158 PLANT PHYSIOLOGY 



The study of the reaction of plants to strong chemical and 

 thermal stimulation is to a considerable extent complicated by 

 the phenomenon of traumatropism, which was discovered by 

 Darwin (1881). If the tip of a root is severely injured by means 

 of a unilateral incision or cauterization with an acid, etc., the 

 irritation produced is transmitted to the zone of elongation and 

 produces a negative curvature. Traumatropic sensitivity per- 

 mits the root to avoid obstacles that may stand in its way. It 

 is, therefore, of ecological importance. The organs above 

 ground, particularly those of seedlings, also exhibit traumatropic 

 bending, but these are chiefly of a positive nature. 



Autotropism is the tendency of a plant to grow in its normal 

 form. Thus, in its growth, the stem tip may nutate from side 

 to side depending upon unequal growth on different sides, or it 

 may bend in response to stimuli. Curvatures formed owing to 

 the transient effect of some external stimulus are again straight- 

 ened out, provided that they have not been definitely fixed by 

 growth. Autotropism also serves to straighten the small curva- 

 tures of a casual nature. 



General References 



Blackman, V. H. The compound interest law and plant growth. Ann. 



Botany, 33: 353-360, 1919. 

 Bonner, J., and F. Addicott. Cultivation in vitro of excised pea roots. 



Bot. Gaz., 99: 144-170, 1937. 

 BoYSEN Jensen, P. "Growth Hormones in Plants." Trans. G. S. Avery, 



Jr., and P. R. Burkholder. McGraw-Hill Book Company, Inc., New 



York. 1936. 

 Brenchley, W. The relation between growth and environmental con- 

 ditions of temperature and bright sunshine. Ann. Appl. Biol, 6: 



211-244, 1920. 

 Harvey, E. M. A study of growth of summer shoots of the apple with 



special consideration of the role of carbohydrates and nitrogen. Ore. 



Agr. Exp. Sta. Bull. 200, 1923. 

 Knudson, L. Observation on the inception, season, and duration of cam- 

 bium development of the American larch. Bull. Torrey Botan. Club, 



40: 271-293, 1913. 

 LooMis, W. E. Translocation and growth balance in woody plants. Ann. 



Botany, 49: 247-272, 1935. 

 Lundegardh, H. "Environment and Plant Development." Trans, and 



ed. E. Ashby. Edward Arnold & Co., London. 1931. 

 Miller, E. C. "Plant Physiology," Chap. XIV. McGraw-Hill Book 



Company, Inc., New York. 1931. 



