The male gametophyte is represented by one or few cells and, except in a few primitive forms where the male cell still retains the motile character as in the Pteridophyta, is carried passively to the macrospore in a development of the pollen grain, the pollen tube.
The independent plant which is generally attached to the soil by hair-like structures is the sexual generation, the sporophyte is a stalked or sessile capsule which remains always attached to the gametophyte from which it derives the whole or part of its nourishment.
The gametophyte is a small thalloid structure which shows varying degrees of independence affording an interesting transition to the next group.
The male gametophyte is sometimes represented by a transitory prothallial cell;, the two male cells are carried passively down into the ovary and into the mouth of the ovule by means of the pollen-tube.
The female gametophyte is extremely reduced; there is a sexual apparatus of naked cells, one of which is the egg-cell which, after fusion with a male cell, divides to form a large siispensorial cell and a terminal embryo.
In the vascular cryptogams and phanerogams it takes place in the spore mother cells and the reduced number is found in all the cells of the gametophyte, the full number in those of the sporophyte.
However, they belong respectively to two different forms in the life-history of the plants; the leaves of the mosses are borne by the gametophyte, those of the club-mosses by the sporophyte.
Thus in the series Bryophyta, Pteridophyta, Phanerogamia, whilst the sporophyte presents progressive development, the gametophyte presents continuous reduction.
The last-mentioned case has been regarded as representing an apogamous development of the sporophyte from the gametophyte comparable to the cases of apogamy described in Ferns.
As is well known, the dividing nuclei of the cells of the sporophyte generation of the higher plants exhibit a double number of chromosomes, while the dividing nuclei of the cells of the gametophyte generation exhibit the single number.
And since this rule has been found to hold good for all the archegoniate series and also for the flowering plants where, however, the gametophyte generation has become so extremely reduced as to be only with difficulty discerned, it is natural that when alternation of generation is stated to occur in any group of Thallophyta it should be required that the cytological evidence should support the view.
If the sporophyte generation is confined to the cystocarp, is the tetrasporiferous plant, as has been suggested, merely a potential gametophyte reproducing by a process analogous to the budformation of the Bryophyta?
Moreover, it is known that the reduction in the number of chromosomes which occurs at the initiation of the gametophyte generation in Pteridophyta occurs of the various constituent groups.
..Bangiaceae - Eti Florideae Eugleneae Chi Iromonadinae Pleurococcaceae - Endosphaeraceae Volvocaceae hlorosphaeraceae ï¿½ CoNJuGA'rAE, Siphonales Tetrasporaceao Ulvaceae Confdyvaleb Characeae in the culminating stage of Fucus, where the oogonium is separated from the stalk-cell, so that unless it be contended that the Fucus is really a sporophyte which does not produce spores, and that the gametophyte is represented merely by the oogonium and antheridium, there is no semblance of alternation of generation in this case.
Soc. (1906) (with bibliography); Lawson, " Sequoia sempervirens," Annals of Botany (1904); Robertson, " Torreya Californica," New Phytologist (1904); Coker, " Gametophyte and Embryo of Taxodium," Bot.
The complete life-history, with its regular alternation of gametophyte and sporophyte, is now known in all except a few rare genera of recent Pteridophyta, and will be described in connexion with the several groups.
On the formation of the spores a reduction to the number characteristic of the gametophyte takes place.
The modifications shown by the gametophyte of Lycopodium will be described below.
The main existing groups of the Filicaceae may now be briefly described, with special reference to the characters of gametophyte and sporophyte, which have been found of value in determining affinities.
The gametophyte is unknown.
In the Mosses the plant-body (gametophyte) is always separable into a radially organized, supporting and conducting axis (stem)
Besides this there is usually a living conducting tissue, sometimes differentiated as leptom, forming a mantle round the hydrom, and bounded externally by a more or less well-differentiated endodermis, abutting on an irregularly cylindrical lacuna; the latter separates the central conducting cylinder from the cortex of the seta, which, like the cortex of the gametophyte stem, is usually differentiated into an outer thick-walled stereom and an inner starchy parenchyma.
The gametophyte, which bears the sexual organs, is either a free-living thallus corresponding in degree of differentiation with the lower liverworts, or it is a mass of cells which always remains enclosed in a spore and is parasitic upon the sporophyte.
The body of the sporophyte in the great majority of the vascular plants shows a considerable increase in complexity over that found in the gametophyte of Bryophytes.
On the other hand, we have (2) an internal differentiation of conducting tissue, the main features of which as seen in the gametophyte of Bryophytes have already been fully described.
The matter is complicated by the apogamous transition from gametophyte to sporophyte in the absence of the ascogonium; also by the fact that there are normally two fusions in the life-history as mentioned earlier.
There is thus in all the Basidiales an alternation of generations, obscured, however, by the apogamous transition from the gametophyte to sporophyte.
Although in the forms without aecidia the two generations are not sharply marked off from one another, we may look up the generation with single nuclei in the cells as the gametophyte and that with conjugate nuclei as the sporophyte.
The gametophyte or prothallial generation is thus extremely reduced, consisting of but little more than the male and female sexual cells - the two sperm-cells in the pollen-tube and the egg-cell (with the synergidae) in the embryo-sac. At the period of fertilization the embryo-sac lies in close proximity tube has penetrated, the separating cell-wall becomes absorbed, and the male or sperm-cells are ejected into the embryosac. Guided by the synergidae one male-cell passes into the oosphere with which it fuses, the two nuclei uniting, while the other fuses with the definitive nucleus, or, as it is also called, the endosperm nucleus.
Green organism is developed; this is the prothallus (gametophyte, sexual generation; fig.
The point common to all Pteridophyta is that from the first the gametophyte is an independent organism, while the sporophyte, though in the first stages of its development it obtains nutriment from the prothallus, becomes physiologically independent when its root develops.
On the one hand from the Bryophyta (in which the sporophyte is throughout its life attached to the gametophyte), and on the other hand from the Gymnosperms and Angiosperms (in which the more or less reduced gametophyte remains enclosed within the tissues of the sporophyte).
Some of the more striking adaptive modifications in the gametophyte and sporophyte, and certain effects of altered external conditions which have been ascertained experimentally, may be briefly mentioned.
The gametophyte is unknown.
The gametophyte in Hymenophyllum is flat and variously lobed; that of Trichomanes may be similar, but in other species is filamentous.
Although the antithetic theory is supported by many facts regarding the lifehistory and structure of the group of plants under consideration, it is quite possible that a stage in which the sporophyte was wholly dependent on the gametophyte may never have been passed through in their evolution.