Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/345

 mere analogical resemblances give no assistance in determining systematic position, but rather tend to lead astray, as they have done in the genus in question. There is one point in which the fossil differs from the living plants, which must at once, if it can be established, separate them widely in systematic position. I refer to the absence of medullary rays in the vascular cylinder of Stigmaria. This cylinder is composed entirely of scalariform tissue pierced only by meshes for the passage from the inner surface of the cylinder of the vascular bundles that supply the rootlets.

It is important to remember that medullary rays are composed entirely of muriform cellular tissue, and keep up the connexion between the internal and external cellular structures — the pith and bark. Medullary rays are confined to and characteristic of exogenous stems. They break up the wood into innumerable fine meshes. But, besides these, there are in exogenous stems other and larger meshes for the passage outwards of the vascular bundles which supply the axial appendages — the leaves and branches. In some stems, as in Phytocrene, Tupa, Cycas, Euphorbia, Cactus, &c., from the size of the vascular bundles these meshes are very large ; but in ordinary trees those formed by the leaves early disappear, because, the leaves being deciduous and produced only on the younger portions of the tree, the vascular bundles supplying them are not continued through the whole woody cylinder. It is of the utmost importance to notice these two sets of horizontal radiating structures in the dicotyledonous stem — the one, entirely cellular, being the medullary ray, the other, vascular, being the vessels of the axial organs.

Among recent cryptogams the only plants which have anything like a continuous woody cylinder in their stems are the Ferns. In the Lycopodiaceoe and Equisetaceoe, as represented among living plants, the vascular bundles of the stem are symmetrically arranged in a large mass of parenchyma, and there is consequently no true separation between the cellular tissue of the medulla and that of the cortical layer. It is however different in many tree ferns, which are composed of a mass of parenchyma traversed by vascular bundles of scalariform tissue, which form a closed circle separating the medulla in the interior from the cortex of the exterior. The tissue of this vascular cylinder is entirely destitute of medullary rays ; but it is penetrated by large meshes, through which pass the vascular bundles that supply the fronds and invariably rise from the inner surface of the cylinder.

The trunk of the cryptogam differs from that of the dicotyledon, as regards the points in question, in having its vascular cylinder penetrated by only one kind of horizontal tissue, namely, the vascular bundles belonging to the fronds, while the exogenous stem has, be- sides, another horizontal tissue of a very different structure and performing a totally different function.

The woody cylinder of Stigmaria possesses only the vascular horizontal layer ; but from not distinguishing aright these two structures, this has been described as a medullary system. Where a medullary ray has been described in addition to the vascular system.