Cell, which is non-motile, is carried to the oosphere by means of a pollen tube.
The eggcell or oosphere is a large cell containing a single large nucleus, and in the green plants the rudiments of plastids.
In some cases the region where the penetration of the male organ takes place is indicated on the oosphere by a hyaline receptive spot (Oedogonium, Vaucheria, &c.), or by a receptive papilla consisting of hyaline cytoplasm (Peronosporeae).
Mycelium present, antheridia with antherozoids, oogonium with single oosphere: Monoblepharidaceae.
The contents of the antheridium are not set free, but that organ penetrates the oogonium by means of a narrow outgrowth, the fertilizing tube, and a male nucleus then passes over into the single oosphere, which at first multinucleate becomes uninucleate before fertilization.
In Cystopus Bliti the oosphere contains numerous nuclei, and all the male nuclei from the antheridium pass into it, the male and female nuclei then fusing in pairs.
We thus have a process of "multiple fertilization"; the oosphere really represents a large From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
Fertilized egg with the central uninucleate cell (o) surrounded by the oosphere and the fertilizing periplasm (p).
(1900); Stevens, "The Compound Oosphere of Albugo Bliti," Bot.
Of the three cells at the micropylar end of the sac, all naked cells (the so-called egg-apparatus), one is the egg-cell or oosphere, the other two, which may be regarded as representing abortive egg-cells (in rare cases capable Of fertilization), are known as synergidae.
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.
This remarkable double fertilization as it has been called, although only recently discovered, has been proved to take place in widely-separated families, and both in Monocotyledons and Dicotyledons, and there is every probability that, perhaps with variations, it is the normal process in Angiosperms. After impregnation the fertilized oosphere immediately surrounds itself with a cell-wall and becomes the oospore which by a process of growth forms the embryo of the new plant.
In such a case the zoogamete is male, is called an antherozoid or spermatozoid, and arises in an antheridium; the larger gamete is an oosphere and arises in an oogonium.
It is interesting to know, on the authority of Oltmanns, that when the oosphere is forming in the oogonium of Vaucheria, there is a retrocession of all the included nuclei but one.
Again, in oogamous reproduction, while in general only one oosphere is differentiated in the oogonium, in Sphaeroplea several oospheres arise in each oogonium; and while the oospheres usually contract away from the oogonial wall, acquiring for themselves a new cell-wall after fertilization, in Coleochaete the oosphere remains throughout in contact with the oogonial wall.
The oosphere is in all cases fertilized while still within the oogonium, the antherozoids being admitted by means of a pore.
There is usually distinguishable upon the surface of the oosphere an area free from chlorophyll, known as the receptive spot, at which the fusion with the antherozoid takes place; and in many cases, before fertilization, a small mucilaginous mass has been observed to separate itself off from the oosphere at this point and to escape through the pore.
In rare cases the oosphere has been known to germinate without fertilization (Oedogonium, Cylindrocapsa).
The oosphere is not differentiated within the wall of the oogonium, but certain cells known as wendungszellen, the significance of which has given rise to much speculation, are cut off from the basal portion of the parent-cell during its development.
Suddenly the attraction ceases, and the oosphere is fertilized, probably at that moment, by the entry of a single antherozoid into the substance of the oosphere; a cell-wall is formed thereupon, in some cases in so short an interval as five minutes.
Remarkable changes of size and outline of the oosphere have recently been described as accompanying fertilization in Halidrys.
In Dictyota the unfertilized oosphere is found to be capable of undergoing a limited number of divisions, but the body thus formed appears to atrophy sooner or later.
From each locule of a plurilocular sporangium there is set free an oosphere, which, being furnished with a pair of cilia, swarms for a time.
This is the carpogonium; it consists of a ventral portion which contains a nucleus, but in which no oosphere is differentiated, and an elongated tubular portion known as the trichogyne, into which the cytoplasm extends.
The doubling process is provided by the act of fertilization, where an antherozoid with the single number of chromosomes fuses with an oosphere also with the single number to provide a fertilized egg with the double number.