XXXIII 



GROWTH MOVEMENTS IN RELATION TO RADIATION 



Earl S. Johnston 



Division of Radiation and Organisms, Smithsonian Institution 



Light-growth response. Light intensity. Wave-length. Sensitivity. Growth sub- 

 stance. Generalization. References. 



Growth movements in plants are effected in a manner distinctly 

 different from amoeboid movements, locomotion of zoospores, turgor 

 movements, etc. Growth differentials arising in the two sides of a cell 

 or organ may be the result of either internal or external conditions, or 

 both. Curvatures resulting in differences in growth caused by asymmet- 

 rical conditions are designated as tropisms. The present discussion is 

 limited to tropic movements in plants and only those tropic movements 

 which are influenced by the action of light — phototropism. Limited 

 space does not permit either an exhaustive treatment of the subject or a 

 complete bibliography. Other interesting references on this subject may 

 be found in the papers mentioned in the present review. In spite of an 

 apparent confusion in the literature (9, 33, 36, 37) there is evidence that 

 phototropism is a special case of the more general light-growth phenome- 

 non, and that intensity, or the energy value of radiation, as w^ell as wave- 

 length and duration, must be carefully considered. Then, too, the plant 

 characteristics must be noted. Its sensitivity, frequently limited to 

 localized regions, its response, sometimes positive, sometimes negative, 

 and its previous history, each has a bearing on the reaction. Further- 

 more, there appears to be ample evidence of a substance which is directly 

 responsible for specific growth responses. 



Many of the experiments dealing with growth responses as influenced 

 by light, have been made with sporangiophores of Phycomyces and 

 coleoptiles of Avena, although a number of other plants have been used. 

 Blaauw (4, 5, 6, 7) laid the foundation for much of the recent work on 

 phototropism. Perhaps the first serious attempt to study phototropism 

 in which quantitative measurements and interpretation of modern 

 physics were used was made by him and published in 1909. Responses 

 were studied in the different spectral regions of sunlight and the carbon 

 arc. Energy values for these regions were calculated from Langley's (29) 

 tables. Blaauw found the most effective region of the carbon spectrum 

 for phototropic response of Avena seedlings to lie between 4660 and 



o 



4780 A, while the red and yellow regions were ineffective. According to 



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