ovule(redirected from ovulum)
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in seed plants, a multicellular formation from which the seed develops. The principal parts of the ovule are the nu-cellus, the integument (or integuments), and the funiculus. The nucellus typically develops as a nodule of placental cells of the megasporophyll, or carpel. The integument arises as a ring-shaped welt at the base of the nucellus and eventually encloses the developing nucellus, leaving a narrow canal-like opening, or micropyle, at the top. Most gymnospermous plants have a pollen chamber under the mircopyle. The funiculus attaches the ovule to the placenta. The basal portion of the ovule, from which the funiculus departs, is called the chalaza.
According to the most popular hypothesis (the synangial hypothesis), the integument arises as a result of the concrescence of sterilized megasporangia surrounding the nucellus. Consequently, the ovule is a megasynangium, that is, a group of con-cresced sporangia, in which only the central sporangium is fertile. This hypothesis is confirmed by the structure of primitive ovules of many now-extinct seed ferns (Lyginopteridopsida). The primitive ovules had segmented integuments with a vascular bundle in each segment and free ends of segments in the region of the pollen chamber; there was as yet no typical micropyle.
Usually one of the cells of the nucellus proliferates and becomes archesporial, forming a tetrad of megaspores after meiosis. As a rule, three micropyle spores die off and the chala-zal spore gives rise to a female gametophyte—the primary endosperm of gymnosperms or the embryo sac of angiosperms —which remains inside the nucellus. Fertilization, development of an embryo, and conversion of the ovule into a seed become possible after pollination, that is, after the entry of male game-tophytes (pollen grains) into the micropyle of the ovule in gymnosperms or placement on the stigma in angiosperms. When this happens the ovules of gymnosperms ensure full development of the male gametophytes in the pollen chambers (formation of gametes) by supplying them with nutrient matter through haustoria embedded in the nucellus. In conifers and Gneticae, male gametes (spermatozoids) pass along the haustoria that grow to the female prothallia (thus consisting of pollen tubules). In angiosperms the pollen tubules, passing through the stigma and style, penetrate the cavity of the ovary and then, usually through the micropyle, enter the ovule. The tubules deliver the spermatozoids, which were formed in the pollen grains, either before pollination (in plants with tricellular pollen) or afterward, with the division of sperm cells in the pollen tubules (if the pollen is bicellular).
Gymnosperms are characterized by atropous and anatro-pous ovules and by their intermediate forms. The most primitive ovules have been discovered in Carboniferous seed ferns. In some of the ferns, for example, the Calymmatotheca, the ovule was protected externally by a cupule, which probably developed from segments of the sporophyll. Embryos have not been found in the ovules of these plants, evidently because the ovules fell off soon after pollination and the embryos developed without the period of dormancy characteristic of the seeds of true seed plants. Consequently, seed ferns were characterized only by the presence of ovules. Cordaitales were also ovule plants. Extant ovule plants include certain conifers (Podo-carpus), some sago palms (whose embryos complete their development in detached ovules), and ginkgoes (whose embryos mature on the maternal plant or after the ovule has fallen). The outer layer of the three-layer integument of sago palms is possibly homologous to the cupule of seed ferns. Some Ben-nettitales also had a cupule on the ovule. In some conifers the ovules are embedded in the tissue of the seed scales and con-cresce with the scale (Araucaria); in others they concresce with the modified seed scales—either epimatia (Podocarpus, Cepha-lotaxus) or arils (some Taxaceae).
The ovules of angiosperms are smaller than those of gymno-sperms. They have relatively thin integuments, since the ovules develop protected by the concresced megasporophylls. The integuments number two (in most monocotyledons and cho-ripetalous dicotyledons) or one (in most sympetalous dicotyledons). In some plants, however, integuments are absent.
In the morphological classification of ovules, consideration is given to the position of the micropyle and the longitudinal axis of the nucellus in relation to the funiculus. Five basic types of ovules and a series of derivatives have been established. Most common in angiosperms are anatropous ovules, in which the micropyle is turned toward the placenta; this type of structure is favorable for penetration of the ovule by the pollen tubule. In crassinucellate ovules the archesporial cell is separated from its epidermis by several layers of cells of the nucellus; in tenuinu-cellate ovules the cell usually lies close to the epidermis. In some plants, including species of the families Compositae, Pyrola-ceae, and Orchidaceae, even the epidermal layer of the nucellus is short-lived. In a few Loranthaceae and Balanophoraceae the ovules are not morphologically expressed; the megaspores and female gametophytes develop in cells of the placenta.
REFERENCESMaheshwari. P. Embriologiia pokrytosemennykh. Moscow, 1954. (Translated from English.)
Takhtadzhian, A. L. Vysshie rasteniia, vol. 1. Moscow-Leningrad, 1956.
Poddubnaia-Arnol’di, V. A. Obshchaia embriologiia pokrytosemennykh rastenii. Moscow, 1964.
Savchenko, M. I. Morfologiia semiapochki pokrytosemennykh rastenii. Leningrad, 1973.
A. N. SLADKOV