Have plants embryos
Evolution and biodiversity of plants
After the first division of the zygote, a short, two-celled cell thread is present. A thread has two ends and the embryo is already bipolar at this stage. In the whole plant kingdom there are essentially two different possible positions for these first two cells in the archegonium. Either one cell is facing the archegonia neck and the other is accordingly turned away, or the partition between the two cells is facing the archegonia neck. The second solution occurs with most ferns (leptosporangiate ferns). In the first solution, one has to distinguish between two important subcases. The future pole of the shoot can arise from the cell pointing outside towards the archegonia neck; this position is called exoscopic. The exoscopic location is e.g. typical of the mosses, in which the opposite pole remains as a haustorium in the archegonium. If, on the other hand, the pole of the shoot is inward and the opposite pole is turned towards the archegonium, this position is referred to as endoscopic. The endoscopic position is characteristic of the seed plants, since the later radicle is formed at the opposite pole and this will emerge first through the micropyle during germination.
Further divisions create a multicellular structure that already has a shoot vegetation point and one or more leaf systems. However, some of the cells of this structure degenerate again at an early stage and are not involved in the formation of the finished embryo that is ready to germinate. Certain parts of the embryo have very special and important tasks in individual development phases.
In moss and fern plants, nutrient uptake is perceived by a so-called foot, whereas in seed plants it is perceived by the cotyledons. In the case of the fern plants, the foot is opposite the point of the shoot vegetation and can be separated from it by a more or less distinct stalk. This stem can perhaps be homologated to the seta (capsule stem) of the mosses. The first root develops at quite different places in different groups of ferns, but it never arises at the other end of an axis of symmetry, as is the case with seed plants. All fern embryos are therefore more or less unsymmetrical.
In the cycads, the zygote first passes through up to 10 simultaneous nuclear divisions. The cell, which then contains up to 1024 nuclei, is usually transformed by wall formation into a cellular, undifferentiated tissue complex, which is referred to as the basal body. Starting from this and facing away from the micropyle, one or more filamentary structures develop, but they are multicellular in cross-section. These structures are called suspensors. A sprout vegetation point develops at the top of each suspensor. This means that from a zygote not just one embryo, but several genetically identical embryos can arise. In addition to this monozygotic polyembryony or fission polyembryony, a polyzygotic polyembryony can also occur wherever a macroprothallium has several archegonia. In the course of development, however, only one of the various embryos in an ovule is usually left; the others are crushed by the strongest embryo and degenerate prematurely.
The phenomenon of polyembryony shows that not all of the tissue emerging from the zygote goes into the formation of a single embryo. On the one hand, several embryos can be formed; on the other hand, tissue complexes such as the basal body and the suspensors perish before semen maturity and are not involved in the formation of the structure that is usually referred to as an embryo. For everything that emerges from a zygote through divisions, the term total embryo has therefore become established.
In the case of Pinus, the zygote initially forms a 16-cell complex in which the cells are arranged in four levels of four cells on top of each other. An embryo then develops from each of the four inwardly oriented cells. Separate, single-row suspensors develop from the cells of the next level. The other two levels form the so-called basal plate.
Polyembryonia normally no longer occurs in angiosperms. The basal body, which is already reduced to an 8-cell basal plate in Pinus, is further reduced and consists only of a single, enlarged basal cell. The suspensor is a single-row thread. If you follow the individual steps of division from the zygote to the embryo, you will notice remarkable and systematically usable differences. In some groups, the basal cell and suspensor arise from the cell lying exoscope after the first division of the zygote. In others, the exoscopic cell only forms the basal cell and does not undergo any further divisions; the suspensor is completely formed by the endoscopic cell. In other groups there are intermediate forms from these two extremes. In angiosperms there is obviously a different and precisely defined sequence of division steps for individual systematic groups in the development of the entire embryo. The study of embryogeny can therefore often provide important information about the phylogenetic relationships.
During germination, the pole opposite the shoot pole is first pushed out through the micropyle. This section, known as the radicle (radicle), is usually the later hypocotyl (see page 140).
The cotyledons of the seed plant initially remain in the seed as haustorial organs. If the whole cotyledon functions as a haustorium, it often remains in the seed as a whole and never assimilates. The first assimilating leaves are then already the following leaves. With this type of germination, the seed remains on or below the surface of the soil and the cotyledons are not visible, which is why it is called hypogean germination. In many cases, however, only the tip of the leaf is haustorial and the base is pushed out of the seed and greened. In this case, the seed coat is first lifted up as a hood on the tips of the cotyledons and only thrown off when the nutrients in the seed are completely absorbed. At the same time, the hypocotyl stretches and lifts the attachment point of the cotyledons above the ground. This type of germination is known as epigeic germination. There are transitions between hypogean and epigeic germination.
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