OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
45 
upwards and outwards, as if to accompany some vascular bundle on its way to the 
leaves. This is seen in Plate I. fig. 5, h'. But in no one instance have I found the 
slightest trace of an actual vessel accompanying them. Though my sections, made in 
various directions, are nearly a hundred in number, I cannot trace a solitary example 
of a divergent vessel. I will not venture to affirm that the leaves of this plant, like 
those of the living Psilotum, were non-vascular ; but it is certain that I cannot detect a 
trace of a vessel, either in the cellular leaves and disks or diverging from the vascular 
axis in the direction of those organs. In all the transverse sections the vessels retain 
the undisturbed arrangement shown in the figures, whether such sections are made at 
the nodes or at the internodes*. It is obvious that these stems have not been jointed or 
articulated in the same way as the Calamites. In the latter plants, the existence of 
each node alters the entire arrangement of all the tissues, vascular and cellular, to the 
very centre of the stem. On the other hand, the appearance of articulations seen in the 
common examples of Asterophyllites found in the shales, is an altogether different thing. 
It is wholly due to modifications of the cortical tissues, which are expanded into disks 
by a centrifugal development, which does not visibly affect the true vascular axis of the 
stem. This is an important physiological distinction, to be borne in mind when endea- 
vouring to determine the systematic relations of these plants. The appearance of the 
cells of the outer bark, as seen in longitudinal sections of the internodes, is represented 
in Plate II. fig. 8. Each internode is rather more than half an inch in length. 
Returning to the circumferential growth of the stem, I find that layers were succes- 
sively added to the preexisting ones in precisely the same regular manner as seen in 
the examples already figured. Plate II. fig. 9 represents one of the finest sections I 
have met with, and which Mr. Dkinkwater, late of Salford, kindly permitted me to make 
from a specimen which he had obtained from the Oldham deposits. In this example 
the central triangle has now increased to a maximum diameter of *08, the result appa- 
rently of a general increase in the dimensions of the smaller vessels, because the largest 
ones still retain the maximum diameter of ’005 already observed in Plate I. fig. 4. 
The exogenous zones are now seven in number on two sides of the stem, and eight on 
the third. With the exception of this intercalation of a zone, the vessels of these exo- 
genous layers maintain the same remarkable regularity of arrangement, both radially 
and circumferentially, that I have already described. The bark ( h ) remaining in this 
specimen is but a portion of the middle layer. Plate II. fig. 10 represents a slightly 
crushed example, in which we find eight exogenous layers on one side, nine or ten on 
the second, and eleven on the third, indicating that, as the stems increased in diameter, 
* Since the above description was written, Professor Renault’s important memoir, entitled “ Recherches sur 
l’Organisation des Sphenophyllum et des Annular ia,” presented to the Academie des Sciences of Paris in May 
1870 and 1873, has reached me. It proves very clearly that vascular bundles are given off from the three 
angles of the central triangular axis, and that these pass outwards, through the exogeneous zones and bark, to 
the leaves. Hence there is little doubt that the arrangement represented in my fig. 5, h', indicates the con- 
dition suggested in the text. — February 5, 1874. 
