BIOLOGICAL CYCLES. - There is no fundamental difference from the concept of rhythm. Life manifests itself rhythmically (v. BIORHYTHMS): such do the physiological processes appear, such as respiration, the cardiac pulse, the circulation, secretion, etc. But in the phenomena of life there manifest themselves more complex rhythms, of longer period, which are better defined as b. c. The life of an organism from birth to death is accomplished with a more or less complex b. c. It presents various phases or periods. The animal organism, for example, which originates from the egg cell, presents a first phase called segmentation, during which the egg segments into a great number of cells without growing. This period is succeeded by the embryonic one in which organogenesis takes place together with a first phase of growth; there follows a more or less long post-embryonic period of growth which leads, with its ceasing, to the adult condition. In the post-embryonic growth phase there may occur an ac-
(drawn from Enc. Catt.)
larval growth or fetal growth; thus growth can continue, in certain species, even in the adult. The adult condition, generally characterized by the cessation of development, corresponds to sexual maturity, which allows the formation of spermatozoa and eggs and the generation of a new progeny. The adult condition is followed by the period of senescence, and the biological cycle closes with the death of the soma.
The environments in which the organism passes through the various phases of its life cycle can be considered. These conditions are particularly interesting in animals in which there is a larval life and a metamorphosis of the larva: in anuran amphibians, for example, one passes from the aquatic life condition of the larva to the terrestrial life of the adult. In some animals, such as mollusks, crustaceans, worms, etc., there may be several larval conditions that succeed one another in different habitats. This phenomenon is particularly complex in those parasites in which the biological cycle is also completed through different hosts (intermediate hosts: those in which the parasite is not in the adult condition; definitive host: that in which the parasite matures the germinal elements: v. PARASSITISMO).
Another important condition in the cycle of life is the alternation of generations, referring to the condition of a single set (haploid condition: n) and of two sets (diploid condition: 2n) of chromosomes in the cells of the body. In plants one speaks of a diploid sporophyte; it matures the haploid spores from which germinate the haploid gametophytes that produce the male and female germinal cells (gametes). From the union of the two gametes comes the diploid zygote, from which the sporophyte germinates. In plants the sporophyte and the gametophyte may be represented by two plants of very different morphological appearance, as for example in ferns (fig. 1), even though the gametophyte (prothallus) is far less conspicuous than the sporophyte. The mechanism by which in plants the haploid spores are formed from the diploid cells of the sporophyte is represented by meiosis (v. GAMETI). In higher plants the greatly reduced prothallus develops on the sporophyte itself, without an independent life, in the flowers.
In animals the diploid generation is represented by the individual itself. In its gonads, by means of meiosis, the haploid gametes (eggs and spermatozoa) are obtained, which are the only representatives of the haploid generation. From their union, through the processes of fertilization, the zygote or fertilized egg, diploid, is obtained, from which the diploid animal originates through development (fig. 2).
Connected with biological cycles are the cyclical combinations undergone by the elements composing living matter, or biogenetic elements, which pass from the mineral world to the organic and living one. Particularly important are the cycles of carbon, nitrogen, phosphorus and sulfur (v. ORGANICAZIONE). In these cycles, organic matter is built by autotrophic organisms (green plants) that use the elements of the mineral world, with an endothermic synthetic reaction, exploiting the energy of the sun (v. BOTANICA). Heterotrophic organisms break down the organic substances thus formed, releasing the energy accumulated in them (exothermic reaction) and returning to the mineral world the elements that had been organic. This process of breakdown is mostly accomplished through oxidations (respiration; fig. 3).