32 



GROWTH HORMONES IN PLANTS 



5. The rack of 12 test plants with their curved coleoptiles is placed 

 directly in front of a sheet of silver-bromide paper, and a shadow picture is 

 taken of the 12 coleoptiles (Fig. 15). 



6. After developing the print, the curvatures are determined by meas- 

 uringthe deflection of the coleoptile tips in degrees, a method first intro- 

 duced by Simon (1912); Went (1928a) suggests a simple measuring protractor 

 (Fig. 14) for this purpose, and the photograph provides a permanent 

 record which can be referred to later if desired. Soding (1934) has 

 described a measuring method in which the photographic step is omitted. 

 Each curved coleoptile is removed from the plant and placed upon a glass 

 plate over a protractor. The angle is determined indirectly. Navez and 

 Robinson (1932a) have described an automatic photographic method. 



Uiiinii 



Fig. 15. — Shadow pictures of Avena coleoptiles which have curved in response 

 to unilateral application of agar blocks containing growth hormone. These 

 curvatures may be measured with a protractor such as is shown in Fig. 14. 



Went (1928a) states that curvatures over a range of about 1 to 20 deg. are 

 strictly proportional to the concentration of growth substance in the agar; 

 hence if the mean curvature of 12 plants is 20 deg. or less, an accurate 

 determination of the concentration is possible (Fig. 2, Went, and Fig. 11). 

 If the curvature is much greater than 20 deg. ("maximum angle"), the 

 direct relationship between curvature and concentration no longer exists. 



7. METHODS OF EXPRESSING THE RESULTS. — Various uuits have been 

 proposed by workers using this technique, each based upon the degree of 

 curvature of the Avena coleoptile: 



One unit is that quantity of growth substance that has to be present in 

 1 cc. of solution to give, after mixing with 1 cc. agar, an angle of 1 deg. 

 at a temperature of 25°C. and a relative humidity of 85 to 90 per cent. 

 The blocks are prepared from the larger rectangular agar plates men- 

 tioned on page 31, and each has a volume of a little over 10 mm.^ One 

 block is applied to each of the 12 test coleoptiles. The average curva- 

 ture of the coleoptiles, in degrees, is multiplied by 12. The product, 

 then, may be expressed as plant units (Dolk and Thimann, 1932). 



One plant unit is the amount of growth substance applied in one agar 

 block, as above, to give an angle of 1 deg. The growth substance has 

 diffused from a plant part into the agar. These blocks are applied to 

 12 test plants also, so the average result is multiplied by 12, as in the 

 above. In this case the actual amount of material in each block 

 applied to the plant is but Moo of that present in 1 cc; hence a plant 

 unit is J^oo unit (Dolk and Thimann, 1932). 



