Exogenous Structure in Coal-Plants

Abstract

THE points at issue between Prof. Williamson and myself remain in the same position as at first. He has not yet answered one of my objections. He still holds that in Lepidodendron we have a vascular medulla, outside which is a series of fibrovascular bundles which are not closed, but go on forming new tissues by means of a cambium layer like a dicotyledonous stem. From my own observations, and from the study of recent Continental authorities, I have no hesitation in stating that the central “medulla” of Prof. Williamson consists of the united closed, fibro-vascular bundles, while the investing cylinder is the modified primitive tissue which increases in diameter by means of the meristem layer of Nägeli. If Prof. Williamson will refer to Sachs' Lehrbuch, Ed. 2, p. 397, he will find good reasons given for the statement there made that Isoëtes contains no cambium in the stem; but that the stem increases in the same way as Dracæna, i.e. by a meristem layer in the primitive tissue. As long as Prof. Williamson believes in a central vascular medulla in these Lycopodiaceous stems, all his other conclusions must likewise be false.