See also:

• REPRODUCTION

An isolated See also:

• LEAF (O. Eng. leaf, cf. Dutch loof, Ger. Laub, Swed. lof, &c.; possibly to be referred to the root seen in Gr. Ma-eta, to peel, strip)

This See also:

• CONVERSION (Lat. conversio, from convertere, to turn or ,change)

When sexual reproduction is suppressed the case is one of See also:

• APOGAMY

In more highly organized plants special organs are differentiated for the production of spores. In the See also:

• MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

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 See also:

• APPEARANCE (from Lat. apparere, to appear)

For instance, the spore of a See also:

• FERN (from O. Eng. fearn, a word common to Teutonic languages, cf. Dutch varen, and Ger. Farn; the Indo-European root, seen in the Sanskrit parna, a feather, shows the primary meaning; cf. Gr. rmep6 , feather, rTepis, fern)

When the organism is unicellular, as in the See also:

• LOWER

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 See also:

• FUSION

(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. (a) 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-See also:

• TUBE (Lat. tuba)

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 See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

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 produces spores and is diploid. Hence all such plants are to this extent polymorphic—that is, the plant assumes these two forms in the course of its life-history. When, as in many Thallophyta, one or other of these forms can reproduce itself by means of gonidia, additional forms may be introduced into the life-history, which becomes the more complicated the more pronounced the polymorphism. The most straightforward life-histories are those presented by the Bryophyta and the Pteridophyta, where there are but the two forms, the sexual and the asexual. In the life-history of a moss, the plant itself bears only sexual organs: it is the sexual form, and is distinguished as the gametophyte. The zygote (oospore) formed in the sexual act develops into an organism, the sporogonium, which is entirely asexual, producing only spores: it is distinguished as the sporophyte.

When these spores germinate, they give rise to moss-plants. Thus the two forms, the sexual and the asexual, regularly alternate with each other—that is, the life-history presents that simple form of polymorphism which is known as See also:

• ALTERNATION (from Lat. alternare, to do by turns)

The male prothallium consists of the few cells formed by the germinating pollen-grain (microspore); and though it is quite independent, since the microspores are See also:

• SHED

To begin with the Algae. In the Dictyotaceae (Brown Algae) there are two very similar forms in the life-history, the one bearing asexual reproductive organs (tetrasporangia), the other bearing sexual organs (oogonia ' and antheridia). It has 'been shown (See also:

• LLOYD, EDWARD (1845— )
• LLOYD, WILLIAM (1627–1717)
• LLOYD, WILLIAM WATKISS (1813–1893)

The second type of life-history has been traced in Nemalion. Here there is no tetrasporangiate form, consequently meiosis takes place at a different stage in the life-history. It has been observed (See also:

• WOLFE, CHARLES (1791-1823)
• WOLFE, JAMES (1727-1759)

The mycelium developed from the aecidiospore, as well as the uredospores and the teleutospores that it bears, shows two conjugate nuclei. When, however, the teleutospore is about to germinate, the two nuclei fuse (thus completing the sexual act) and meiosis takes place. As a result the promycelium developed from the teleutospore, and the sporidia that it produces, are uninucleate: so are also the mycelium developed from the sporidium, and the female organs (archicarps) borne upon it. Hence the limits of the sporophyte are the aecidiospore and the teleutospore: those of the gametophyte, the teleutospore and the aecidiospore. Similar observations have been made upon other Uredineae with a more contracted life-history. Phragmidium Potentillaecanadensis is a See also:

• RUST (O.E. rust, a word which appears in -many Teutonic languages, cf. Du. roest, Ger. rost); in origin it is allied with " ruddy " and " red," the reddish-brown powdery substance which forms on the surface of iron or steel exposed to atmospheric corrosio

The life-histories sketched in the preceding paragraphs show that one of the complexities met with in the Thallophyta is that meiosis does not always take place at the same point in the life-history. In the higher plants the incidence of meiosis is generally, though not absolutely, See also:

• CONSTANT, BENJAMIN (1845-1902)

This stage can still be observed in some of these Algae (e.g. Ulothrix, Ectocarpus) where the zoospores (gonidia) may either germinate independently, or fuse in pairs to form a zygote. Gradually the sexuality of these cells became more pronounced: losing the capacity for independent germination, they acquired the external characters of more or less differentiated sexual cells, and the gametangia producing them developed into male and female sexual organs. But this advancing sexual differentiation did not necessarily deprive the plant of the primitive mode of propagation: the sexual organism still retained the See also:

• FACULTY (through the French, from the Lat. facultas, ability to do anything, from facilis, easy, facere, to do; another form of the word in Lat. facilitas, facility, ease, keeps the original meaning)

In the Pteridophyta and the Phanerogams, the zygote gives rise to the highly developed sporophytic plant. Thus the See also:

• EVOLUTION

Abnormalities of Reproduction. In what may be regarded as the type of normal life-history, the transition from the one generation to the other is marked by definite processes: there is the meiotic development of spores by the sporophyte, and the sexual production of a zygote, or something analogous to it, by the gametophyte. But it has been mentioned in the preceding pages that the transition may, in certain cases, be effected in other ways, which may be regarded as abnormal, though they are constant enough in the plants in which they occur, in fact as manifestations of reproductive degeneration. In the first place, the sporophyte may be developed either after an abnormal sexual act, or without any preceding sexual act at all, a condition known as apogamy. In the second, the gametophyte may be developed otherwise than from a post- meiotic spore, a condition known as apospory.