OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
397 
vascular bundles. These layers present very different appearances when viewed in 
transverse and in vertical sections: in the former the outermost layer exhibits the 
greatest peculiarities of structure ; in the latter the peculiarities are the most striking- 
in the middle one. The innermost layer exhibits the same appearances in both sections. 
The latter (Plate XXVIII. fig. 30, g, & Plate XXIX. fig. 35, g) exhibits so close a general 
resemblance to the innermost bark-layer of the Lepidodendroid plants, that I do not 
hesitate to assign to both the same functions. It has a rather uniform thickness of -008, 
occasionally swelling out to as much as - 025. It consists of delicate parenchyma, the 
largest cells of which range between -0023 and -0028, their size being moderately uniform: 
sometimes they are arranged in radiating lines ; at others they appear as regular paren- 
chyma. This layer is a very distinct one, both in the definiteness of its boundaries, in the 
delicacy of its texture, and in the general uniformity of its aspect: in Plate XXVIII. 
fig. 30 it is seen describing a semicircle at g g, and in the vertical section at Plate XXIX. 
fig. 32, g. In every one of the numerous examples which I have examined, this inner bark 
appears too large for the cellulo-vascular axis which it invests. Sometimes the excess 
projects at one or both ends of the compressed axis, enclosing a vacant space, as in 
Plate XXVIII. fig. 30 & Plate XXX. fig. 36 ; at others it appears in the transverse sec- 
tion as a caudate appendage, occupying the same position as in most of the figures on 
Plate XXX. This condition has doubtless resulted from a contraction of the central axis 
that had no existence during the life of the plant. The layer has evidently been at once 
flexible and firm ; since, though compressed into innumerable forms and detached from 
the central axis in a variety of ways, I have rarely seen its continuity interrupted, save 
where some vascular bundle passed through its tissues to reach the periphery of the stem. 
The middle bark-layer (Plate XXVIII. fig. 30, A, & Plate XXIX. figs. 32, A, & 35, h) 
is a much more variable one. It consists of masses of very regular parenchyma, inter- 
mingled with horizontal layers of darker-coloured cells, which latter constitute the cha- 
racteristic feature of this tissue. In transverse sections we only detect these darker layers in 
the form of dark isolated patches (Plate XXVIII. fig. 30, A') and wavy lines (Plate XXIX- 
fig. 35, A'), surrounded by or enclosing clearer spaces occupied by regular parenchyma 
(Plate XXIX. fig. 35, A") ; but in the vertical section the former appear in the shape of 
very regular, parallel, horizontally disposed bars (Plate XXIX. fig. 32, A'). The ordinary 
parenchyma consists of cells having a maximum diameter of from -0028 to -004. In 
Plate XXXI. fig. 45, which represents this tissue in a vertical section of the two outer- 
most layers of the bark of a very young stem, each cell has been drawn with scrupulous 
accuracy, the dark-coloured transverse bars being represented at A' A', and the intervening 
parenchyma at A". In vertical sections of this tissue in older stems the latter cells con- 
stantly exhibit a disposition to arrange themselves in vertical and often slightly curved 
lines (Plate XXIX. fig. 32, A"), approaching, in the former respect, to the condition of 
the tissue so common in the meclullse of the Lepidodendroid plants*. 
* Mr. Binney has proposed (“ Observations on the Structure of Fossil Plants found in the Carboniferous 
Strata. — Part III. Lepulodendron ”) to assign to this tissue the name of “ orthosenchyma.” It is but one of the 
