life cycle

life cycle

the series of changes occurring in an animal or plant between one development stage and the identical stage in the next generation

Life cycle

The consecutive, interlinked stages of a product or structure and its fixtures, beginning with raw materials acquisition and manufacture and continuing with its fabrication, manufacture, construction, and use, and concluding with any of a variety of recovery, recycling, or waste management options.

life cycle

see LIFE COURSE.

Life Cycle

 

or cycle of development, the group of developmental phases through which an organism passes in attaining maturity and becoming capable of giving rise to the next generation (thus completing the life cycle).

The length of a life cycle is determined by the number of generations that develop during the course of a year or the number of years required to complete the cycle; it also depends on the length of the period of dormancy or diapause. In animals one may distinguish a simple life cycle, in which there is direct development of individuals, and a complex life cycle, with metamorphosis or alternation of generations. In development with metamorphosis the life cycle can be observed in the development of a single individual. For example, in the swine tapeworm, ovum → oncosphere → cysticercus → adult tapeworm; in the cockchafer, ovum → larva → pupa →imago.

In development with alternation of generations or of the modes of reproduction the life cycle can be observed in two or more individuals belonging to different generations, until the initial form reappears. For example, in the Scyphozoa, ovum → planula → scyphistoma → ephyra → medusa; in the liver fluke, ovum → miracidium → rediae → cercaria → adolescaria → adult worms; in aphids, ovum → stem-mother → migrants sexupara → bisexual insects. Thus, the unit in studying the life cycle may be either the individual ontogeny or a series of alternating ontogenies. In higher plants one may distinguish annual, biennial, and perennial life cycles. The alternation of the gametophye and sporophyte generations is characteristic of the life cycles of many lower plants and ferns. In parasitic fungi the life cycle is similar in its complexity to the cycles of certain parasitic worms. In rust molds the life cycle is complex; there are forms that yield aeciospores, urediospores, and teleutospores, as well as a basidial stage. In the protozoans the most complex life cycles are found in the Sporozoa (for example, in Gregarinida and Haemosporidia).

M. S. GILIAROV

life cycle

[′līf ‚sī·kəl]

(biology)

The functional and morphological stages through which an organism passes between two successive primary stages.

life cycle

lierne ribs

That period of time over which a building or piece of equipment can be reasonably expected to carry out its intended function.