DETECTION AND QUANTITATIVE DETERMINATION 29 



I is determined with millimetric paper, and t with a micrometer screw; the 

 diameter of the organ is usually about 1.5 to 1.7 mm. To measure r, arcs 

 with various radii (0.6 to 10 cm.) are drawn upon paper. If the curved 

 Avena coleoptile is compared with the arcs, the radius of curvature of the 

 coleoptile can be measured, and the d value computed in millimeters from 

 the equation given (p. 28). (For applications of the radius of curvature for 

 curvature measurements, see Rothert, 1894.) 



4. The number of experimental plants to be used for a determination 

 depends, of course, upon the degree of accuracy that one wishes to achieve. 

 Various experimental series of Nielsen (19306) give detailed information 

 concerning the fluctuations in magnitude of curvature in the experimental 

 plants. From these data it has been computed that when d is equal to 

 0.56 mm., the standard deviation of a single measurement is about 



±0.09 mm. A mean error of about ±0.03 mm. has been found when using 

 9 plants; the error is reduced to approximately ±0.013 mm. with 50 plants. 

 Measurements for general purposes of orientation can be made with about 

 6 to 8 plants; 10 to 12 and preferably 30 to 40 plants should be used for more 

 exact measurements. 



5. The unit of growth substance used in the Copenhagen laboratory 

 (Boysen Jensen, 19316) is that amount, dissolved in 50 cc. water plus 50 cc. 

 3 per cent agar, which will produce a d value of 1 mm. when the curvature 

 of the Avena coleoptile takes place at a temperature of 21 to 22°C., and 

 the magnitude of this curvature is measured after 3 hours. This amount is 

 designated as a growth-substance Avena unit (Wuchsstoff A-Einheit = WAE). 

 The size of the blocks should be uniform (2 by 2 by 1 mm.), although small 

 deviations have no influence upon the size of the curvature (as Nielsen 

 (19306) and van der Weij (1932) have shown) since this is dependent upon 

 the growth-substance concentration and not upon the amount of growth 

 substance (see also van der Weij (1932) and Thimann and Bonner (1932) 

 on concentration vs. amount); however, the amount of contact surface 

 between the agar and the coleoptile should always be the same (Fig. 8C). 

 The degree of curvature is much greater when the block is placed over a 

 bundle than when it is placed on parenchymatous tissue (Laibach and 

 Kornmann, 19336); hence the block should be placed in contact with a 

 vascular bundle if consistent results are to be obtained (Fig. 13). 



6. If the growth-substance content of a solution is to be measured in 

 WAE, a number of dilutions are made from the solution, e.g., 3'2 (is., 1 cc. 

 solution + 1 cc. agar), J4 (1 cc. solution, 1 cc. water, 2 cc. agar), etc., in 

 order to find the dilution that produces a d value of about 0.5 mm. If, for 

 example, the d value is 0.55 mm. with a 3^ dilution, then the original solution 

 contains 4.4 WAE in 100 cc. 



7. If it is desirable to determine how much growth substance moves from 

 a plant organ into an agar block in a definite time, the block must have a 

 very definite size, such as mentioned above. The following will serve as an 

 example for computation of the amount of extracted growth substance: 

 If a root tip is placed upon a block of dextrose agar 4 mm.^ in size and 

 allowed to remain for 2 hours, and the block produces in the Avena coleoptile 

 a curvature with a d value of about 1 mm., then the root tip has given off 



