Page:EB1911 - Volume 23.djvu/143

Rh the question arose as to its morphological significance; whether it is to be regarded as modified form of the gametophyte, or as an altogether new form intercalated in the life-history: in other words, whether the alternation is “homologous” or “antithetic.” In certain plants there is a succession of forms which are undoubtedly homologous: for instance, in Coleochaete where a succession of individuals without sexual organs is produced by zoospores (gonidia). The main fact that has been established is that the sporophyte, from the simple zygote of the Thallophyta to the spore-bearing plant of the Phanerogams, is characterized by its diploid nuclei; that it is a diplophyte, in contrast to the haplophytic gametophyte. Were these nuclear characters absolutely universal, there could be no question but that the sporophyte is an altogether new antithetic form, and not an homologous generation. But certain exceptions to the rule have been detected, which are described under Abnormalities of Reproduction: at present it will suffice to say that such things as a diploid gametophyte and a haploid sporophyte have been observed in certain ferns. It can only be inferred that alternation of generations is not absolutely dependent upon the periodic halving in meiosis and the subsequent doubling by a sexual act, of the number of chromosomes in the nuclei, though the two sets of phenomena usually coincide. It must not, however, be overlooked that these exceptional cases occur in plants presenting an abnormal life-history: the fact remains that where there is both normal spore-formation with meiosis, and a subsequent sexual act, the haploid form is the gametophyte, the diploid the sporophyte. But the actual observation of a haploid sporophyte and of a diploid gametophyte makes it clear that however generally useful the nuclear characters may be in the distinction of sporophyte and gametophyte, they do not afford an absolute criterion, and therefore their value in determining homologies is debatable.

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.

.—The cases to be considered under this head may be arranged in two groups:—

1. Pseudapogamy: sexual act abnormal.—The following abnormalities have been observed:—


 * (a) Fusion of two female organs: observed (Christman) in certain Uredineae (Caeoma nitens, Phragmidium speciosum, Uromyces Caladii) where adjacent archicarps fuse: male cells (spermatia) are present but functionless.
 * (b) Fusion between nuclei of the same female organ: observed in the ascogonium of certain Ascomycetes, Humaria granulata (Blackman), where there is no male organ; Lachnea stercorea (Fraser), where the male organ (pollinodium) is present but is apparently functionless.
 * (c) Fusion of a female organ with an adjacent tissue-cell; observed (Blackman) in the archicarp of some Uredineae (Phragmidium violaceum, Uromyces Poae, Puccinia Poarum): male cells (spermatia) present but functionless.
 * (d) There is no female organ: fusion takes place between two adjacent tissue-cells of the gametophyte; the sporophyte is developed from diploid cells thus produced, but there is no proper zygote as there is in a, b and c: observed (Farmer) in the prothallium of certain ferns (Lastraea pseudo-mas, var. polydactyla): male organs (and sometimes female) present but functionless. Another such case is that of Humaria rutilans (Ascomycete), in which nuclear fusion has been observed (Fraser) in hyphae of the hypothecium: the asci are developed from these hyphae, and in them meiosis takes place; there are no sexual organs.

2. Eu-apogamy: no kind of sexual act—


 * (a) The gametophyte is haploid:
 * The sporophyte is developed from the unfertilized oosphere: no such case of true parthenogenesis has yet been observed.
 * The sporophyte is developed vegetatively from the gametophyte and is haploid: observed in the prothallia of certain ferns, Lastraea pseudo-mas, var. cristata-apospora (Farmer and Digby), and Nephrodium molle (Yamanouchi).
 * (b) The gametophyte is diploid (see under Apospory):
 * The sporophyte is developed from the diploid oosphere: observed in some Pteridophyta, viz. certain ferns (Farmer), Athyrium Filix-foemina, var. clarissima, Scolopendrium vulgare, var. crispum-Drummondae, and Marsilia (Strasburger); also in some Phanerogams, viz. Compositae (Taraxacum, Murbeck; Antennaria alpina, Juel; sp. of Hieracium (Rosenberg): Rosaceae (Eu-Alchemilla sp., Murbeck, Strasburger): Ranunculaceae (Thalictrum purpurascens, Overton).
 * The sporophyte is developed vegetatively from the gametophyte: observed (Farmer) in the fern Athyrium Filix-foemina, var. clarissima.


 * In all the cases enumerated under Eu-apogamy, apogamy is associated with some form of apospory except Nephrodium molle, full details of which have not yet been published.


 * Many other ferns are known to be apogamous, but they are not included here because the details of their nuclear structure have not been investigated.

.—The known modes of apospory may be arranged as follows:—

1. Pseudapospory: a spore is formed but without meiosis, so that it is diploid—observed only in heterosporous plants, viz. certain species of Marsilia (e.g. Marsilia Drummondii) where the megaspore has a diploid nucleus (32 chromosomes) and the resulting prothallium and female organs are also diploid (Strasburger); and in various Phanerogams, some Compositae (Taraxacum and Antennaria alpina, Juel), some Rosaceae (Eu-Alchemilla, Strasburger), and occasionally in Thalictrum purpurascens (Overton), where the megaspore (embryo-sac) is diploid; in some species of Hieracium it has been found (Rosenberg) that adventitious diploid embryo-sacs are developed in the nucellus: these plants are also apogamous.

2. Eu-apospory: no spore is formed—of this there are two varieties:
 * (a) With meiosis: this occurs in some Thallophyta which form no spores; the sporophyte of the Fucaceae bears no spores, consequently meiosis takes place in the developing sexual organs; the Conjugate Green Algae also have no spores, meiosis taking place in the germinating zygospore which develops directly into the sexual plant.
 * (b) Without meiosis: the gametophyte is developed upon the sporophyte by budding; that is, spore-reproduction is replaced by a vegetative process: for instance, in mosses it has been found possible to induce the development of protonema, the first stage of the gametophyte, from tissue-cells of the sporogonium: similarly, in certain ferns (varieties of Athyrium Filix-foemina, Scolopendrium vulgare, Lastraea pseudo-mas, Polystichum angulare, and in the species Pteris aquilina and Asplenium dimorphum), the gametophyte (prothallium) is developed by budding on the leaf of the sporophyte, and in some of these cases it has been ascertained that the gametophyte so developed has the same number (2x) of chromosomes in its nuclei as the sporophyte that bears it—that is, it is diploid.
 * Apospory has been found to be frequently associated with apogamy; in fact, in the absence of meiosis, this association would appear to be inevitable.

Combined Apospory and Apogamy.—Instances have been given of the occurrence of both apospory and apogamy in the same life-history; but in all of them there is a regular succession of sporophyte and gametophyte. The cases now to be considered are those in which one or other of the generations gives rise directly to its like, sporophyte to sporophyte, gametophyte to gametophyte, the normally intervening generation being omitted.

It is possible to conceive of this abbreviation of the life-history taking place in various ways. Thus, a sporophyte might be developed from a haploid spore instead of a gametophyte as is the normal case, but this has not been observed: again, a sporophyte might be developed from a diploid spore (as distinguished from a zygote or a diploid oosphere), a possibility that is to some extent realized in the life-history of some Uredineae in which successive forms of the polymorphic sporophyte are developed from diplogonidia. Similarly a gametophyte might be developed from a fertilized or an unfertilized