Mechanics of Auxin-induced Growth 



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TIME IN HOURS 



Fig. 5. Progress curves of lAA-induced Avena coleoptile section — growth as influ- 

 enced by addition of calcium ions (10 mequiv/1) as well as by subsequent with- 

 drawal of calcium. Basal medium contained sucrose 0.09M and lAA 2 X lO'W. 

 A. Sections in basal medium throughout. B. Sections transferred to basal medium 

 plus calcium at first arrow and thence to KCl (1 mequiv/1) at second arrow. C. 

 Sections transferred to basal medium plus calcium at first arrow and thence to 

 basal medium minus salt at second arrow. D. Sections transferred to basal me- 

 dium plus calcium at first arrow and left in this solution. E. Sections in basal 

 medium plus calcium throughout. Modified after Cooil and Bonner (12). 



posed external load. The results of such measurements confirm and 

 extend the conclusions reached above on the basis of growth rate 

 studies, namely, that lAA increases cell wall deformability while cal- 

 cium ions decrease it. A convenient measurement is that of the rate 

 of bending of a coleoptile which is rigidly supported at one end and 

 loaded with a weight on the other. The mechanical analysis of this 

 method, which was first used by Heyn (15), is as follows: The turgid 

 cylindrical section of plant tissue, composed of cells, may be thought 

 of as a structure composed of bags of water, surrounded by intercon- 

 nected walls. When the section is subjected to a force normal to its 

 long axis, one side of the structure is placed in compression, the other 

 in tension. The side which is under compression cannot, as a first 

 approximation, compress. It is composed of incompressible liquid. The 



