Reproduction

REPRODUCTION, or the capacity for self-replication, one of the basic characteristics of living organisms, ensuring the preservation of the species. Among the externally infinite diversity of methods of R., two basic types can be outlined: R. by means of a single cell, or cytogony, and R. by means of multicellular bodies--vegetative R. (hydrozoans, worms, many plants). In its simplest form, cytogony is found in cell multiplication, carried out by cell division, which occurs in unicellular organisms (bacteria, algae, protozoa) and in the tissue cells of multicellular organisms. The reproduction of the latter is also mostly reduced to cytogony, for which agametes (special cells formed in the multicellular body) serve in the case of asexual, or agamic, reproduction, and gametes in the case of sexual reproduction, or gamogony. Gamogony in turn can undergo secondary reduction, passing into parthenogenesis (see), in which the fusion of male and female gametes into a zygote (fertilization) does not occur, but the female gamete is capable of virgin reproduction (insects, worms). Finally, combinations of different types of reproduction are very common--alternation of generations. Thus, heterogony is distinguished, when sexual reproduction alternates with parthenogenetic (insects, worms, lower plants), metagenesis, when sexual reproduction alternates with vegetative (for example, hydrozoans). Agamic cytogony occurs at the lowest stages of evolution, but here also the prototype of gamogony is found in the form of copulation (see) of protozoa, in which both gametes are similar to each other (isogamy). A special modification of copulation is conjugation (see) of protozoa, in which two unicellular individuals exchange parts of their nuclear apparatus. The same is found in lower multicellular organisms in lower algae, where along with isogamy, heterogamy also appears, and a distinction is made between the larger, mostly immotile macrogamete (female sex cell) and the motile microgamete (male sex cell). This differentiation of gametes reaches its highest development in higher multicellular organisms (egg and sperm), in which these cells are produced by special glands, and the meeting of gametes is often facilitated by the presence of special organs intended for this purpose. In higher plants, in addition to gametes, additional nuclei (synergids, polar nuclei, antipodes) also participate in fertilization, of which the polar nuclei give rise to the endosperm (double fertilization). Agamic cytogony is especially widespread, besides unicellular organisms, in lower plants. Here it is carried out by means of special cells, spores, formed from sporangia (algae, fungi, mosses, ferns), and sometimes a distinction is made between immotile spores-akinetes and motile-zoospores. Macrospores, giving rise to the female plant, and corresponding microspores are also found (some ferns). Here, therefore, sexual differentiation is outlined. Vegetative reproduction is carried out by means of various formations: statoblasts (bryozoans), buds (hydrozoans), cuttings (plants), gemmules (sponges), as well as by division of the multicellular organism with subsequent regeneration of the missing parts of the body (for example, worms). In this case, the newly formed individuals may not separate from the parent body, as a result of which complex colonial forms can be formed (for example, hydrozoans). Vegetative reproduction is made possible by the presence of embryonic cells (neoblasts) in the body, retaining the ability to develop, which is especially characteristic of plants. Sometimes (for example, in Hymenoptera), from one egg, many embryos develop at once due to vegetative reproduction of the areas separating within it (polyembryony). Very often the same form can possess several methods of reproduction. This especially applies to parasitic forms, both unicellular and multicellular. In these cases, developmental cycles are observed, sometimes reaching a high degree of complexity. Here we are dealing with adaptation to living conditions, experimental changes of which can also have a huge influence on the form of reproduction. Plant physiology is particularly rich in examples. Of external conditions, temperature, light (especially red and violet rays), the composition of the nutrient medium, and the concentration of hydrogen and other ions (for example, iron) can be noted. By varying these conditions, it is possible to modify the form of reproduction in both unicellular and multicellular plants. Thus, Chlamydomonas can reproduce asexually in Knop's solution, but when transferred to distilled water, sexual reproduction occurs (formation of gametes). On the other hand, in a nutrient solution with a high salt concentration, gametes do not copulate but reproduce parthenogenetically. In algae, sexual reproduction is stimulated by an increase in the intensity of illumination and a decrease in the concentration of nutrient salts. For higher plants, especially in recent years, a method has been found to influence the duration of the growing period by means of radiant energy and chemical treatment of seeds (vernalization), which has great national economic importance. A similar influence of external environmental factors on the form of reproduction is found in animals (insects, worms, hydrozoans). Thus, unfavorable environmental conditions (starvation, cold) can lead, in hydrozoan polyps, to the predominance of sexual reproduction, while under other conditions-asexual reproduction by budding. The appearance of males in parthenogenetically reproducing rotifers and daphnia is also stimulated by unfavorable environmental conditions (drying up of bodies of water, starvation, etc.). Nevertheless, the function of reproduction is regulated by internal factors determined by the structure and hereditary properties of the organism, which together with external environmental factors form a single integrated complex, the parts of which influence each other.- Of internal factors of reproduction, hormones can be mentioned, which determine not only the growth and development of the organism, but also the period associated with it when the capacity for reproduction appears. In some animals, however, this capacity can appear already at the embryonic stage of development (pedogenesis), an example of which is the Mexican axolotl, newt, gall wasps, aphids, etc. As for plants, here also, especially in recent years, many are inclined to attribute decisive importance to plant hormones as internal factors determining the form of reproduction.