GINKGO A FLOWERLESS SEED PLANT

Abstract

After studying Ginkgo in relation to its position in the evolutionary scale, the writer became convinced that there could be no strobili in a plant, supposedly descended from the Ptenophyta, on such a low level unless they were of considerable size. The conclusion followed naturally, therefore, that the reproductive structures usually called strobili in most recent textbooks must be nothing more than ordinary sporophylls. The difference between the delicate herbaceous stalk of the supposed flowers and the woody dwarf and long branches is too great if Ginkgo really is a close relative of the eusporangiate ferns, pteridosperms, and cycads. Furthermore, the fact that these supposed branches would have to be regarded as secondary branches on the dwarf shoots indicated that such a condition is probably not present. A careful morphological study was, therefore, made of certain characters in order to obtain definite information as to the actual nature of the structures bearing the ovules and pollen sacs. This study showed that the supposed carpellate and staminate strobili are simply ordinary, slightly modified sporophylls, and that Ginkgo is a completely flowerless plant with no determinate axes whatever, and in this respect in the same condition as the typical ferns, quillworts, and lower lycopods. The petiole of the fern-like foliage leaf of Ginkgo has two leaf traces, and the leaf scar shows two distinct bundle scars (text fig. i). The stalks of the microsporangiate and megasporangiate reproductive organs present a similar condition. If they represented stems of strobili they should have a circle of bundles with a central pith. The branched stamen or microsporophyll shows several vascular strands in the petiole, although two are larger than the others, but at the very base, just below the surface of the cortex of the dwarf branch, these merge into two bundles as in an ordinary leaf (text figs. 2, 3). The petiole of the fruiting carpel or megasporophyll shows some enlargement of the vascular tissue that might, on superficial examination, be mistaken for -a circle of bundles with a central pith, but the two bundles can still be distinguished and in the very base there are two distinct leaf traces as in the leaf and microsporophyll. In autumn, when the mature carpels with seeds are cut off by an abscission layer, the two bundle scars often show with great distinctness and can be seen even