TROPISMS. — The phenomenon of tropisms is closely connected with that of the excitability of living matter. Excitability is a fundamental or elementary property of organisms and is manifested when a physical manifestation of the environment becomes a stimulus.
Living matter does not react to all physical modifications of the environment, but only to certain categories of stimuli. These are of a mechanical, vibratory nature (radiations of specific wavelengths), or chemical. There are also stimuli of a biological nature, but most often these stimuli translate into chemical or physical stimuli. Stimuli can provoke a defined and specific reaction in relation to the type of cell differentiation. Thus, for example, the same stimulus, if it acts on a glandular cell, provokes a secretion, while if it acts on a muscle cell or fiber, it provokes a contraction (specificity of the reaction). It is well known that for an action to have the value of a stimulus, it must have a certain intensity, that is, it must exceed a "threshold" value, and it must not exceed a maximum limit of intensity, otherwise it ceases to be a stimulus and instead has a harmful effect. Furthermore, a stimulus must have a certain duration and not exceed II.
One form of reaction to stimuli is movement, in the sense of displacement of the cell or organism.
Tropea, diocese of — The Resurrection. Marble bas-relief from the school of the Gagini (15th-16th century) — Tropea, Cathedral.
cf. Gab. fot. naz.)
TROPEA, DIOCESE OF — The Resurrection. Marble bas-relief from the school of the Gagini (15th-16th century) — Tropea, Cathedral.
When this movement has a defined direction due to the exciting action, it is called a tropism. It can be positive, if the movement is toward the stimulus, and negative if it is in the opposite direction. Some authors use the term "tactism" when the movement is of a free organism that displaces itself, while tropism would refer only to the orientation of a fixed organism in relation to the stimulus. However, this distinction in meaning between the two words, especially used by botanists, is generally not followed by zoologists, who consider them synonyms. The enormous importance of tropisms is clear, both for the study of the "behavior" of organisms, demonstrating the entirely reflex nature of many movements, and for the analysis of the causes that provoke the phenomenon. The study of ecology, or the behavior of various species in their typical environment, places the problem of tropisms at the forefront. Thus, the distribution of organisms in various habitats is regulated by tropic conditions, taking into account the optimal stimulus condition. Finally, in embryonic and post-embryonic development, tropic phenomena are of primary importance in the morphogenesis of numerous structures.
Thus, nerve connections, fundamental for the life of the organism, occur through neurotropic actions (which resolve into chemo- and thigmotropic actions). In the analysis of tropisms, the effects due to mechanical stimuli are first considered. The material support effect for the growth of many tissues, called thigmotropism, is clearly demonstrated by many experiments. For example, to allow the proliferation of cells cultured "in vitro," it is necessary to provide them with a support, such as the fibrin stroma that forms in the plasma clot on which the explanted cells are placed. Thus, in the experience of "reconstitution" of a sponge passed through a sieve, as Wilson did, it is necessary to provide the cells with a support on which to reassociate. Similarly, a substratum is required for the growth of nerve fibers in the embryo, the notochord for the regeneration of the caudal spinal cord in urodele amphibians, and a support for the growth of climbing plants.
Other aspects of mechanical tropisms include geotropism, determined by gravity, which is positive for plant roots and negative for the stem; rheotropism, determined by a current, by which organisms move against or with the current. Thus, independently of any aspect of will, the ascent of fish up rivers for spawning (salmon, lampreys, etc.) or the descent to the sea of others (eels) is explained.
Very often, behavior is the result of the combined effect of two or more stimuli. Thus, the direction of a root is determined by the geotropic component and
Even the normal mechanism of nutrition of amoebae through phagocytosis is determined by chemical tropisms. Of great importance are, finally, chemiotropisms in pathology, not only in the mentioned phenomena of defensive phagocytosis, but also in the elective sites of parasites. Thus, for example, the rabies virus localizes in nervous sites (Di Vestea and Zagari), and the cholera bacillus in the lymphoid organs of the digestive system (Sanarelli).
Of fundamental biological importance are thermotropisms and phototropisms. Every form has its own temperature optimum, and thus moves toward this optimum. One factor, if not the only one, in many migratory phenomena (fish, birds) is the search for the optimum of temperature and light.
Phenomena of phototropism of great magnitude and extreme importance, even from a practical standpoint for fishing, are the bathymetric migrations of fish and plankton in the sea and lakes. Phenomena of phototropism and heliotropism can be easily observed in all plants equipped with chloroplasts and in which photosynthesis processes occur.
Of great importance are the phenomena of dilation or concentration of the pigment cells of many animals, in the dark or in the light (reptiles, among which the
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TROPISMS — TROYA CARLO
reaction upon encountering a paramecium with a source of stimulus that causes it to retreat; the appearance is one of continuous correction toward wrong directions. According to Jennings, these reactions eventually become habitual, and the protozoa behave in the same way, with this coming and going, even without the modifying causes of direction.
Even in the behavior of higher animals, many phenomena that appear conscious or voluntary are the result of an automatism that has been established through the habit of a series of responses to stimuli, which must be considered as tropisms.