Darwin and Modern Science [145]
of the ordinary leaves--the most primitive arrangement known in any living seed-plant. The whole Order is relatively primitive, as shown most strikingly in its cryptogamic mode of fertilisation, by means of spermatozoids, which it shares with the maidenhair tree alone, among recent seed-plants.
In all the older Mesozoic rocks, from the Trias to the Lower Cretaceous, plants of the Cycad class (Cycadophyta, to use Nathorst's comprehensive name) are extraordinarily abundant in all parts of the world; in fact they were almost as prominent in the flora of those ages as the Dicotyledons are in that of our own day. In habit and to a great extent in anatomy, the Mesozoic Cycadophyta for the most part much resemble the recent Cycadaceae. But, strange to say, it is only in the rarest cases that the fructification has proved to be of the simple type characteristic of the recent family; the vast majority of the abundant fertile specimens yielded by the Mesozoic rocks possess a type of reproductive apparatus far more elaborate than anything known in Cycadaceae or other Gymnosperms. The predominant Mesozoic family, characterised by this advanced reproductive organisation, is known as the Bennettiteae; in habit these plants resembled the more stunted Cycads of the recent flora, but differed from them in the presence of numerous lateral fructifications, like large buds, borne on the stem among the crowded bases of the leaves. The organisation of these fructifications was first worked out on European specimens by Carruthers, Solms-Laubach, Lignier and others, but these observers had only more or less ripe fruits to deal with; the complete structure of the flower has only been elucidated within the last few years by the researches of Wieland on the magnificent American material, derived from the Upper Jurassic and Lower Cretaceous beds of Maryland, Dakota and Wyoming. (G.R. Wieland, "American Fossil Cycads", Carnegie Institution, Washington, 1906.) The word "flower" is used deliberately, for reasons which will be apparent from the following brief description, based on Wieland's observations.
The fructification is attached to the stem by a thick stalk, which, in its upper part, bears a large number of spirally arranged bracts, forming collectively a kind of perianth and completely enclosing the essential organs of reproduction. The latter consist of a whorl of stamens, of extremely elaborate structure, surrounding a central cone or receptacle bearing numerous ovules. The stamens resemble the fertile fronds of a fern; they are of a compound, pinnate form, and bear very large numbers of pollen-sacs, each of which is itself a compound structure consisting of a number of compartments in which the pollen was formed. In their lower part the stamens are fused together by their stalks, like the "monadelphous" stamens of a mallow. The numerous ovules borne on the central receptacle are stalked, and are intermixed with sterile scales; the latter are expanded at their outer ends, which are united to form a kind of pericarp or ovary-wall, only interrupted by the protruding micropyles of the ovules. There is thus an approach to the closed pistil of an Angiosperm, but it is evident that the ovules received the pollen directly. The whole fructification is of large size; in the case of Cycadeoidea dacotensis, one of the species investigated by Wieland, the total length, in the bud condition, is about 12 cm., half of which belongs to the peduncle.
The general arrangement of the organs is manifestly the same as in a typical Angiospermous flower, with a central pistil, a surrounding whorl of stamens and an enveloping perianth; there is, as we have seen, some approach to the closed ovary of an Angiosperm; another point, first discovered nearly 20 years ago by Solms-Laubach in his investigation of a British species, is that the seed was practically "exalbuminous," its cavity being filled by the large, dicotyledonous embryo, whereas in all known Gymnosperms a large part of the sac is occupied by a nutritive tissue, the prothallus or endosperm;
In all the older Mesozoic rocks, from the Trias to the Lower Cretaceous, plants of the Cycad class (Cycadophyta, to use Nathorst's comprehensive name) are extraordinarily abundant in all parts of the world; in fact they were almost as prominent in the flora of those ages as the Dicotyledons are in that of our own day. In habit and to a great extent in anatomy, the Mesozoic Cycadophyta for the most part much resemble the recent Cycadaceae. But, strange to say, it is only in the rarest cases that the fructification has proved to be of the simple type characteristic of the recent family; the vast majority of the abundant fertile specimens yielded by the Mesozoic rocks possess a type of reproductive apparatus far more elaborate than anything known in Cycadaceae or other Gymnosperms. The predominant Mesozoic family, characterised by this advanced reproductive organisation, is known as the Bennettiteae; in habit these plants resembled the more stunted Cycads of the recent flora, but differed from them in the presence of numerous lateral fructifications, like large buds, borne on the stem among the crowded bases of the leaves. The organisation of these fructifications was first worked out on European specimens by Carruthers, Solms-Laubach, Lignier and others, but these observers had only more or less ripe fruits to deal with; the complete structure of the flower has only been elucidated within the last few years by the researches of Wieland on the magnificent American material, derived from the Upper Jurassic and Lower Cretaceous beds of Maryland, Dakota and Wyoming. (G.R. Wieland, "American Fossil Cycads", Carnegie Institution, Washington, 1906.) The word "flower" is used deliberately, for reasons which will be apparent from the following brief description, based on Wieland's observations.
The fructification is attached to the stem by a thick stalk, which, in its upper part, bears a large number of spirally arranged bracts, forming collectively a kind of perianth and completely enclosing the essential organs of reproduction. The latter consist of a whorl of stamens, of extremely elaborate structure, surrounding a central cone or receptacle bearing numerous ovules. The stamens resemble the fertile fronds of a fern; they are of a compound, pinnate form, and bear very large numbers of pollen-sacs, each of which is itself a compound structure consisting of a number of compartments in which the pollen was formed. In their lower part the stamens are fused together by their stalks, like the "monadelphous" stamens of a mallow. The numerous ovules borne on the central receptacle are stalked, and are intermixed with sterile scales; the latter are expanded at their outer ends, which are united to form a kind of pericarp or ovary-wall, only interrupted by the protruding micropyles of the ovules. There is thus an approach to the closed pistil of an Angiosperm, but it is evident that the ovules received the pollen directly. The whole fructification is of large size; in the case of Cycadeoidea dacotensis, one of the species investigated by Wieland, the total length, in the bud condition, is about 12 cm., half of which belongs to the peduncle.
The general arrangement of the organs is manifestly the same as in a typical Angiospermous flower, with a central pistil, a surrounding whorl of stamens and an enveloping perianth; there is, as we have seen, some approach to the closed ovary of an Angiosperm; another point, first discovered nearly 20 years ago by Solms-Laubach in his investigation of a British species, is that the seed was practically "exalbuminous," its cavity being filled by the large, dicotyledonous embryo, whereas in all known Gymnosperms a large part of the sac is occupied by a nutritive tissue, the prothallus or endosperm;