the more inclined to this hypothesis 
from the fact that the tissue filling the gaps is conspicuously different from the sur- 
* The same point is shown in a similar section of another specimen, C.N. 132*. In one of the 
preparations kindly lent to us by Count Soums-Lavusacu, this structure is, if possible, even more 
beautifully preserved than in the section figured. 
+ Asin C.N. 20z, and 1554, 
¢ Sce Srrassuranr, ‘ Histologische Beitrige,’ 3, p. 437, 
