Page:EB1911 - Volume 23.djvu/140

PLANTS] product is not a single cell, but a fructification termed cystocarp (Red Algae), or ascocarp (Ascomycetes) or aecidium (Uredineae), containing many spores (carpospores).

As a consequence of the diversity in the sexual organs and cells, in the details of the sexual act, and in the product of it, several modes of the sexual process have to be distinguished, which may be conveniently summarized as follows:—

I. Isogamy: the sexual process consists in the fusion of either two similar sexual cells (isogarnetes), or two similar sexual organs (gametangia): it is termed conjugation, and the product is a zygospore. Its varieties are:—

(a) Gametes ciliated and free-swimming (planogametes), set free into the water where they meet and fuse: lower Green Algae (Protococcaceae, Pandorineae, most Siphonaceae and Confervaceae); some Brown Algae (Phaeosporeae):

(b) Gametangia fuse in pairs, and a gamete is differentiated in each: the gametes of each pair fuse, but are not set free and are not ciliated (the Conjugate Green Algae): or, no gametes are differentiated, the undifferentiated contents)of the gametangia fusing (Mucorinae among the Fungi).

II. Ooamy: male and female organs distinct: the protoplasm of the female organ is differentiated into one or (rarely) more oospheres which usually remain enclosed in the female organ: the contents of the male organ are usually differentiated into one or more male cells: the process is fertilization, the product is an oospore.

(A) The sexual organs are unicellular (or coenocytic as in certain Siphonaceous Green Algae and in the Oomycetous Fungi); the female organ is an oogonium.

The male organ is an antheridium giving rise to one or more free-swimming ciliated spermatozoids:


 * (1) The oogonium contains a single oosphere which is fertilized in situ: higher Green Algae (Volvox, Vaucheria, Oedogonium, Coleochaete, Characeae); some Brown Algae (Tilopteris); among the Fungi, Monoblepharis, the only fungus known to have spermatozoids:
 * (2) The oogonium produces a single oosphere which is extruded and is fertilized in the water: Dictyota and some Fucaceae (Brown Algae):
 * (3) The oogonium contains several oospheres which are fertilized in situ: Sphaeroplea (Siphonaceous Green Alga):
 * (4) The oogonium produces more than one oosphere (2-8) which are extruded and are fertilized in the water: certain Brown Algae (Pelvetia, Ascophyllum, Fucus):

The male organ is a pollinodium which applies itself closely to the oogonium: the amorphous male cell is not ciliated and is not set free:
 * (1) The oogonium contains a single oosphere which is fertilized in situ: Peronosporaceae (Oomycetes):
 * (2) The oogonium contains several oospheres; Saprolegniaceae: but it is debated whether or not fertilization actually takes place.

(B) The male and female organs are (as a rule) multicellular; the male organ is an antheridium, the female an archegonium: the archegonium always contains a single oosphere which is fertilized in situ.

The male cell is a free-swimming ciliated spermatozoid: the antheridium produces more than one (usually very many) spermatozoids, each of which is developed in a single cell: all Bryophyta (mosses, &c.) and Pteridophyta (ferns, &c.): the only Phanerogams in which spermatozoids have been observed are the gymnospermous species Ginkgo biloba, Cycas revoluta, Zamia integrifolia.

The male cell is amorphous and passes directly from the pollen-tube into the oosphere (siphonogamy): all Phanerogams except the species just mentioned.

It must be explained that in the angiospermous Phanerogams, the male and female organs are so reduced that each is represented by only a single cell: the male, by the generative cell, formed in the pollen-grain, which usually divides into two male cells: the female, by the oosphere. The gradual reduction can be traced through the Gymnosperms.

Attention may here be drawn to the fact (see ) that, in several cases, the second male cell has been seen to enter the embryo-sac from the pollen-tube, and its nucleus to fuse with the definitive nucleus (endosperm-nucleus) or with one of the polar nuclei. The significance of this remarkable observation is discussed in the section on the Physiology of Reproduction.

III. Carpogamy: the sexual organs are (as a rule) differentiated into male and female: the protoplasm of the unicellular or multicellular female organ (archicarp, procarp) is never differentiated into an oosphere: in many cases definite male cells, spermatia, are produced and are set free, but they are not ciliated, and frequently have a cell-wall: the process is fertilization: the product is a fructification derived essentially from the female organ containing several (sometimes very many) spores (carpospores): characteristic of the Red Algae and of the Ascomycetous Fungi.

(A) There are definite male cells (spermatia):
 * The female organ is a procarp, consisting of an elongated, closed, receptive filament, the trichogyne, and of a basal fertile portion, the carpogonium: on fertilization the latter grows and gives rise directly or indirectly to a cystocarp: the spermatia are each formed in a unicellular antheridium and have no cell-wall at first: they fuse with the tip of the trichogyne: Red Algae (Rhodophyceae or Florideae):
 * The female organ (archicarp) resembles the preceding: in fertilization the fertile portion (ascogonium) develops into an ascocarp containing one or more asci (sporangia) each containing usually eight ascospores: the spermatia are formed by abstraction from the filaments (sterigmata) lining special receptacles, the spermogonia, which are the male organs: certain Ascomycetous Fungi (e.g. Laboulbeniaceae, some Lichen-Fungi, Polystigma). For the Uredineae, see Abnormalities of Reproduction, below).

(B) There are no definite male cells: the more or less distinct male and female organs come into contact, and their undifferentiated contents fuse: the product is an ascocarp:
 * The male and female organs are obviously different: the female organ is an ascogonium, the male a pollinodium: e.g. Pyronema, Sphaerotheca (Ascomycetes):
 * The male and female organs are quite similar: e.g. Eremascus, Dipodascus (Ascomycetes).

It may be explained that carpogamy is the expression of sexual degeneration. In the cases last mentioned, when the sexual organs are quite similar, they have reverted to the condition of gametangia. Still further reduction is observable in other Ascomycetes in which one of the sexual organs, presumably the male, is either much reduced or is altogether wanting. Again in the rusts (Uredineae), there are spermatia, but they are functionless (see section on Abnormalities of Reproduction). In the highest Fungi, the Autobasidiomycetes, no sexual organs have been discovered.

Details of the Sexual Act.—It has been already stated that the sexual act consists in the fusion of two masses of protoplasm, commonly cells, derived from two organs of opposite sex: but this is only the first stage in the process. The second stage is the fusion of the nuclei, which usually follows quickly upon the fusion of the cells; but nuclear fusion may be postponed so that the two sexual nuclei may be observed in the zygote, as “conjugate” nuclei, and even in the cells of the organism developed from the zygote (e.g. Uredineae). The result of nuclear fusion is that the nucleus of the zygote contains the double number of chromosomes—that is, if the number of chromosomes in each of the fusing sexual nuclei be x, the number in the nucleus of the zygote will be 2x. Moreover, this double number persists in all the cells of the organism developed from the zygote, until it is reduced to one-half by meiosis preceding either the development of the spores, or, less commonly, the development of the sexual cells. But there is yet a third stage, which consists in the temporary fusion of the chromosomes belonging to the two sexual nuclei. This always takes place as a preliminary to meiosis; it may be in the germinating zygote, or after many generations of cells have been formed from it. At the onset of meiosis the (2x) chromosomes are seen to be double, one of each pair having been derived from the male and the female cell respectively: the chromosomes of each pair: then fuse so that their chromomeres unite along their length, constituting the pseudo-chromosomes. The paired chromosomes separate and eventually go to form the two daughter-nuclei, one to each, which thus have half (x) the original number of chromosomes. The daughter-nuclei at once divide homotypically, retaining the reduced (x) number of chromosomes to form the four nuclei of a tetrad of spores (more rarely, e.g. Fucus, of sexual cells).

It will have been gathered from the foregoing sections that plants generally are capable of both sexual and asexual reproduction; and, further, that in different stages of their life-history they possess the diploid (2x) number of chromosomes in their nuclei, or the haploid (x) number. It may be at once stated that, in all plants in which sexual reproduction and true meiotic spore-formation exist, these two modes of reproduction are restricted to distinct forms of the plant; the sexual form bears only the sexual organs and is haploid; the asexual form only