ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURKS. 873 
In fact, all the evidence goes to show that the internodal canals of Calamites ave 
identical in nature and origin, as well as in position, with the carinal passages of 
Lquisetum. The demonstration of this homology is a considerable step towards 
establishing the essential agreement between the stem of a Calamite and that of an 
Equisetum.* 
We will now go on to consider the structure of the remainder of the primary wood, 
1.€., of those xylem-elements which lie on the outer side of the canal. Here we often 
find a difficulty in distinguishing between primary and secondary xylem. In trans- 
verse sections the regular radial series can often be traced inwards, as far as the canal, 
while, in other cases, the elements nearest to the latter are irregularly placed. Where 
all the tissue in question is radially arranged, such sections do not enable us to draw 
the line between primary and secondary wood. The same difficulty exists in many 
recent plants with an early development of secondary tissue. 
Longitudinal sections, however, especially those in the radial plane, show us the 
elements which are characteristic of the primary xylem. The trachez just outside 
the canal are either densely spiral, or reticulated. Spirals capable of unrolling seem 
only to occur in the protoxylem of the canal itself. Scalariform trachez come next, or 
may themselves adjoin the canal. (See Plate 77, fig. 5; Plate 78, figs. 7 and 10.) 
Their pits are bordered, as in similar elements of recent plants. This is shown in 
fig. 94, on Plate 78, which is taken from a tangential section passing so near the pith 
that we regard it as showing the structure of the primary wood. Where one of the 
trachez is in contact with a parenchymatous cell, the pits are only bordered towards 
the tracheze, just as in so many recent plants. Short parenchymatous cells are 
embedded here and there among the trachew. The latter seem to be tracheides, but 
further histological details will be more conveniently considered when we come to the 
secondary wood. 
The sections of the very young stems afford little information as to the phloém, 
which is in no case preserved in such specimens. There is, however, no doubt that 
the bundles were collateral. In all the corticated specimens there is a gap between 
xylem and cortex, which is only obliterated where the latter has evidently been 
crushed in upon the former. (See the various transverse sections shown on Plates 72, 
77,and 78.) In this gap, disorganized carbonaceous matter, sometimes showing signs 
of cellular structure, is present. In very early stages (see Plate 77, fig. 2) the 
interfascicular tissue is sometimes continuous from pith to cortex, but more usually 
it is interrupted by a disorganized layer, which presumably represents the delicate 
pericyclic tissue from which the interfascicular cambium would have arisen. 
The preparation which shows the most satisfactory remains of the phloém, is one 
from which photograph 3, on Plate 72, and figs. 12, 18, and 14, on Plate 78 are taken. 
In the group shown in fig. 13 the whole tissue, though much crushed, is preserved 
* After the above was written, Count Soums-Lausacu very kindly lent us some beautiful sections of a 
Calamite from Halifax, which afforded additional evidence of the facts just stated. 
5 T 2 
