Page:EB1911 - Volume 23.djvu/139

 It is important to note that in all the Bryophyta and in some of the Pteridophyta (most of the Filicinae, all existing Equisetinae, and the Lycopodiaceae and Psilotaceae) there is but one kind of sporangium and spore, the plants being homosporous or isosporous, whereas the rest of the Pteridophyta (Hydropterideae, Selaginellaceae) and the Phanerogams are heterosporous, having sporangia of two kinds; some produce one or a few large spores (megaspores), and are hence termed megasporangia, while others give rise to a larger number of small spores (microspores) and are hence termed microsporangia. In the Phanerogams the two kinds of sporangia have received special names: the megasporangium, which produces as a rule only one mature spore (embryo-sac), is termed the ovule; the microsporangium, which produces a large number of microspores (pollen-grains), is termed the pollen-sac.

The development of spores, except in the simpler Thallophyta, is more or less restricted to definite parts of the body. Thus in the Red Algae (Florideae) there are the organs known as stichidia, nemathecia. In the fungi the number and variety of such organs is very great; they may be described generally as simple and compound sporophores: but for a description the article should be consulted. In the higher plants the organs are less various. In the Bryophyta the production of spores is restricted to the sporogonium. In the vascular plants (Pteridophyta, Phanerogams) the development of sporangia, speaking generally, is confined to the leaves. In most ferns the sporangiferous leaves (sporophylls) do not differ in appearance from the foliage leaves; but in other Pteridophyta (Equisetaceæ, Marsiliaceae, some species of Lycopodium and Selaginella) they present considerable adaptation, and notably in the Phanerogams. In the Phanerogams the specialization is so great that the sporophylls have received special names; those which bear the microsporangia (pollen-sacs) are termed the stamens, and those which bear the megasporangia (ovules) are termed the carpels. The sporophylls are usually aggregated together on a short stem, forming a shoot that constitutes a flower.

Many terms are employed to indicate the nature of the various kinds of spores, especially among the fungi, but the endless varieties of asexual (and asexually produced) reproductive cells may be grouped under two heads—(1) Gonidia, (2) Spores proper.

The distinction between these two kinds of asexual reproductive cells is as follows.

The gonidium is a reproductive cell that gives rise, on germination, to an organism resembling the parent. For instance, among the algae, the “zoospore” of Vaucheria develops into a Vaucheria-plant. There is thus a close connexion between vegetative multiplication and multiplication by means of gonida. The production of gonida is entirely limited to the Thallophyta, and is especially marked in the fungi, though the nature of all the many kinds of reproductive cells formed in this group has not yet been fully investigated. It is, however, wanting in certain algae (Conjugatae, Fucaceae, Characeae) and fungi (some Peronosporeae and Ascomycetes).

The spore proper is a reproductive cell that as a rule gives rise, on germination, to an organism unlike that which produced it. For instance, the spore of a fern when it germinates gives rise, not to a fern-plant, but to a prothallium. The apparent exceptions to this rule occur only among the Thallophyta, and are explained below in the section on Life-history.

The true spore is developed, usually in a sporangium, after a process of division which presents certain features that call for special notice.

Observation of the process of division of the nucleus (karyokinesis) in plants generally has shown (for details see ) that the linin-reticulum of the resting nucleus breaks up into a definite number of segments, the chromosomes, each of which bears a series of minute bodies, the chromatin-disks or chromomeres, consisting largely of a substance termed chromatin. In the ordinary homotype divisions of the nuclei the characteristic number of chromosomes is always observable; but when the spore-mother-cells are being formed the number of chromosomes

is reduced to one-half. This, if the number of chromosomes of the parent plant be expressed as 2x the number in the spore will be x. To take a concrete case: it has been observed by Guignard and others that in the early divisions taking place in the developing anther and ovule of the lily the number of chromosomes is 24; whereas in the later divisions which give rise to the pollen-mother-cells in the one case and to the mother cell of the embryo-sac in the other, the number of chromosomes is only 12. Thus the development of a spore (as distinguished from a gonidium) is always preceded by a reducing- or heterotype-division, a process now more generally termed meiosis (Farmer). The reduced number of chromosomes in the nucleus of the spore-mother-cell persists in the spore, and in all the cells of the organism to which the spore may give rise. (Meiosis is discussed below in the section on Sexual Reproduction.)

It should be explained that cells, to which the name “spore” has also been applied, are formed as the result of a sexual act: such are zygospores, oospores, and some carpospores. But these cells differ from spores proper not only in their mode of origin but also in that their nuclei contain the full double number (2x) of chromosomes; hence they may be distinguished as diplospores.

Sexual Reproduction.—Sexual reproduction involves the development of sexual organs (gametangia) and sexual cells (gametes). When the organism is unicellular, as in the lower Green Algae (e.g. Protococcaceae, Conjugatae), the cell becomes a sexual organ and its whole protoplasm gives rise to one or more sexual cells: in the higher forms certain parts of the body are specialized as sexual organs. In many of the lower plants the organs present no external distinction of sex (e.g. lower Green Algae: the Chytridiaceae, Mucorinae, and some Ascomycetes among the fungi): it is impossible to distinguish between the male and female organs, although it cannot be doubted that the essential physiological difference exists; consequently the organs are merely described as gametangia. The gap between these plants and those with differentiated sexual organs is, however, bridged over by intermediate forms, as explained in the article.

When the sexual organs are more or less obviously differentiated into male and female, they present considerable variety of form in different groups of plants, and accordingly bear different names. Thus the male organ is a pollinodium in most of the fungi, a spermogonium in others (certain Ascomycetes, Uredineae); in all other plants it is an antheridium. Similarly the female organ is an oogonium in various Thallophyta (Green and Brown Algae: Oomycetous Fungi); a procarp in the Red Algae; an archicarp in certain Ascomycetous Fungi and in the Uredineae; an archegonium in all the higher plants.

It is generally the case that the protoplasm of the sexual organ is differentiated into one or more sexual cells. Thus the gametangium usually gives rise to cells which, as they are externally similar, are termed isogametes or simply gametes. Certain forms of the male organ, the spermogonium and the antheridium, give rise to male cells which are termed spermatia when they are non-ciliate, spermatozoids when they are ciliated and free-swimming. Again, the female organs termed oogonia and archegonia produce one or more female cells called oospheres. But there are important exceptions to this rule. Thus the protoplasm is not differentiated into cells in the gametangium of the Mucorinae; in the male organ (pollinodium), of fungi generally; and in the female organ (procarp) of the Red Algae and (archicarp) of the Ascomycetes and Uredineae.

The immediate product of the fusion of cells, or of undifferentiated protoplasm, derived from sexual organs of opposite sex may be generally termed the zygote; but it is not always of the same kind. Thus when two isogametes, or the undifferentiated contents of two gametangia, fuse together, the process is designated conjugation, and the product is usually a single cell termed zygospore. When an oosphere fuses with a male cell, or with the undifferentiated contents of a male organ, the process is fertilization, and the product is a single cell termed oospore. When, finally, a female organ with undifferentiated contents receives a male cell, the process again is fertilization; here the