284 PROCEEDINGS OF SOCIETIES.
' medaliary slieatli' in recent plants. The consideration of the extinct forms of higher Cryptogams showed him that the term Acrogens must be given up, and led him to propose their division into two groups, correla- tive with those of Phanerogams. Amongst Cryptogamic Exogens he would include Equisetacece, Lycopodiacea, hoetacefs. The Ferns would constitute the Endogeus. Mr. Carruthers congratulated Professor Wil- liamson on his acquaintance with Professor King's paper, which he (Mr. Carruthers) had been the means of bringing to his notice. The so-called ' vascular pith,' according to his view, was entirely composed of vessels, some very short ; anything like the truncated cellular tissue shown in the diagram was not shown by the specimens. Dr. Hooker had suggested the term ' utricles' for these, as a kind of compromise between vessels and cells. The structure of Fern stems was precisely that of Stigmaria ; from the vascular cylinder pass off vascular bundles carrying,' with them a considerable quantity of cellular tissue. In llywenoj)hjUam, however, there was the opposite arrangement, a vascular axis instead of a cylinder; so that, according to Professor Williamson, Ferns woidd be split up into two groups. The vascular bundles were scattered in some Lycopodlacece, but in Lycopodiam they were united into a central axis, and this was precisely the same as in Lepldodcndron. Classification must be based
entirely on the reproductive organs. Professor Dickson thought the
' vascular pith' was undoubtedly a central axis. Be should like to know
what distinct evidence there was of the existence of a cambium layer.
Prof. McNab could not agree with Prof. Williamson in his interpretation of the structure of these stems. Botanists are all agreed in this, that Lepldodendrou and their allies are closely related to the Lycopods. Now we know that the Lycopods, like the Ferns, have closed fibro-vascular bundles Avhich can only grow for a certain time, and then, all the cambium being converted into permanent tissue, growth must cease. The key to these structures is to be met with in Lycopndinm ChanKTcyparissus, in which we have a cylinder of wood-cells surrounding the central cylinder of united fibro-vascular bundles. This cylinder of wood-cells represents and is a mere modification of the cellular tissue met with in the ordinary stems of Lycopods. In this wtiy it follows that the central portion is not a pith, but consists of the central group of fibro-vascular bundles. It also follows that the wood cylinder in these stems is not the homologue of the wood cylinder of an ordinary exogen. He thought the classification of these
plants proposed by Prof. Williamson quite untenable. Prof. Thiselton
Dyer was glad that Prof. Williamson had at last brought his jiapers to the section where they could be properly discussed. He thought it was a great mistake to dissociate, as was so frequently done, the study of extinct from that of recent forms. The proposed arrangement seemed to him to violate all the canons of natural classitication. It was impossible to separate Eqiusetacefe from Ferns. It was a caricature of evolution to hint any analogy between Prof. Williamson's ' medullary zone ' and the medullary sheath.
{To he continued.)
��Belfast Naturalists' Field Club. — The third excursion for this season took place on Saturday, July 8th, the locality selected being the Cave Plill Quarries and Deer Park. The sections exposed here are very instruc-
�� �
