106 GROWTH HORMONES IN PLANTS 



addition of wall materials. The walls can change their elastic 

 and plastic properties by hardening, by increasing wall substance, 

 and in many other ways. 



Methods for Measuring Extensibility. — A technique by which 

 cell-wall extensibility can be measured may be described briefly 

 as follows: A decapitated coleoptile or flower stalk can be sus- 

 pended perpendicularly, and the changes in length in a definite 

 region may be determined when a weight, for example, 2 to 10 g., 

 is attached to the lower end of the organ. The plant part being 

 tested must not be turgid, because the change in length resulting 

 from a definite pull on a turgescent coleoptile is only a small 

 fraction of the change in length of a coleoptile which is in a state 

 of plasmolysis. For plasmolyzing, one may use a 50 per cent 

 glycerin solution. The measurements are made with the horizon- 

 tal microscope. If the original length of the region marked 

 previously on the coleoptile is termed a, the length after attach- 

 ment of the weight h, and after removal of the weight c, then the 

 total increase in length (extension) is & — a (elastic and plastic 

 extension), the remaining increase in length, c — a (plastic exten- 

 sion) . If weights of 2 g. are used on the oat coleoptile, the exten- 

 sion that remains is negligible; therefore, only elastic extensibility 

 is measured (Soding, 1931). When a weight of 10 g. is used 

 (Heyn, 193 1&), the increase in length remaining after the weight 

 is removed (c — a) is about 30 per cent of the total length increase 

 (6 - a). 



Heyn (19316) used the following method to measure plastic 

 extensibility: Excised coleoptiles, with the primary leaf removed, 

 were fastened at one end and placed in a horizontal position 

 (Fig. 35). A weight of 250 mg. was placed upon the free end for a 

 given length of time. After the weight was removed, the size of 

 the bend formed in the coleoptiles was determined. This 

 curvature was used as an index of plastic extensibility. 



It is possible, also, to measure the extension of the cell wall 

 which is produced by osmotic pressure (Schmid, 1923). The 

 plant part is first placed in water and afterward plasmolyzed. 

 The difference in length before and after plasmolysis is a measure 

 of the extension of the cell wall due to osmotic force. 



Hypotheses on the Method of Action. — Since growth substances 

 may have a controlling effect upon the rate of growth, the relation 

 that they have to the growth of the cell waU is closely bound 



