BIOLOGICAL CYCLES. — There is no fundamental difference from the concept of rhythm. Life manifests itself rhythmically (v. Biorhythms): such are the physiological processes, such as respiration, heartbeat, circulation, secretion, etc. But in the phenomena of life more complex rhythms appear, with longer periods, which are better defined as biological cycles. The life of an organism from birth to death unfolds in a more or less complex biological cycle. It presents various phases or periods. The animal organism, for example, which originates from the egg cell, exhibits a first phase called segmentation, during which the egg divides into a large number of cells without growing. This period is followed by the embryonic period, in which organogenesis and an initial phase of growth occur; then follows a more or less prolonged post-embryonic growth period which, with its cessation, leads to the adult condition. During the post-embryonic growth phase, larval or fetal growth may occur; 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 for the formation of spermatozoa and eggs and the generation of new offspring. 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 also be considered. Particular attention must be given to the alternation of generations, which is a characteristic of many plants and some animals. In the plant kingdom, for example, the sporophyte and gametophyte generations alternate (v. Alternation of Generations). In the animal kingdom, the alternation of generations is less common but can be observed in certain groups, such as cnidarians (v. Metagenesis).
The biological cycle also involves the exchange of matter and energy between the organism and its environment. Autotrophic organisms, such as green plants, synthesize organic substances from inorganic ones, using solar energy (v. BOTANY). Heterotrophic organisms break down the organic substances thus formed, releasing the energy stored within them (exothermic reaction) and returning the elements to the mineral world. This process of breakdown is mostly accomplished through oxidation (respiration; fig. 3).