of the Fossil Plants of the genus Sigilaria. 129 
any distinct medullary sheath as in Sigillaria. This is granted, but there 
seems to be no difficulty in the way of the vessels of such a sheath 
being so mixed with the tissue of the ligneous cylinder, that is, on its 
inner side, and consequently falling into the radial arrangement of the 
latter, as to loose all appearance of individuality, or, in other words, be 
prevented shewing themselves under a distinctive form. This I strongly 
suspect is the case with the vessels of the vascular system of Stigmaria : 
the fact of the fibril cords having originated on the inner side of the 
cylinder, as shewn by the groove of the radiating plates, forms, in my 
opinion, a strong argument in fayour of this supposition.* And it seems 
to be further supported by three transverse sections of this fossil at pre- 
sent before me, in each of which the vessels are somewhat irregularly 
arranged on the inner side of the wedge-shaped bundles: in fact, there 
is displayed the same want of regularity in the radial arrangement as 
characterises the tissues situated on the inner side of the ligneous zone of 
some Conifers, that is, where the spiral vessels are mixed with the disci- 
gerous tubes. t 
From what has now been stated, the following general character of 
the histology of Stigmaria is proposed ; and, with the view of aiding our 
description, an enlarged restoration of the’cylinder,'exhibiting its consti- 
tuent parts, is given in fig. 3, Plate V. 
The tissues of Stigmaria are of three kinds,—vascular, ligneous, and 
parenchymous, 
The vascular tissue enters into the composition of the inner part (7) 
of the cylinder ;{ the ligneous composes the remaining portion, b; and 
the parenchymous forms those parts of the fossil which have been pre- 
viously named the “ broad zone” and the “ centre” (c and d, fig. 1, Plate 
Y.): the two former parts represent the medullary sheath and the wood 
—the vascular and the ligneous system ;. while the two latter represent 
the pith and the bark—the parenchymous system. The tissue of the 
ligneous system consists of very enlongated tubes, which, on the trans- 
verse section, are arranged in lines radiating from the pith, while in the 
longitudinal sense, they follow an oblique undulating course: their walls 
* The width of the cords in fig. 2, Plate V., appears to militate against their 
having originated on the inner side of the cylinder; but it may be observed, 
that the cords were probably made up, as is usually the case in other plants, of 
a mixture of vascular and woody tissue. In this case, the vascular tissue may 
have occupied but a small portion of the inner side of a cord. 
t Perhaps the longitudinal section of Stigmaria represented in the “ Fossil 
Flora,” (Vol. iii., Plate 166, fig. 2), will throw some light on this point. It is 
much to be desired that a more detailed account of this section were published, 
} The line on the inner side of the cylinder in the restoration is merely given 
to indicate the situation of the vascular tissue. 
VOL. XXXVIII. NO. UXXV.—JAN. 1845. I 
