66 



FOSSIL PLANTS. 



the Botanical Department of the British Museum, and of Mr. Alexander Bryson, enable 

 me to give a somewhat complete description of its singular structure. 



" The axis of the stem cannot be considered as a true medulla or pith, inasmuch as it 

 is composed, not of simple cells, but of elongated utricles, of various sizes, irregularly 

 arranged, and having their walls marked with scalariform bars (pi. Ivi, fig. 2). 

 These utricles indeed differ from the vascular tissue of the woody cylinder which surrounds 

 them, only by their length. The tissue of the woody cylinder consists of long scalariform 

 vessels, which increase in size from the inner margin to the outer ; this increase being 

 sufficient to meet the requirements of tlie enlarged circumference, wdth the help of only a 

 few additional series of vessels. As there is no true medullary cellular tissue in the axis, 

 so there are no medullary rays passing through this cylinder. In radial sections an 

 appearance is seen singularly resembling, to the naked eye, the silver grain produced in 

 dicotyledonous w^oods by the medullary rays, but this arises from a very different cause. 

 The diameter of the vesselsjs so great that, on a polished surface, only the scalariform wall 

 of the vessel that lies on or near the surface is exhibited ; and when the upper wall of a 

 vessel is cut away, the lower wall is often so deeply buried in the opaque substance that 

 the peculiar structure is obscured. In the case of sections prepared for microscopic 

 examination, both surfaces of some vessels are often removed, and the scalariform markings 

 on the lateral walls, or on any horizontal walls wdiich by chance occupy a medial position 

 between the polished surfaces, only are seen. This absence of the scalariform bars gives 

 at first sight the appearance produced by medullary rays. 



"The continuous cylinder of scalariform vascular tissue appears to be penetrated by 

 the vascular bundles which ultimately supply the leaves. These bundles apparently 

 originate either in the scalariform tissue of the axis or on the inner surface of the woody 

 cylinder. They have been mistaken for, or misnamed, medullary rays. 



" The woody cylinder is surrounded by a great thickness of cellular tissue, which 

 extends to the exterior of the stem, and is composed of three distinct and separate zones. 

 The inner zone has never, so far as I know, been perfectly preserved in any specimen, yet 

 traces of it sometimes may be seen, and it is rightly restored in Brongniart's drawing of 

 Lepidodendron Harcourtii, in the ' Archives du Museum,' vol. i, pi. xxxi. Its absence in 

 fossils is owing to its extremely delicate structure. The cells of the middle zone have 

 thicker walls, and they have consequently frequently resisted decomposition before fos- 

 silization made them permanent. In the outer zones the cells are very much lengthened, 

 and have a smaller diameter. They nearly resemble true vascular tissue ; but the progress 

 of lengthening may easily be traced from the interior outwards, and no distinction can be 

 drawn between the true cells and the long and slender ones of the outer circumference. 

 The cell-walls of all the three zones are without markings of any kind. 



" These three cellular zones are traversed by the vascular bundles which rise from the 

 outside of the interior woody cylinder, if they do not actually pass through it, and pass to 

 the leaves and branches. These bundles separate from the woody cylinder a long way 



