OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
215 
of long, narrow, square-ended cells of variable length and about *0005 in diameter ; but 
interspersed amongst them are many scattered cells of larger dimensions, as shown in fig. 13. 
Cortex . — External to the vascular bundle-sheath we have a thick mass of ordinary 
parenchyma, figs. 4, e, 14, e, & 20, e, the cells of which are remarkably uniform, both in 
texture and size, their diameters chiefly ranging between ’005 and '003. This paren- 
chyma is invested by a thin outer layer about '04 in thickness (figs. A,f, & 2 ,f), com- 
posed of denser and darker-coloured cells. 
Secondary bundle . — I have already remarked that, besides the large central vascular 
cylinder, each transverse section of the stem reveals two or more secondary bundles. 
Of these the largest and most conspicuous is the usually solitary one of this type repre- 
sented in fig. 4, c ; and in the subsequent figures the same bundle is indicated by the 
same italic letter. An enlarged representation of this bundle, as it appears in the 
section, fig. 4, is seen in fig. 17. Its outline is reniform. The greater part of the 
vessels, g, of which it is composed are nearly as large as those of the central cylinder ; but 
those seen clustered together at each extremity of the transverse section, g', are very much 
smaller than the others. This vascular bundle is surrounded by a sheath of narrow 
elongated cells (fig. 4, g"), identical with that investing the axial cylinder, and this, in 
turn, is enclosed within the ordinary parenchyma of the inner cortex (fig. 4, c ). 
The origin and destination of this bundle would be alike obscure were our study of it 
limited to a single-section ; but by following the ascending series of sections included in the 
figures 2-12 we obtain information on both these points. In the sections from figs. 2-8, c, 
we see but little change in the position and surroundings of the bundle ; but in fig. 9 
we find that the outermost cortical layer of the stem (fig. 9, f) is obviously bending 
inwards at two points, f',f, tending towards the enclosure and detachment of the bundle 
from the main stem. In fig. 10 this enclosure is complete. The bundle c is now the 
centre of what has every appearance of being the base of a leaf-petiole. In fig. 11, c, 
the bundle has been accidentally broken away in the preparation of the section ; but at 
f we find the petiole freeing itself from the central stem, whilst in fig, 12 the part of 
the stem, f, from which the orientation took place is now encircled by the normal 
investment of outer cortex. In its original state the outer surface of the specimen 
exhibited several rounded areas not marked by defined cicatrices, but irregularly 
fractured, from which points similar petiolar extensions seem to have been broken off. 
Fig. 18 represents a section of one of these detached petioles enlarged 7^ diameters. 
I am indebted for this section to Mr. Butterworth, by whom it was prepared. Its 
central vascular bundle ( c ) is virtually identical in size and contour with that of fig. 17, 
whilst the inner (e) and outer (f) cortical layers are, in like manner, identical with those 
of the parent stem. The large rounded lacunas in the inner bark are merely accidental 
results, probably of partial desiccation before the specimen was imbedded in its matrix. 
Origin of the primary bundles . — On returning to the series of sections, we observe 
that at the point b 1 of fig. 6 the central vascular cylinder is undergoing a change. The 
arrangement of the tissues at this point is seen in fig. 19, which is an enlarged repre- 
2 h 2 
