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HYDROPTERIDEAE

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Originally appearing in Volume V22, Page 614 of the 1911 Encyclopedia Britannica.
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HYDROPTERIDEAE .—TWO very distinct orders of heterosporous Filicales, the Salviniaceae and the Marsiliaceae, are included in this See also:

group. The difficulty of determining their exact relationship to the other orders of Ferns is increased by the more or less completely aquatic See also:habit of the See also:plants and the modifications and reductions in structure associated with this. The See also:absence of an annulus from their indehiscent sporangia makes it impossible to compare them with the other Ferns in respect of this important See also:character. It has been suggested with considerable See also:probability that the Marsiliaceae-are allied to the Schizaeaceae, while the Salviniaceae may possibly be related tc the Hymenophyllaceae or to some other See also:family of the Gradatae. Space will only permit of a brief See also:general See also:account of the more Obvious features of the several genera, the structure and See also:life-See also:history of which are known in See also:great detail. Unlike as they are in many respects, the two orders agree in being heterosporous. The microspores on germination produce a small, greatly reduced male prothallus bearing one or two antheridia which give rise to a number of spirally coiled, multiciliate spermatozoids. The single large megaspore contained in each megasporangium produces a small prothallus, which bears one or a few archegonia; these are exposed on the See also:surface of the prothallus at the See also:summit of the germinated megaspore (fig. r, i). r. The Salviniaceae include the two genera Salvinia (fig. so) and Azolla. The small See also:dorsiventral plants are in both cases floating aquatics. Azolla has roots depending from the See also:lower surface of the See also:stem into the See also:water, while these See also:organs are completely wanting in Salvinia, their See also:place being taken functionally by highly divided leaves See also:borne on the ventral surface of the stem.

Nostoc colonies are constantly See also:

present in a See also:special cavity of the dorsal See also:lobe of the See also:leaf in Azolla. The sporangia in both genera are associated in sori enclosed by indusia springing from the See also:base of the receptacle. In Salvinia (fig. 2, h) the sori are borne towards the base of the submerged leaves, in Azolla on the reduced ventral lobe of the leaf. They cons; st either of microsporangia or megasporangia, which are arranged in basipetal See also:succession on the receptacle. In the megasorus of Azolla there is only the one terminal, functional sporangium. The micro-spores are See also:united by means of hardened See also:protoplasm into one or more masses, while the solitary megaspores have a more or less complicated episporium. d. (Reduced. After Bischoff from Strasburger's Lehr&uch der Botanik.) A, From the See also:side. B, From above. 2.

The Marsiliaceae also include two genera, Marsilia and Pilularia, the latter of which is found in See also:

Britain. The plants grow as a See also:rule in marshy places, though some See also:species of Marsilia are xerophytic. The creeping stem produces roots from the ventral surface and leaves from the dorsal surface; the leaves when See also:young are circinately coiled. The leaves are See also:simple and linear in Pilularia, but in Marsilia See also:bear a pinnate four-lobed lamina. The highly specialized sporocarps are borne on the basal portions of the leaves, as a rule singly, but in some species of Marsilia in See also:numbers. The development of the sporocarp shows that it corresponds to a pinna, although when mature it may appear to occupy a ventral position in relation to the vegetative portion of the leaf. It has a complicated structure in both genera; in Pilularia its shape is nearly spherical, while in Marsilia it is elongated and See also:bean-shaped. The sort are See also:developed in depressions and are thus protected within the resistent See also:outer See also:wall of the sporocarp. There are usually four sori in Pilularia, while in Marsilia they See also:form two See also:longitudinal rows. Each sorus includes both microsporangia, with numerous spores, and megasporangia, each of which contains a single megaspore with a complicated wall. Enclosed within the sporocarp they can endure a See also:period of drought, but on the return of moist conditions are extruded from the sporocarp by the swelling of a special mucilaginous See also:tissue and the spores become See also:free. The development of the prothalli is in general similar to that of the Salviniaceae, though the resemblance may be homoplastic.

The stem in the less reduced forms is solenostelic with sclerenchymatous ground tissue occupying the centre of the See also:

stele. In the absence of See also:direct See also:evidence from See also:Palaeobotany, and bearing in mind the modifications associated with See also:adaptation to an aquatic life in other plants, the recognition of any more definite See also:affinity for these heterosporous ferns than that indicated above appears to be inadvisable. Further evidence is necessary before they can be removed from such a position of convenience as is assigned to them here and placed in proper relation to the See also:series of the Filicaceae. The several phyla of See also:Pteridophyta having now been briefly described, their relationship to one another remains for See also:con- Pnylogeny. sideration. The available evidence does not suffice to solve this question, although certain indications exist. In the earliest See also:land vegetations of which we have any sufficient See also:record specialized forms of Equisetales, Lycopodiales, Sphenophyllales and Filicales existed, so that we are reduced to hypotheses founded on the careful comparison of the See also:recent and See also:extinct members of these See also:groups. In this connexion it may be pointed out that the See also:fuller study of the extinct forms has as yet been of most use in emphasizing the difficulty of the questions at issue. It has thus led to a See also:condition of uncertainty as regards the relationship of the great groups of Vascular Cryptogams, in which, however, lies the See also:hope of an ultimate approach to a satisfactory See also:solution. The study of the Sphenophyllales, how- ever, as has been pointed out above, appears to indicate that the Equisetales and Lycopodiales may be traced back to a com- mon ancestry. As to the relationship of the Filicales to the other phyla, evidence from extinct plants appears to be wanting. If, as has been suggested by See also:Bower, the strobiloid types are relatively See also:primitive, the large-leaved Pteridophyta must be supposed to have arisen See also:early from such forms. The question cannot be discussed fully here, but enough has been said above to show that in the See also:light of our present knowledge the See also:main phyla of the Vascular Cryptogams cannot be placed in any serial relationship to one another.

It may even be regarded as an open question whether some of them may not have arisen independently and represent parallel lines of See also:

evolution from Bryophytic or Algal forms. This leads us to consider the question whether any indications exist as to the manner in which the Pteridophyta arose. It will be evident that no direct record of this evolution can be expected, and recourse must be had to hypotheses founded on the indirect evidence available. There appears to be no See also:reason to doubt that the sexual See also:generation is homologous with the thallus of a Liverwort, or of such an Alga as Coleochaete. It is with regard to the origin of the spore-bearing generation of the Pteridophyta that See also:differences of See also:opinion exist. This, though at first dependent on the prothallus, soon becomes See also:independent. It may be regarded as derived from a wholly dependent sporogonium not unlike that of some of the simpler See also:Bryophyta; the latter are assumed to have arisen from primitive Algal forms, in which, as the first step in the See also:interpolation of the second generation in the life See also:cycle, the fertilized ovum gave rise to a group of swarm spores, each of which developed into a new sexual plant. On this view the origin of the sporophyte is looked for in the See also:gradual development of sterile tissue in the generation arising from the fertilized ovum, and a consequent postponement of spore-formation. Certain See also:green See also:Algae (e.g. Oedogonium, Coleochaete), the Bryophyta, and the simpler Pteridophyta, such as Phylloglossum, have been regarded as illustrating the method of progression, though there is no reason to regard the existing forms as constituting an actual series. For a discussion of this view, which regards the See also:alternation of generations in Pteridophytes as antithetic and the two generations as not homologous with one another, reference may be made to the See also:works of Celakovsky and Bower. Although the antithetic theory is supported by many facts regarding the life-history and structure of the group of plants under See also:consideration, it is quite possible that a See also:stage in which the sporophyte was wholly dependent on the gametophyte may never have been passed through in their evolution.

The spore-bearing generation may throughout its phylogenetic history have been independent at one See also:

part of its life, and have been derived by modification of individuals homologous with those of the sexual generation, and not by the progressive sterilization of a structure the whole of which was originally devoted to asexual See also:reproduction. A number of facts regarding the Algae, and also those See also:relating to such deviations from the normal life cycle as See also:apogamy or apospory, may be regarded as lending support to this view, which, in contrast to the theory of antithetic alternation, has been called that of homologous alternation. Without entering further into the discussion of these alternative theories, for which the literature of the subject must be consulted, it may be pointed out that on the latter view the strobiloid forms of Pteridophyta would not necessarily be regarded as primitive relatively to the large-leaved forms, and also that the early stages of the origin of the sporophyte in the two cases may have proceeded on different lines. Another question of great See also:interest, which can only be touched upon here and may fitly See also:close the consideration of this See also:division of the See also:Vegetable See also:Kingdom, concerns the evidence as to the derivation of higher groups from the Pteridophyta. The most important See also:positive evidence on this point indicates that the most See also:ancient See also:Gymnosperms were derived from the Filicales rather than from any other phylum of the Vascular Cryptogams. Extinct forms are known intermediate between the Ferns and the Cycads, and a number of these have been shown to bear seeds and must be classed as Pteridospermae. These forms will, however, be found discussed in the articles treating of extinct plants and the Gymnosperms, but their recognition will serve to emphasize, in conclusion, the important position the Pteridophyta hold with regard to the existing See also:flora. Cultivation.—Numerous species of ferns, both temperate and tropical, are cultivated as valued ornamental plants. Species of the other groups are occasionally grown for scientific purposes in the larger botanic gardens, but their cultivation, which often presents special difficulties, need not be referred to here. While a number of ferns can be multiplied vegetatively, by buds formed on the leaves and in other ways, the See also:regular mode of See also:propagation is by See also:sowing the spores See also:shed from the ripe sporangia. The spores should be thinly sprinkled on the surface of the See also:soil in well-drained pots, which should stand in saucers filled with water and be covered with See also:glass plates. After the prothalli have attained some See also:size and bear sexual organs the pots should be occasionally sunk in water so as to See also:flood the prothalli for a few minutes and facilitate fertilization.

The young plants developed on the prothalli should be carefully pricked out into other pans and later transferred to 3-in. pots. When the pots are fairly filled with roots the plants may be shifted into larger ones. The best See also:

time for a general repotting of ferns is in See also:spring, just before growth commences. Those with creeping rhizomes can be propagated by dividing these into well-rooted portions, and, if a number of crowns is formed, they can be divided at that See also:season. In most cases this can be performed with little See also:risk, but the Gleichenias, for example, must only be cut into large portions, as small divisions of the rhizomes are almost certain to See also:die; in such cases, however, the points of the rhizomes can be led over and layered into small pots, several in succession, and allowed to remain unsevered from the See also:parent plant until they become well rooted. In potting the well-established plants, and all those of considerable size, the soil should be used in a rough turfy See also:state, not sifted but broken, and one-See also:sixth of broken crocks or See also:charcoal and as much See also:sand as will insure free percolation should be mixed with it. The See also:stove ferns require a See also:day temperature of 65° to 75°, but do not thrive in an excessively high or close dry See also:atmosphere. They require only such shade as will shut out the direct rays of the See also:sun, and, though abundant moisture must be supplied, the atmosphere should not be loaded with it. The water used should always be at or near the temperature of the See also:house in which the plants are growing. Some ferns, as the different kinds of Gymnogrammae and Cheilanthes, prefer a drier atmosphere than others, and the former do not well bear a lower See also:winter temperature than about 6o° by See also:night. Most other stove ferns, if dormant, will bear a temperature as See also:low as 55° by night and 6o° by day from See also:November to See also:February. About the end of the latter See also:month the whole collection should be turned out of the pots and redrained or repotted into larger pots as required.

This should take place before growth has commenced. Towards the end of See also:

March the night temperature may be raised to 6o°, and the day temperature to 70° or 75°, the plants being shaded in See also:bright See also:weather. Such ferns as Gymnogrammas, which have their surface covered with See also:golden or See also:silver See also:powder, and certain species of scaly-surfaced Cheilanthes and Nothochlaena, as they cannot bear to have their fronds wetted, should never be syringed; but most other ferns may have a moderate sprinkling occasionally (not necessarily daily) and as the season advances sufficient See also:air and light must be admitted.

End of Article: HYDROPTERIDEAE

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