germ(redirected from dental germ)
Also found in: Dictionary, Thesaurus, Medical, Legal.
(1) In animals and man, the germ, or embryo, is the organism in its early period of development, from the fertilized ovum (zygote) until the appearance of the capacity for independent feeding and active locomotion. The development of the embryo usually occurs in the egg membranes or in special organs of the mother’s body.
(2) In plants, the germ, or embryo, is the rudiment of a new individual that develops from the zygote, which originates as a result of the fertilization of the egg cell in the embryo sac, or from the cells of the nucellus or integument (in the event of apomixis). The most simple type of embryonic development is that of the green algae; for example, spirogyra, whose zygote does not form a sporophyte filament, but immediately begins meiosis. In brown algae, such as fucus, the zygote attaches itself to a substrate and divides, after which it forms an embryonal body from the growth point of the thallus. In bryophytes the embryo grows on the maternal body, the gametophyte, on which it feeds and then forms sporogenic tissue in which the spores are formed. This type of embryo is not found among other groups of higher plants. In the oldest higher plants, the family Psilotaceae, development of the embryo occurs in the tissue of the gametophyte, where a series of successive divisions of the zygote take place. From the cells of the lower part a stalk is formed and it becomes embedded in the tissue of the gametophyte; in the upper part, the growth point of the shoot is laid down, with two meristematic nodules. The germ here is a polarized structure, one end of which forms the superterrestrial shoot, and the other the rhizome. In Lycopodiales the embyro has two symmetrical nodules of future germ layers, between which there is a growing point in the form of an apical cell. Embryonic development in horsetails is characterized by a regular arrangement of segments in four zones, which give rise to the shoot, the root, the first leaf, and the stalk. In Filicales the first four cells that originate as a result of the first two divisions of the zygote have a definite place for establishing the apex of the shoot, the first leaf, the root, and the stalk. In gymnosperms the initial cycle of embryonic development is characterized by a free nuclear, or coenocytic, stage (in sago palms, for example, there are 1,024 free nuclei), which is replaced by the formation of cells; then there is the establishment of the growing points of the shoot, root, and cotyledon nodules (in some species of pine there are as many as 18) in the forming embryo. In the majority of angiosperms the zygote divides into two cells: the apical cell, which gives rise to the embryo itself, and the basal cell, which gives rise to the suspensor.
Two phases of development are distinguished in embryo-genesis: proembryonic (from zygote to formation of the embryoderm) and embryonic (establishment of the basic structures: growing point of the shoot, cotyledon, hypocotyl, and embryo rootlet). The classifications of embryo types used by taxonomists in solving the problems of the evolution and phylogeny of plants were created on the basis of the first stages of division of the proembryo. The proembryonic stage of development is the same in monocotyledons and dicotyledons. In later stages the apical zone of the monocotyledon’s proembryo gives rise to only one cotyledon nodule; in dicotyledons there may be two, and in some species three or even more such nodules—which leads to the formation of polycotyledony. In angiospermous plants, contrary to gymnosperms, the development of the embryo is accompanied by the formation of cell membranes. Only in peonies is there revealed a new type of embryogenesis, in which first there emerge free nuclei in the zygote and then this multinuclear structure becomes cellular and proembryonic nodules arise on its periphery; of these only one develops into the embryo proper. Angiosperms are divided into two groups, according to the presence or absence in the embryo of the green pigment chlorophyll: chloroembryophytes (such as legumes and crucifers) and leucoembryophytes (such as Ranunculaceae and Gramineae). In the process of evolution, reduction of the embryo is observed among some groups of angiosperms, for example, in parasitic plants (Orobanchaceae) and saprophytes (Orchidaceae). The endosperm serves as the source of nutrition for the embryo; in some species (legumes) it is completely absorbed by the growing embryo, and in others (cereals) it is retained. Sometimes the embryo in the seed is surrounded not only by endosperm but also by other reserve tissue—the perisperm, which develops from the cells of the nucellus (in sugar beet, black pepper, and so forth).
REFERENCESBaranov, P. A. Istoriia embriologii rastenii v sviazi s razvitiem predstavlenii o zarozhdenii organizmov. Moscow-Leningrad, 1955.
Iakovlev, M. S. Embriogenez i ego znachenie dlia filogenii rastenii. Moscow-Leningrad, 1960.
Poddubnaia-Arnol’di, V. A. Obshchaia embriologiia pokrytosemennykh rastenii. Moscow, 1964.
Takhtadzhian, A. L. Osnovy evoliutsionnoi morfologii pokrytosemennykh. Moscow-Leningrad, 1964.
M. S. IAKOVLEV