Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/346

 it has arisen from some error of observation. I exclude from this the specimen figured by Cotta under the name oi Stigmaria Ficoides, the vascular tissue of which is composed of Cycadian-like or of dotted vessels traversed by muriform cells of medullary rays ; for this is certainly not a specimen of Stigmaria.

Brongniart, in his memoir on Sigillaria elegans, figures a specimen of Stigmaria from the cabinet of Robert Brown, which is now in the collection of the British Museum, and which I have repeatedly and carefully examined. In an enlarged representation of a portion of the cylinder (pl. v. fig. 3) there is shown an axial vascular bundle passing outwards through a mesh between the large wedges of the cylinder. Several open spaces separating the parallel series of vessels on the outer aspect of the cylinder are described as small medullary rays ; but the examination of this and of an extensive series of other specimens has convinced me that these are only disruptions of the tissue produced along the line of least resistance from desiccation or some other cause before or during the process of fossilization. Brongniart was unable to detect any remains of structure in these spaces. Such openings are altogether absent in many specimens which I have examined ; and in one in the collection of the British Museum they occur on the one half of the specimen and not on the other.

Dr. Hooker, in his paper on the structure of Stigmaria, makes no reference to the small cellular medullary rays of Brongniart, which he evidently could not detect in his specimens, and which are not indicated in the least in his beautiful drawings. He, however, describes structures to which he applies the name of medullary rays. " The cylindrical axis," he says, " is divided into elongated wedge-shaped masses, rounded at their posterior or inner extremity by numerous medullary rays of various breadths, some much narrower than the diameter of the tubes, others considerably broader, but none are conspicuous to the naked eye, except towards the outer circumference. The medullary rays, even the narrowest, are traversed by bundles of tubes half the diameter of the largest vessels of the axis, or even less " (p. 434). In the explanation of plate ii, fig. 6, he further says, " The vascular tissue is divided by broad and narrow medullary rays ;" and fig. 8 is a " highly magnified view of one of the narrow medullary rays forming a bundle of tubes of less diameter than those forming the wood." The structures described in these extracts are evidently the vascular bundles which belong to the axial appendages. These bundles take their origin in the inner surface of the vascular cylinder, and pass upwards and outwards through a mesh, which increases in size as it reaches the circumference of the cylinder. The vascular bundle did not completely fill the mesh, but was accompanied by a quantity of cellular tissue which is very rarely preserved, the space occupied by it being generally filled with the amorphous material of the matrix in which the specimen is preserved. This parenchyma is composed of roundish cells. In the fern -stem a similar structure exists. The vascular tissue of each frond has a parenchyma associated with it which is continuous with the medulla.