34 



PHYTOHORMONES 



zone is a circular arc. It is, however, apparent from Dolk's 

 (1930) measurements of plants curved under the influence 



of auxin that this is far from being 

 the case, and that the radius of 

 curvature decreases smoothly from 

 the straight base to a region of maxi- 

 mum curvature some way below 

 the tip (cf. X H). The values 

 adopted for r and I are therefore of 

 necessity arbitrary. 



If the test is carried out as above, 

 the optimum time for photograph- 

 ing is about 90 minutes after ap- 

 plication of the agar block {cf. Fig- 

 ure 20) ; after this time the curvature 

 is no longer proportional to the concentration of hormone 

 in the agar, because the curvatures produced by low con- 

 centrations decrease or remain stationary, while larger ones 

 continue to increase. 



Fig. 14. Purdy's method 

 of measuring the difference 

 in growth between the two 

 sides of a curved coleoptile. 

 Explanation in text. (From 

 Purdy, Kgl. Danske Viden- 

 skab. Selskab., Biol. Medd. 3: 

 3-29, 1921.) 



C. 3. Preparation of the Agar 



A good quality of agar is well washed and made up to a 3 per cent gel. 

 To prepare the blocks containing auxin for the test, the agar may be 

 either first cut into sheets and these soaked in the test solution or it 

 may be melted and mixed with an equal volume of the test solution. 

 The preparation of agar sheets of uniform thickness for the former 

 method requires some precautions. The most satisfactory procedure is 

 to cut a block of agar about 40 X 20 X 20 mm. and to surround this 

 on 3 of its long sides with paraffin (see Figure 15). The agar is then 

 cut in a microtome using a safety razor blade, and the resulting book 

 of sheets (1 or 1.5 mm. thick) preserved in 60 per cent alcohol. Before 

 use, these must be washed free of alcohol (1 hour). They are then cut 

 into rectangles, with a special cutter, and these are placed in the test 

 solution for at least 1 hour; sufficient solution must be present to 

 ensure that the concentration is not changed appreciably by addition 

 of the agar. For the second method, usually ]4, cc of the melted agar 

 is mixed with 3^ cc. of the test solution, and }^cc. oi the resulting mix- 

 ture quickly poured into a shallow circular mould with removable base 

 whose dimensions, 10.3 mm. radius X 1.5 mm. deep, are such that it is 



