FERNS 105 The anlheridium is morphologically a trichome, and is an outgrowth of a superficial cell. The wall when mature consists of a single layer of cells, inclosing a central cell, from which the antherozoids are developed in separate mother cells. The antheridium eventually bursts owing to the absorption of water, and the spirally-twisted motile ciliated antherozoids (fig. 9) are set free (fig. 10). FiR. 10. Fig. 9. FIG. 8. Germinating spore (.<) of Pleris longifolia, producing a cellular prothal- lium (p) with a root-hair (r). FIG. 9. Antherozoid of Aiplenium scp eritrionale. FIG. 10. Antheridia from the prothalllnm of 1 teris aquiUna. a, antheridium unopened ; b, one discharging cells, each of which contains an antherozuid ; f, empty tmtheridiuin. Fio. 11. Archegonium of Asplenivm septentrionale. a, canal; c, oospherc, with nucleus (e). The (trcheyonia are produced on a cellular cushion imme diately behind the notch. They are developed by repeated divisions from a superficial cell, and, when mature, each is a bottle-shaped structure (fig. 11), the rounded base of which is buried in the prothallium, and contains a central cell, the oosphere. The canal of the neck is at first filled by a cell, which eventually deliquesces, and leaves the passage open for the access of the antherozoids to the oosphere, which, after fertilization by blending with them, becomes the oospore. The oospore first divides by a division-wall oblique to the prothallium, and then divides again so as to form four cells. Two of these give rise to a mass of tissue which remains in contact with the pro- thallium, and is called the foot. Of the other two, one de velops into the stem (first leaf), the other into the root. As soon as the young fern-plant (sporophore) is fairly established (fig. 12) the prothallium (oophore) disappears. Its existence is therefore remark ably contrasted by its brevity with the oophore stage in the Bryophyta. There are, however, cases in which the conditions of the one group are approached by the other. In Qsmunda the prothallia admit of vegetative reproduction by the de tachment of marginal shoots. In Qymnogramme leptophylla, Goebel has shown that the prothallium vegetates in a persistent way, com parable to that of the Oophore Stage Fio. W. f, Young Fern -plant in a moss. The sporophore is very SS&SSftS ! St transient in its existence, only living Hzcd archegonhun of p, the for a few months, and being entirely P - thallium < 00 " hore >- devoted to the production of spores. Amongst Bryophyta, in Anthoceros the sporophore exhibits a continued produc tion of spores, which places it nearly on a level with Gymuo- fjramme. Farlow has made the remarkable observation that the sporophore may in some cases originate from the prothallium without the intervention of a sexual process, by a process of budding. 2. The Stii>uUitai include two orders often included among ferns, but differing from them in some very import ant points. Ophioglussacew include Ophioylossum (adder n tongues) and Botrychium (moon-worts), both widely dis tributed, and Helminthostachys, which is found from tho Himalayas to Queensland. The leaves as soon as they have attained a certain age bear a fertile branch from the axial side. The sporangia are developed in a manner totally different from those of other Filicince. Instead of being pro ducts of single epidermal cells their origin resembles that of the pollen-sacs of anthers. The sexual generation (oophore) is tuberous and subterranean. Ufarattiacece is a group of ferns including about four genera. All are essentially tropical, but run more into the south than the north temperate zone. Dancea is exclu sively American, Kaulfussia and Angiopteris Old World, Marattia cosmopolitan. The sporangia are placed on lateral veins of the pinnae, and contain two rows of loculi, each of which corresponds to a single sporangium of Ophioglossum, and produces a number of spores originating from many primary mother-cells. The development of the sporangia in Anyiopteris, which differs from other Marat- tiacece in the sporangia not being confluent, is still a desideratum for the determination of its affinities. 3. Jthizocarpece are plants whose affinities have long been obscure, but which are now gradually being seen to be re lated in many essential points to ferns, though departing from them in other respects. In the growth of the roots, mode of branching, and circinate vernation there is entire agreement, and the fructification, though apparently very different, clearly admits of comparison with the true fern type. For a detailed account of their structure reference must be made to the works referred to below. It will be sufficient here to briefly refer to the fructification. Rhizo- carpece include two orders, Marsilece and Salviniea . The former consists of the two genera Pilularia and Marsilea. The former consists of three species of small creeping plants with filiform leaves, one of which is not uncommon in marshy places in Britain ; the fruit (sporocarp) is globular, shortly stalked, and springs from the axils of the leaves. The sporocarp is divided by partitions into from one to four cavities, and each of these bears on its outer wall a projecting ridge to which the sporangia aro attached. The species of Marsilea, are widely distributed mostly in hot countries; M. macropus is the nardoo of Australia. They all agree in hav ing leaves and long stalks and two pairs of leaflets arranged cross-wise; one pair is developed by lateral branching, the other pair by dicho tomy of the growing point. The sporocarp is variously placed, but always in relation to a leaf. Sachs considers that in both Marsilea and Pilidaria the sporocarps are modified leaves bearing the sporangia on their upper sides in definite relation to the fibro-vascular bundles. With re gard to position he compares them to the fertile fronds of Ophioglossacece In Marsilea the sporocarp is bean shaped, and the sporangia are attached to transverse ridges in small compartments which are torn away in the peculiar dehiscence of the sporocarp (fig. 13). IX. 14 gelatinous cord (p) to which the contents of the compart ments of the sporocarp are attached ; these compart ments (/) contain both macrosporangia and micro- sporangia.
Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/115
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