Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/512

 or slits passed upwards and outwards through the cylinder ; and in transverse section they are seen either completely breaking up by a thin film of cellular tissue the continuity of the cylinder, or only penetrating it for a short distance. Such partial penetrations sometimes proceed from the inside and sometimes from the outside of the cylinder, depending upon the part of the particular mesh which has been cut. When the mesh is cut near its origin, the cylinder is complete on its outer margin; while when it is cut through the upper portion, the cylinder is complete on its inner margin. Dr. Ogilvie, in a valuable paper on the vascular and woody tissues of ferns (Annals and Magazine of Nat. History, 3rd ser. vol. vi. (1860) p. 320, pl. v.), has somewhat misinterpreted this vascular cylinder in Osmunda regalis. He says the cylinder is made up of about eight fasciculi, having the same crescentic section as the vascular bundle in the petiole, and having their concavities all turned inwards. This is figured and described in plate v. fig 1. His error arises from his entertaining the notion that the vascular cylinder is composed of the fasciculi of the petioles, which, when they descend into the stem, branch out and anastomose with those derived from former petioles. This cylinder, however, is formed independently of the food-producing leaves, or at all events in advance of them ; and the meshes are really the openings through which the fasciculi pass to the leaves. These meshes do not in any way represent the reticulations caused by the medullary rays in the wood of Dicotyledons. They are homologous with the meshes produced by the tissues which pass through the wood into the foliar or ramal appendages.

The vascular cylinder of the stem is surrounded by a thin layer of pale parenchyma similar to that composing the axis. Dr. Ogilvie calls this a cambium layer; but he overlooks the fact that the vascular bundles in ferns are definite and simultaneous, and that a cambium layer does not exist in this position.

The vascular bundles of the petioles spring from the meshes in the vascular cylinder. At their origin they have the crescentic form which characterizes them throughout their whole course in the petiole and rachis, as well as all the branches in the secondary rachides. Each petiole is composed of its vascular bundle, having the concavity of the crescent looking towards the axis of the stem. This is imbedded in a pale parenchyma, composed of slightly oblong cells ; and the whole is surrounded by a cortical layer of smaller but more elongated cells, deeply coloured by the deposit in their interior of the brown-coloured substance so extensively distributed in ferns, and which is said, from its abundance in them, to give a peculiar rusty tint to the vegetation of those districts in which (as in some parts of New Zealand) ferns form a prevailing feature. Near the base the petiole is furnished with a wing, which is composed of white cellular tissue enclosed in a thin epidermis. In addition to the colouring- matter in the cortical layer of the petiole, a few small bundles of elongated cells occur irregularly scattered through the central parenchyma, both in the hollow concavity of the vascular bundle and on