36 Fundamentals of Auxin Action 



paper toweling place the seeds, grooved side down, with the embryo end projecting 

 slightly over the edge. Place the glass in a germinating dish in the darkroom; add 

 water to keep seeds moist but not wet; cover. One day after planting, germinating 

 seeds should be exposed to two hours of red light to inhibit elongation of the first 

 internode. 



Cutting 



Three days after planting, when coleoptiles are 20 to 30 mm. long, cut imiform 

 sections 3 to 5 mm. in length. Discard the apical 4 mm. of the coleoptile. For 

 greatest sensitivity, the coleoptiles should be prepared with the primary leaf re- 

 moved. Leaving the primary leaf within the cylinder is permissible when a supply 

 of growth substances from the primary leaf is not objectionable. Two sections may 

 be taken from each coleoptile provided that the distribution of first and second 

 sections is constant from treatment to treatment. 



Place the sections on comb teeth as shown in figure 17. This step is op- 

 tional, but is particularly convenient when a guillotine or van der Weij cutter 

 is used. Two out of every three comb teeth are renroved, and the remaining teeth 

 are filed down to fit inside the coleoptile cylinders. 



Place the sections directly into the solutions to be tested, or for more exact 

 studies to determine auxin activity, the sections should be placed in water for an 

 hour before testing. If combs are used, 40 ml. of test solution per Petri dish will 

 be necessary. If the sections are floating individually, 10 ml. of solutions are suffi- 

 cient. The sections must break surface. 



Reading 



Measure growth after 24 hours, when 80 to 90 per cent of growth has been 

 completed, or at 48 hours when growth is essentially complete. If the growth rate 

 is the function to be determined, growth is measured after 12 hours (McRae and 

 Bonner, 1952). The giowth rate is approximately constant for the first 18-hour 

 period. 



PROCEDURE FOR THE PEA STRAIGHT-GROWTH TEST 



Planting 



In a porcelain-ware tray place about one-half inch of dry vermiculite or sand. 

 Saturate with water. Scatter seeds of Alaska pea evenly over the vermiculite; cover 

 with dry vermiculite to a depth of about 2 inches. Place in the darkroom (25° C). 

 Planting should be 8 days before the test. The seedlings may be given some red 

 light. 



Cutting 



Select seedlings in which the stem internode above the first leaf node is 14 

 to 1/2 inch long. (If completely etiolated peas are used, select seedlings in which 

 the internode above the second scale leaf node is 1/2 to 1 inch long.) Uniformity of 

 plants used is important. Cut off the seedling near the base and then cut a section 

 of uniform length (a size between 3 and 5 mm. is satisfactory) beginning at a 

 uniform place such as i/j inch below the leaf node. Place directly in Petii dishes 

 containing solutions. The use of 10 ml. of solution keeps all sections at the surface. 

 Submerged sections grow crooked. 



