but the male organs had been thrown off before the complete
development of the gynoecium. This fact suggests the possibility
that the flowers described by Mr Wieland, in which the male
organs are mature and the gynoecium is composed of very short and
immature ovuliferous stalks and inter seminal scales, arc not
essentially distinct from those which have lost the staminate leaves
and possess mature seeds. It is probable that the flowers of
Bennettites were normally hermaphrodite, and they may have been
markedly protandrous. We cannot decide at present whether the
gynoecium in a flower, such as that represented in fig. 15, 7, has
partially aborted or whether it would have matured later after the
fall of the male organs.
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| Fig. 15. |
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1, Bennettites stem: portion of transverse section of stem; a, vascular cylinder; b, leaf-traces; c, pith; d, cortex. |
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2, Bennettites stem, tangential section; e, flower-peduncles. |
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3, Bennettites stem, leaf-traces attached to the vascular cylinder and passing as simple strands through the cortex; d, cortex. |
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4, Williamsonia, Wealden, England. |
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5, Young leaf of Bennettites. |
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6, Ramenta of Bennettites in transverse section. |
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7, Bennettites, female flower in longitudinal section; f, apex of peduncle; g, bracts (shown in surface view in 4); h, seeds and seminiferous pedicels; i, interseminal scales. |
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8, Bennettites, synangium of male flower, showing line of dehiscence, k, and microspores, l. |
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9, Synangium, in transverse section, showing sporangial groups, m, and microspores, l. |
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10, Bennettites flower in vertical section, showing the central female portion, n, two sporophylls bearing synangia (male), o, and hairy bracts, g. |
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11, Bennettites seed in longitudinal section, showing the dicotyledonous embryo; p, cotyledons; r, radicle; s, testa. |
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(1-3, after Carruthers; 5, 8, 9 and 10, after Wieland; 7, after Scott; 11, after Solms-Laubach.) |
|
| Fig. 16.—Frond of Williamsonia gigas. |
| Inferior Oolite, England. |
It is clear that Bennettites differed in many essential respects from the few modern survivors of the Cycadophyta. Fossil flowers of a type more like that of modern Cycads are few in number, and it is not by any means certain that all of those described as Cycadean flowers and seeds were borne by plants which should be included in the Cycadophyta; a few female flowers have been described from Rhaetic rocks of Scania and elsewhere under the name Zamiostrobus—these consist of an axis with slender pedicels or carpophylls given off at a wide angle and bearing two ovules at the distal end; the structure is in fact similar to that of a Zamia female flower, in which the internodes of the peduncle have been elongated so as to give a looser arrangement to the carpels. It has been suggested that one at least of the flowers, that originally described by Mr Carruthers from the Inferior Oolite of Yorkshire as Beania gracilis, may have been borne by a member of the Ginkgoales. From Jurassic rocks of France and Italy a few imperfect specimens have been described as carpels of Cycads, like those of the recent genus Cycas (see Gymnosperms); while a few of these may have been correctly identified, an inspection of some of the original examples in the Paris collections leads one to express the opinion that others are too imperfect to determine. Pinnate fronds of the Cycas type, characterized by the presence of a midrib and no lateral veins in the linear pinnae, are recorded from Rhaetic rocks of Germany, from Wealden strata in England (fig. 11) and Portugal, and from Liassic beds in Dorsetshire. One large specimen is figured by Heer from Lower Cretaceous rocks of Greenland, and by the side of the frond is shown a carpel with lateral ovules, as in the female flower of Cycas; but an examination of the type-specimen in the Copenhagen Museum led the present writer to regard this supposed carpel as valueless. Professor Nathorst, as the result of a more recent examination of Heer's specimen, found that the segments of the frond are characterized by the presence of two parallel veins instead of a single midrib, with a row of stomata between them; for this type of Cycadean leaf he proposed the generic name Pseudocycas. Another well-known Cycadean genus is Williamsonia, so named by Mr Carruthers in 1870, and now applied to certain pinnate fronds—e.g. those previously described as Zamites gigas (fig. 16), and others known under such names as Pterophyllum or Ptilophyllum pecten, &c., both common Jurassic species—as well as to stems bearing peduncles with terminal oval flowers, similar in form to those of Bennettites. There is good evidence for supporting Professor Williamson's conclusions as to the organic connexion between the flowers, originally described from Inferior Oolite rocks of Yorkshire and subsequently named Williamsonia (fig. 15, 4), and the fronds of Zamites gigas, now known as Williamsonia gigas (fig. 16). There can be little doubt that the majority of the Cycadean fronds of Jurassic and Wealden age, which are nearly always found detached from the rest of the plant, were borne on stems of the Bennettites type. Williamson was the first to express the opinion that the Bennettitean flowers known as Williamsonia were borne on the trunks which terminated in a crown of pinnate fronds of the type long known as Zamites gigas; this view was regarded by Saporta and others as incorrect, and the nature of the Bennettitean foliage was left an open question. A re-examination of the English material in the museums of Paris and elsewhere has confirmed Williamson's conclusions. Mr Wieland has also described young bipinnate fronds, very like those of recent species of Zamia and Encephalartos, attached to a Bennettites stem, and exhibiting the venation characters of many recent Cycads (fig. 15, 5). In Williamsonia the stem bore comparatively long fertile shoots, which, in contrast to those of Bennettites, projected several inches beyond the surface of the main trunk, and terminated in a flower which appears to have resembled those of the true Bennettites. Nathorst has recently described specimens of Williamsonia from the Jurassic rocks of Whitby with micro-Sporophylls like those of Wieland's species. Williamsonia occurs in the Upper Gondwana rocks of India; it is recorded also from strata ranging from the Rhaetic to the Lower Cretaceous period in England, Portugal, Sweden, Bornholm, Greenland, Italy and North America. Professor Nathorst has described another type of stem from the Rhaetic beds of Scania. It consists of a comparatively small and repeatedly forked axis bearing in each fork a flower; the flowers, which are regarded as male and female, appear to be similar to those of Bennettites. The leaves, borne on the regions between the false dichotomies, are those of Anomozamites minor, a type of Cycadean frond originally determined
