Historic fossil cycads ; Accelerated cone growth in Pinus / by G.R. Wieland.
- George Reber Wieland
- Date:
- 1908
Licence: In copyright
Credit: Historic fossil cycads ; Accelerated cone growth in Pinus / by G.R. Wieland. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
13/15 page 104
![of Wel/witschia arisen ? It is certainly thinkable that an ini- tial period of sporophyll reduction and accelerated hrancliing in some ancient pteridospermous ancestor characterized by large bractlike leaves, could result in a growth of fertile axes in compacted groups more or less analogous to that before us, and that a secondary course of reductions may then have inter- vened and produced the condition we now see. In other words, we imagine that in the dioecious Welwitschia single flowers originally derived from fertile terminal buds of vege- tative form have, following increase in number, undergone extreme reduction and modification into “ cones,” the ovulate forms of which have lost their microsporophylls. Further- more, if each cone of a group more essentially like that illus- trated were to be reduced so as to produce but a single seed, another interesting type of conoid inflorescence would be out- lined ; though such a form could scarcely be as capable of wide variation as a still more primitive series of spirally inserted multiovulate sporophylls.* It is thus of decided interest to have clearly brought home to us by such a simple form of accelerated branching as that of Pinus, the very interesting fact that by reason of new emplacements wholly new series of modifications in organs of reproduction may take place. These hypothetical en masse changes following the earlier evolutionary stages of primitive sporophylls pointedly suggest how the endless variety of angio- spermous fructifications can have arisen from plant types not very remote from but still more primitive than the Cvcade- oidese ; while contrariwise in races with few ovules to the carpel and little phytologic plasticity like the conifers, such realignments and modifications have in post-Triassic time seldom appeared or become habitual with a resultant displace- ment of the plastic stock by its better equipped descendants. [April 25, 1906.] * The gymnospenn cone itself, both simple and compound, is throughout derived by branching from a primitive main axis. The sporophylls of both sexes are of course all derivatives of pteridospermous fronds ; and dorsal, ventral, or lateral insertion of sporangia is of purely secondary import, since resulting in the simplest possible manner during the process of extreme reduction in which all eircinnate pretioration was displaced by obscured con- duplicate, or direct forms, with formation of various types of appression faces. Finally, in the Couiferales, selerotizatiou into regularly interlocking spirally inserted prismoids resulted in the nearly aplastic bi-acts and carpels, which have changed but little since the Jura. Appositely it is hypothesized that a diminution in the number of primitive sporophylls, fol- lowed only secondarily by spore decrease in number, would conserve the requirements of carpellary plasticity.](https://iiif.wellcomecollection.org/image/b2241017x_0016.jp2/full/800%2C/0/default.jpg)
