File:Pfeffer's clinostat, "Plant physiology" (1918).jpg

English: Title: Plant physiology

Identifier: cu31924001222706 Year: 1918 (1910s) Authors: Palladin, Vladimir Ivanovich, 1859-1922; Livingston, Burton Edward, 1875-; Hopping, Aleita, 1890- Subjects: Plant physiology Publisher: Philadelphia, P. Blakiston's Son & Co Contributing Library: Cornell University Library Digitizing Sponsor: MSN

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Text Appearing Before Image: 264 PHYSIOLOGY OF GROWTH AND CONFIGURATION certain region of the rotated plant lies underneath for a short time, this region soon conies to lie above for the same period, so that gravitaition acts succes- sively in opposite directions upon each portion of the plant, and a tendency to bend toward one side is ofiset by an equal tendency to bend toward the opposite side. Thus no geotropic bending occurs in such an experiment. Geotropic bendings are due to unequal growth on the two sides of the bend- ing organ, and they occur only in the growing regions of stems and roots; after the tissues have become mature and have ceased to grow these bendings are no longer possible. Also, the more rapidly an organ is growing the more quickly it bends in response to gravitation, and all conditions that retard growth also retard the geotropic response.

Text Appearing After Image: Fig.'139.—Pfeffer's clinostat. A, arranged for rotation of potted plant on horizontal axis; B, glass moist chamber, for rotating germinating seeds, .etc. The effect of-gravitation upon the geotropically stimulated plant is to release certain chemical and physical reactions and these, in their turn, lead to the bend- ing itself, but only after a certain time has elapsed. The time period extending from the beginning of the application of the stimulus to the beginning of the visible response is termed the reaction time, and its'length varies with different organs and plants, from about forty minutes to several hours. It is not neces- sary, however that the stimulus be continued throughout all of this reaction period. If a plant is stimulated for a period shorter than its reaction time, as by lying quietly on its side, and is then rotated on the clinostat so as to equalize the lateral pull of gravity, geotropic bending finally occurs, providing the original period of stimulation was of adequate length. The shortest possible time of stimulation that is sufficient to bring about the later response is called the minimum presentation time of the geotropic stimulus. Generally tliis is only from two to four minutes, rarely longer, and the fact that this period is so short

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