Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/442

Rh 424 REPRODUCTION [VEGETABLE. sxual and such cells are asexually produced spores. In the latter case the reproductive organs are such that they do not singly give rise to cells capable, each by itself, of ms * developing into a new organism. These are sexual repro- ductive organs. In some instances the sexual organ does not give rise to reproductive cells at all until it has received into itself more or less of the protoplasmic contents of another different, at least physiologically, sexual organ ; and the cells which it then produces are capable, each by itself, of developing into a new organism. In others the sexual organ produces reproductive cells without any such previous fusion of protoplasm, but the cells thus produced are incapable, each by itself, of developing into a new organism. Such cells are sexual reproductive cells. It is only by the fusion of two such cells, physiologically different, that a reproductive cell is formed which is capable of developing into a new organism. The fusion either of the protoplasmic contents of two different sexual organs or of two different sexual cells constitutes the sexual process. It may take place, according to circumstances, either within the organs or, in those cases in which sexual cells are produced and are set free, externally to them. The resulting cells are sexually produced spores. In some exceptional cases the normal production, sexual or asexual, of spores does not take place, but the new organism is developed vegetatively from the parent. When the sexual ogamy production of spores is suppressed, the case is one of apogamy ; [ when the asexual production of spores is suppressed, the case is one spory. of apospory. The following are instances of apogamy : in curtain Ferns (Pteris cretica, Aspidium falcatum) the pro thallium produces no sexual organs, but the fern-plant rises vegetatively as a bud upon it. Apospory has been observed to occur in Mosses and in some Ferns (Athyrium Filix-fcemina, Polystichum angulare) : in Mosses a new plant may be developed vegetatively from the tissue of the sporogonium ; in Athyrium the sporangium, instead of producing spores, develops into a fern-prothallium. In the Characeae apospory appears to be the rule. Apogamy and apospory will be more fully discussed subsequently in connexion with the subject of alternation of generations. Asexual Reproduction. Reproduction by means of asexually produced spores is common to nearly all families of plants. It is wanting, among the Algae, in the Conjugatae, the Fucacese, and the Characeae ; among the Fungi, in a few Peronsporese (Pythium vexans, Arto- trogus), in Ancylistes Closterii, in Aplanes Braunii, among the Saprolegnieae ; and, among the Ascornycetes, in Ere- mascus, Sordaria (Hypocopra), Ascobolus furfuraceus, Pyronema (Peziza) confluens, Gymnoascus, the Collemaceae, and most other Lichen-Fungi. relop- In the simplest case the spore is developed from a ntof single cell of the plant, which surrounds itself with the res - characteristic thick wall. This occurs only in plants of low organization ; Nostoc and Bacillus are examples of it. In other cases the contents of the cell undergo division, each portion of the protoplasm constituting a spore. Examples of this are afforded, among unicellular plants, by Yeast and Protococcus, and in multicellular plants by the Confervacese, the Ulvaceae, and some Floridese. >r- In this case each cell, the protoplasm of which divides jia. to form spores, may be regarded as a rudimentary reproductive organ of the nature of a sporangium. In more highly organized plants special organs are differen- tiated for the production of spores. In the majority of cases the special organ is a sporangium, that is, a hollow capsule in the interior of which the spores are developed. In the Thallophytes the sporangium is a single cell. In the Muscineae it is a multicellular capsule ; in Riccia, in which the structure of the capsule is simple, the whole of the internal cells give rise by division to spores ; in other Liverworts, and in the Mosses, in which the structure of the capsule becomes progressively more and more complex, a portion only of the internal cells give rise to spores. In the Ferns, Equisetaceic, and Lycopodiaceaj the sporangium is multicellular, but simple in structure. This is true also in the Rhizocarpos and in the Ligulatos (Selaginella, Isoetes), but in these plants there is this peculiarity that there are sporangia of two kinds, some, namely, which produce one, or a few, large spores, macrospores, and are hence termed macrosporangia, and others which give rise to a large number of small spores, microspores, and are hence termed microsporangia. The Phanerogams also bear two kinds of sporangia, which have received special names : the macrosporangium, which produces only one mature spore, is termed the ovule ; the microsporangium, which produces a large number of microspores, is termed the pollen-sac. In some cases among the Fungi the spores are Forma- not produced in the interior of a sporangium, but are tion of formed by abstriction. This occurs in some Mucorini, s P r< such as Chaetocladium, in the Ustilagineae, the Entomoph- ^ O a n ' thorete, the Peronosporese, the Ascomycetes, the Rusts (Uredineae), and the Basidiomycetes. These asexually produced reproductive cells are com mouly spoken of simply as spores, but in many cases some addition has been made to the word, or an altogether different name is applied to them, in order to mark some peculiarity in their mode of origin, to indicate their order of development, or to assign them without periphrasis to a particular group of plants. Thus, as has been mentioned, Tennir zoospores are motile spores unprovided, for a time at least, ology t with a cell- wall; stylospores are spores which are developed, 8 P res - not in sporangia, but by abstriction as mentioned above ; tetraspores is the name given to the spores of the Florideae to denote the fact that four spores are produced by the division of the mother-cell. The uredospores of the Uredineae are those which are produced during the summer, whereas the teleutospores of these plants are those which are formed in the autumn, at the end of the period of growth. It was the custom, at one time, to speak of the spores of Fungi as conidia; and at the present time the macrospores and the microspores of Phanerogams are better known as embryo-sacs and pollen-grains respectively. The organs which give rise to the asexually produced spores are usually not confined to a particular part of the plant in the Thallophytes. Instances of this do, however, occur among the Ascomycetous Fungi namely, in the Pyrenomycetes. Here the production of the stylospores Pycnii takes place in definite receptacles known as pycnidia. In the vascular plants (Pteridophyta, Phanerogams), the development of sporangia, speaking generally, is confined to the leaves. In many of the Pteridophyta the sporan- giferous leaves do not differ in appearance from the foliage-leaves ; but in other cases they undergo consider- able modification, as in the Equisetacese, Marsiliaceae, some species of Lycopodium and Selaginella, and notably in the Phanerogams. In the Phanerogams the modification is so Natur great that the sporaugiferous leaves have received special a flow names ; those which bear the microsporangia (pollen-sacs) are termed the stamens, and those which bear the macro- sporangia (ovules) are termed the carpels. When the sporangiferous leaves are thus modified they are usually aggregated together, and such an aggregate of sporangi- ferous leaves constitutes a flower. Sexual Reproduction. In nearly all classes of plants No se above the Protophyta spores are formed by a sexual process ; and in those in which no such process can be detected its absence is due, not, as in the Protophyta, to the fact that sexuality has not yet been developed, but to its gradual disappearance. The phenomena of sexual reproduction will be most intelligibly stated by tracing them in the different main divisions of the Vegetable Kingdom the Algre, the Fungi, the Archegoniata, and the Phanerogams.