34 



GROWTH HORMONES IN PLANTS 



Table 1. — Comparison of the Units Used by Different Investigators 

 FOR Expressing Growth-substance Concentrations 



The computations in Table 1 were carried out in the following 

 way: A comparison between the WAE and the AE shows that 

 the former is dissolved in 100 cc, the latter in 2 cc; since the 

 result in the first case is four times greater than that in the second, 

 one may conclude that 1 WAE corresponds to about 200,000 AE. 

 If the size of the agar block is considered, as is done by duBuy and 

 Nuernbergk (1932), the results are different; they compute that 

 1 WAE corresponds to approximately 54,000 AE. Since the 

 curvature is not proportional to the size of the agar block, this 

 value is probably too low (the size of the block used by Boysen 

 Jensen is, moreover, 1 by 2 by 2 mm. = 4 mm.^ and not 7.5 mm.^, 

 as stated by duBuy and Nuernbergk). 



Other Quantitative Methods. Growth in Length of Decapitated 

 Coleoptiles. — If an agar block containing growth substance is 

 placed over the entire cut end of a newly decapitated coleoptile, 

 its rate of growth in length is accelerated, in comparison with 

 the controls (Fig. 1, Went) . Although this method can be used as 

 a measure of the growth-substance concentration in the agar 

 block, it is not so accurate as the procedures outlined above. 



Pea-test Method. — This means of determining concentrations of 

 the growth hormone in solutions was described by Went (19346). 

 Seeds of Pisum sativum are grown in sand cultures in the dark- 

 room until the epicotyl is 5 to 20 cm. in length. Pieces 2 to 20 cm. 

 long are cut from the stem about 5 cm. below the growing point 

 and may be used immediately or in 4 to 8 hours after cutting. 

 Just before immersing the stem segments in the growth-substance 

 solutions of unknown concentration, the distal end of the seg- 

 ment is split lengthwise by an exactly median cut for a distance of 

 1 to 3 cm. Subsequent immersion in the solutions causes the 

 following: If no growth substance is present, each half will curve 

 outward; if growth substance is present, the free ends will start to 



