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The light growth responses of Avena

, : The light growth responses of Avena. Rec Trav Bot Neerland 24(1/3): 307-582

The light growth responses are often of a complicated nature and can be analyzed into 2 or 3 subresponses a long light growth response, a short light growth response and a dark growth response. The long response may consist of diminished or of increased growth. It attains its 1st maximum (or minimum) after 1-2 hr. This long response is obtained only by exposure of the tip. It results from the typical function of the tip, that is, the secretion of growth-promoting substances ("auximones"). An exposure of the tip inhibits this secretion if under 8000 meter-candle-seconds (M.C.S.) and promotes it if above 8000 M.C.S. From this long response to 1-sided exposure the existence of 3 regions of curvature (1st positive, negative, and 2nd positive) and of 2 indifferent regions can be detected. The long response is obtained even with very small quantities of light (under 25 M.C.S.). The short response begins with diminished growth and results in a minimum in about 1/2 hour. The short light growth response may be caused by exposure not only of the tip but also of the subapical regions. For 1-sided exposure, the well known oscillatory movements may be due to the difference in strength of the short light growth-responses of the illuminated and the darkened sides. A rather strong quantity of light (for a zone of 2 mm. between 800 and 8000 M.C.S.) is necessary to obtain a short reaction. A dark growth response is caused by a sudden exposure to darkness after a continuous exposure of the subapical zone. This dark response is opposite to the short response in effect, for it is an acceleration, which reaches its maximum after about 1/2 hour. The dark response, however, is weaker than the short light response. When the subapical zone is exposed a very short time (a light-push), the exposure and the succeeding darkness each produce a separate reaction, so that there ensues a retardation (light response), followed by an acceleration (dark response). After an exposure of some hours, the tip does not show any perceptible dark response. The result of a total exposure during a short time is produced by the combination of these 3 elementary responses, and thus the great variability of phototropical curvatures at a total exposure may be explained. Finally, the author investigated the changes of permeability caused by a definite quantity of light (200X400 M.C.S.) using Helianthus; the result being a positive determination. An initial decrease of permeability was followed by an increase and then again by a weaker minimum; at last the curve of permeability became rather regular. This curve follows in a remarkable way the curves of light responses at exposures to rather large amounts of light.


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