6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 97 



the block. Therefore the larger the block the longer the time during 

 which the induced growth will be proportional to the original con- 

 centration of applied growth substance. Much larger blocks can be 

 applied terminally than can be applied unilaterally. 



With blocks as large as 26 mm^ it has been found that the straight 

 growth rate of the decapitated coleoptiles remains constant for at least 

 6 hours. As blocks of this size can be manipulated very conveniently 

 they have been adopted. 



The test blocks are applied so as to cover the entire cut surface of 

 the coleoptile stump. A small drop of water or gelatin solution may be 

 previously applied to the cut surface in order to insure good contact 

 with the block. About 20 minutes are required for the application of 

 100 blocks. 



Schneider and Went (1938) have shown that the length of time 

 between decapitation and application of the blocks is of considerable 

 importance in the curvature test. This has been confirmed by Thimann 

 and Schneider (1938). In the straight growth method, on the other 

 hand, the response to applied growth substance has not been fovmd to 

 be significantly influenced by the interval between decapitation and 

 application of the blocks, at least within the limits of 5 to 120 minutes, 



LENGTH OF TEST PERIOD 



The greatest sensitivity is obtained with the longest test period 

 during which the growth in excess of the control is proportional to 

 the concentration of applied growth-promoting substance. That is, 

 under such conditions the absolute useful amount of growth is the 

 greatest, and consequently the measurement can be made with great- 

 est accuracy. Actually, it has been found that a test period of 3 to 4 

 hours is quite adequate. In the present study a 4-hour period has 

 been generally used. In practice it is not essential to use a test period 

 of any exactly predetermined length. Hence, if it is inconvenient to 

 terminate the test at precisely 4 hours, there is no objection to making 

 the test period several minutes shorter or longer. In any comparable 

 series, of course, the test periods for all the sets should be the same. 



ENVIRONMENTAL CONDITIONS OF TEST 



Thimann and Schneider (1938) have reported that the growth 

 response of coleoptile sections to indole-3-acetic acid depends upon 

 the conditions of illumination of the seedlings during the previous 

 development ; maximal response was found when the plants received 

 red light during the first several hours of germination and were kept 



