Polarity sometimes is present in an egg cell before fertilization, but it more commonly arises as a result of the penetration by a spermatozoid. In Protozoa, polarity is manifested in the location of organoids along the anteroposterior or dorsoventral axis. In nerve cells, polarity is expressed by the location of the axon and dendrites. In epithelial cells a distinction is made between the basal and distal parts, with a characteristic disposition of individual structures, such as the nucleus, the Golgi apparatus, and secretory granules. In animals, polarity is observed both in cells and in the entire organism. This changes the gradient of hormonal and trophic processes, which in turn determine the polarization of morphological and physiological structures. However, in some cases it is possible to disrupt the polarity by altering ambient conditions (light, heat, moisture, chemicals). The polarity of organs already formed generally is preserved even when their normal position is sharply disrupted (experiments on the inversion of grafts). The growth of aboveground parts is activated in stem grafts by the action of gibberellins, and the establishment and growth of roots are promoted by auxins. The addition of auxins to the implantation zone sharply increases the polarity. Thus, the implantation of a lilac bud in an undifferentiated callus tissue causes the polar formation of xylem strands. The leading role in the polarization of cells and tissues and in the orientation of organs in space belongs to plant hormones. The polarization and differentiation of each cell depend on the position the cell occupies with respect to other cells. In a developing plant organism, polarity appears in the predominant direction of cell divisions, growth, and differentiation. In seed plants, polarity is displayed even in the zygote and developing seed, where two rudimentary organs form, namely, the leaf bud and the root. The cells begin to break down, giving rise to a new organism oriented in a certain plane. Polarity arises in the spores of algae, fungi, mosses, horsetails, and ferns only after an appropriate external stimulus. Even the multicellular strands of green algae and the hyphae of fungi are marked by polarity: their constituent cells are oriented in one direction. Polarity is manifested with particular clarity in plants. The specific orientation of processes and structures in space, which is inherent in organisms and leads to the appearance of morphological and physiological differences at opposite ends or on opposite sides of cells, tissues, organs, and the organism as a whole.
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