The development of the ovule, which represents the embryo- Gymnosperms; when mature it consists of one or two sac. coats surrounding the central nucellus, except at the apex where an opening, the micropyle, is left.
The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or macrospore.
In some cases the embryo or the embryo-sac sends out suckers into the nucellus and ovular integument.
In Coelebogyne (Euphorbiaceae) and in Funkia (Liliaceae) polyembryony results from an adventitious production of embryos from the cells of the nucellus around the top of the embryo-sac. In a species of Allium, embryos have been found developing in the same individual from the egg-cell, synergids, antipodal cells and cells of the nucellus.
As the development of embryo and endosperm proceeds within the embryo-sac, its wall enlarges and commonly absorbs the substance of the nucellus (which is likewise enlarging) to near its outer limit, and combines with it and the integument Fruit and to form the seed-coat; or the whole nucellus and even the integument may be absorbed.
In some plants the nucellus is not thus absorbed, but itself becomes a seat of deposit of reserve-food constituting the perisperm which may coexist with endosperm, as in the water-lily order, or may alone form a food-reserve for the embryo, as in Canna.
Ovular characters determine the grouping in the Dicotyledons, van Tieghem supporting the view that the integument, the outer if there be two, is the lamina of a leaf of which the funicle is the petiole, whilst the nucellus is an outgrowth of this leaf, and the inner integument, if present, an indusium.
The Insemineae include forms in which the nucellus is not developed, and therefore there can be no seed.
The plants included are, however, mainly well-established parasites, and the absence of nucellus is only one of those characters of reduction to which parasites are liable.
Ovules naked, rarely without carpellary leaves, usually borne on carpophylls, which assume various forms. The single megaspore enclosed in the nucellus is filled with tissue (prothallus) before fertilization, and contains two or more archegonia, consisting usually of a large egg-cell and a small neck, rarely of an egg-cell only and no neck (Gnetum and Welwitschia).
An ovule consists of a conical nucellus surrounded by a single integument.
At an early stage of development a large cell makes its appearance in the central region of the nucellus; this increases in size and eventually forms three cells; the lowest of these grows vigorously and constitutes the megaspore (embryo-sac),which ultimately absorbs the greater part of the nucellus.
Meanwhile the tissue in the apical region of the nucellus has been undergoing disorganization, which results in the formation of a pollen-chamber (fig.
In the ripe seed the integument assumes the form of a fleshy envelope, succeeded internally by a hard woody shell, internal to which is a thin papery membrane - the apical portion of the nucellus - which is easily dissected out as a conical cap covering the apex of the endosperm.
A young ovule consists of a conical nucellus surrounded by a single integument terminating as a two-lipped micropyle.
A large pollen-chamber occupies the apex of the nucellus; immediately below this, two or more archegonia (fig.
The megaspore (embryo-sac) continues to grow after pollination until the greater part of the nucellus is gradually destroyed; it also gives rise to a vertical outgrowth, which projects from the apex of the megaspore as a short, thick column (fig.
A papery remnant of nucellus lines the inner face of the woody shell, and, as in cycadean seeds, the apical portion is readily separated as a cap covering the summit of the endosperm.
Each ovule is enclosed at the base by an envelope or collar homologous with the lamina of a leaf; the fleshy and hard coats of the nucellus constitute a single integument.
Abies), on each side of which is situated an inverted ovule, consisting of a nucellus surrounded by a single integument.
C, seminiferous scale as fused pair of leaves (1 1, 1 2, l 3, first, second and third leaves; b, shoot; Br, bract), D, cone-scale of Araucaria (n, nucellus; i, integument; x, xylem).
A pollen-grain when first formed from its mother-cell consists of a single cell; in this condition it may be carried to the nucellus of the ovule (e.g.
The ovule is usually surrounded by one integument, which projects beyond the tip of the nucellus as a wide-open lobed funnel, which at the time of pollination folds inwards, and so assists in bringing the pollen-grains on to the nucellus.
In Araucaria and Saxegothaea the nucellus itself projects beyond the open micropyle and receives the pollen-grains direct.
During the growth of the cell which forms the megaspore the greater part of the nucellus is absorbed, except the apical portion, which persists as a cone above the megaspore; the partial disorganization of some of the cells in the centre of the nucellar cone forms an irregular cavity, which may be compared with the larger pollen-chamber of Ginkgo and the cycads.
A complete female flower consists of a nucellus (fig.
Several embryo-sacs (megaspores) are present in the nucellus of a young ovule, but one only attains full size, the smaller and partially developed megaspores (fig.
Gazette, xlvi., 1908, regards this tissue as belonging to the nucellus.) At the time of pollination the long tubular integument secretes a drop of fluid at its apex, which holds the pollen-grains, brought by the wind, or possibly to some extent by insect agency, and by evaporation these are drawn on to the top of the nucellus, where partial disorganization of the cells has given rise to an irregular pollen-chamber (fig.
The integument of the sterile ovule is prolonged above the nucellus as a spirally-twisted tube expanded at its apex into a flat stigma-like organ.
The megaspore of Welwitschia is filled with a prothallus-tissue before fertilization, and some of the prothallus-cells function as egg-cells; these grow upwards as long tubes into the apical region of the nucellus, where they come into contact with the pollen-tubes.