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the period in the life of a plant beginning with the laying down of the rudiments of the flowers and inflorescences in the buds and ending with the withering of the perianth and the stamens. The principal purpose of flowering is to carry out the reproductive process. The period of flowering may be observed externally from the opening of the first flowers to the death of the last ones. Annual plants flower in the first year, and biennial plants in the second. Perennial herbaceous and arboreal plants flower only after having attained a certain age. For example, many trees flower after 20 or 30 years, and many herbs flower after two to five years. A great number of plants flower many times during their lifetime (polycarpous plants); agaves and certain palms, like annuals and biennials, flower only once (monocarpous plants). Many arboreal plants are characterized by periodicity of flowering. For example, many fruit trees flower abundantly several times every other year. Oak flowers abundantly only once every five to seven years. Some tropical plants, for example, Caesalpinia and coconut palm, flower continuously. In some plants the flowers do not close until they wither. In others the flowers may open and close repeatedly; for example, saffron flowers open and close ten to 12 times. The flowering process may last anywhere from 20 or 25 minutes (royal water lily) to 70 or 80 days (some orchids). As soon as pollination occurs, the flowers rapidly wither. The flowers of various plant species open at a definite time in the morning, day, or night (when the weather is fair and clear).
In the course of evolution, in accordance with the basic function of reproduction, plants developed the adaptive reactions of vernalization and photoperiodism (as a result of which flowering is confined to the most favorable season). They also developed numerous mechanisms that ensure the onset of reproduction. For example, flowers pollinated by insects attract them by means of nectar, pollen, odor, and color. During the flowering period many flowers emit a fragrance precisely when their insect-pollinators appear. Petunias, honeysuckles, pelargoniums, and other flowers pollinated by moths have a weak fragrance during the day, whereas flowers pollinated by bees and butterflies cease to emit a fragrance after sunset. Color attracts certain insects: bees prefer dark blues and violets, whereas moths prefer white and pale yellows. The flower of some orchids resembles the female of the pollinating insects, thus attracting the males; pollination of such flowers goes on until females that can “compete” with the flowers appear.
Internal factors that condition flowering have been studied since the mid-18th century. In 1798, J. W. von Goethe developed the theory of the flower as a modified shoot; his theory influenced the continuation of research in that direction. In 1880 the German botanist J. Sachs elaborated the physiological concept of flower-forming substances; in 1913 the German scientist G. Klebs showed the significance of nitrogen compounds. In 1920 the American scientists H. A. Allard and W. W. Garner discovered the phenomenon of photoperiodism, and the Soviet botanist M. Kh. Chailakhian advanced the idea of the hormonal nature of flowering. According to Chailakhian’s theory, flowering is regulated by a hormonal complex—florigen—which apparently induces the laying down of flower rudiments. In connection with this theory, research has concentrated on the meristem in the growing point of the shoot (at the site of the direct formation of flowers) and the leaves as the site of formation of the phytohormones regulating flowering.
REFERENCESChailakhian, M. Kh. Faktory generativnogo razvitiia rastenii. Moscow, 1964.
Aksenova, N. P., T. V. Bavrina, and T. N. Konstantinova. Tsvetenie i ego fotoperiodicheskaia reguliatsiia. Moscow, 1973.
Terekhin, E. S., and R. M. Fedorov. Zhizn’ tsvetka. Moscow, 1975.
Lang, A. “Physiology of Flower Initiation.” In Encyclopedia of Plant Physiology, vol. 15, part 1. Berlin et al., 1965. Pages 1,380–1,536.
V. Z. PODOL’NYI