Tropisms

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 comes into play when a manifestation of the physical world, of the environment, becomes a stimulus.

Living matter does not react to all physical modifications of the environment, but to determinate categories of stimuli. These are of a mechanical, vibratory (radiations of determinate wavelengths), or chemical nature. There are, moreover, stimuli of a biological nature, but most of the time these stimuli are translated into chemical or physical stimuli. Stimuli can provoke a definite and specific reaction in relation to the type of differentiation of the cell. Thus, for example, the same stimulus, if it acts on a glandular cell, provokes a secretion; if it acts on a cell or a muscle fiber, it provokes a contraction (specificity of the reaction). It is 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 limit of maximum intensity, lest it no longer be a stimulus but have a harmful effect. Furthermore, a stimulus must have a certain duration and not exceed it.

One form of reaction to stimuli is movement, in the sense of displacement of the cell or of the organism.

When this movement has a direction defined by 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. For some authors, the term "tactism" is used when the movement is that of a free organism that displaces itself, whereas tropism would be only orientation with respect to the stimulus in a fixed organism. But this difference in the meaning of the two words, especially as used by botanists, is generally not followed by zoologists, who regard them as synonyms. The enormous importance of tropisms is evident, both for the study of the "behavior" of organisms, demonstrating the wholly reflex nature of many movements, and for the analysis of the causes that provoke the phenomenon. The study of ecology, that is, of the behavior of the various species in their typical environment, places the problem of tropisms in the foreground. Thus the distribution of organisms in the various habitats is regulated by tropistic conditions, taking into account the optimal condition of the stimulus. Finally, in embryonic and postembryonic development, tropistic phenomena are factors of the first importance in the morphogenesis of very numerous structures.

Thus the nervous connections, fundamental for the life of the organism, come about through neurotropic actions (which resolve into chemotropic and thigmotropic actions). In the analysis of tropisms, the effects of tropism due to mechanical stimuli are recalled here in the first place. The effect of a material support for the growth of many tissues, called thigmotropism, is clearly demonstrated by many experiments. For example, to permit the proliferation of cells cultivated "in vitro," it is necessary to provide them with a support, such as the stroma of fibrin that forms in the coagulation of plasma on which the explanted cells are placed. Thus, in the experiment 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. Thus a substructure is needed for the growth of nerve fibers in the embryo, the notochord for the regeneration of the caudal spinal cord in urodele amphibians, a support for the growth of climbing plants.

But other aspects of mechanical tropisms are geotropism, determined by gravity, which is positive for the roots of plants and negative for the stem, and rheotropism, determined by a current, by which organisms go against the current or in favor of the current itself. Thus is explained, independently of any aspect of will, the ascent of fish along rivers for the deposition of eggs (salmon, lampreys, etc.) or the descent to the sea of others (eels).

Very often behavior is the result of the combined effect of two or more stimuli. Thus the direction of a root is according to the component geotropism and t. + (or —) with respect to water (moisture). The curious experiment of Rose (fig. 1) shows the effect of these two t. The root of the seedling that sprouts in a box with a wire-mesh bottom, through the effect of + geotropism, comes out through the mesh, but then the moisture draws the roots back inside the box, so again the geotropism moves the root outside, and so on. Thus in the upstream migration of sea fish into rivers, certainly besides rheotropism chemotropism also plays its role, determined by fresh water (or salt water for fish that descend to the sea). Likewise in the case of spermatozoa, they ascend the female genital tracts both by + rheotropism and by chemotropism determined by the gynogamones emitted by the eggs. Clear examples of chemical stimuli are obtained by studying the behavior of protozoa in the presence of gas bubbles of various substances (air, oxygen, carbon dioxide, etc.; fig. 2). Such is the action of malic acid, which attracts the antherozoids of ferns (discovery of Pfeffer). Many cases of sexual attraction are nothing other than + chemotropisms determined by peculiar chemical substances even in extremely faint concentrations. The olfactory sense of insects manifests itself through these effects of chemotropism (experiments of Prufer with butterflies). Thus the deposition of eggs in determined sites is also a chemotropism (e.g., meat for flesh flies).

Chemotropism is fundamental in the defensive processes of higher organisms and of man. Leukocytes rush to the sites of infection, attracted by chemical substances, whether emitted by the germs (toxins) or by the cells of the organism itself in the process of destruction. The classic observations of Metschnikoff, of Bordet, of Buchner are recalled. There are nevertheless bacteria, such as those of chicken cholera, that act negatively on leukocytes.

The normal mechanism of nutrition of amoebas by phagocytosis is also determined by chemical tropisms. Finally, chemotropisms are of great importance in pathology, not only in the aforementioned phenomena of defensive phagocytosis, but also for the sites of predilection 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 tract (Sanarelli).

Thermotropisms and phototropisms are of fundamental biological importance. Each form has its own optimum of temperature and therefore moves toward this optimum. A 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 of extreme importance, also from the practical point of view, for fishing, are the bathymetric migrations of fish and of plankton in the sea and in lakes. Phenomena of phototropism and of heliotropism + are easily observed in all plants provided with chloroplasts and in which processes of photosynthesis take place.

Of very great importance are the phenomena of dilation or concentration of the pigmentary cells of many animals, in darkness or in light (reptiles, among which typical-

(from G. Cotronei, op. cit., p. 221)
TROPISMI - Fig. 3. Galvanotropism.

the chameleon, fish, amphibians, crustaceans, etc.). Of interest are the galvanotropisms, that is, the t. determined by a galvanic electric current. One can observe how the condition can be + in some forms, as in many flagellates (Polytoma uvella according to Verworn), or —, as in Amoeba proteus (Verworn, fig. 3).

It has been observed how the sign can at times change, the sign of the galvanotropism being conditioned by other factors. Thus paramecia (ciliate Protozoa), to give an example, have a negative galvanotropism in fresh water and a positive one in salt water. Of interest are certain theories that have been proposed to explain the phenomenon of t. The theory of Verworn-Loeb, for instance, places in the foreground the symmetry of organisms and the symmetry of receptors. If the stimuli act on one side, they provoke an asymmetrical reaction that will bring the animal to be equally excited from both sides. Such movement is absolutely involuntary. Much importance has been attributed by Child and his school to the chemical and metabolic gradient of organisms, on account of which there is a different behavior between the anterior and caudal parts. The different susceptibility of the axial parts is fundamental in the phenomena of forward and backward movement. Also of interest is Jennings's theory "of trial and error." The material used by this author is represented by protozoa. If one examines paramecia in a culture, one observes the behavior of the animal, which goes in one direction, at a certain moment stops, moves backward, and then takes a new direction. According to Jennings, the fact is in relation to the paramecium's encounter with a source of stimulus that makes the animal move backward; the appearance is one of a continuous correction toward wrong directions. According to Jennings, these reactions in the end become habitual, and the protozoa behave in the same way, with this coming and going, even without the causes that modify direction.

Also in the behavior of higher animals, many phenomena that are apparently conscious or voluntary are the fruit of an automatism that has been established through the habit of a series of responses to stimuli, which are to be considered as t.

Alberto Stefanelli