32 



Fundamentals of Auxin Action 



and cover with dry vermiculite or sand to a depth of about two inches. Place in the 

 darkroom (25° C). Planting should be eight days before the test. The seedlings 

 must receive some red light. Three hours of red light per day is very satisfactory. 



Cutting 



Select seedlings in which the stem above the first leaf node (the 3rd node) 

 is i/^ to i/o inch long. Uniformity of plants used is important. Cut off the seedlings 

 about 2 inches from the top, decapitate 14 inch below the leaf node, and slit the 

 stem from the tip to standard distance (3 cm.). For uniform results it is essential 

 to have stems slit exactly down the middle. 



Wash 



Place the slit sections in a dish of distilled water for one hour. 



Place in solutions 



Make up the solutions to be tested at a volume of 25 ml. in 100 X 20 mm. 

 Petri dishes. Place 5 or 6 slit peas in each dish, choosing carefully for symmetry 

 and uniformity of negative (outward) curvature. Do not use stems with less than 

 60° negative curvature on each arm. 



ABC 



Fig. 15. The slit pea test. A, a stem section before exposure to auxin. B, after 

 exposure to auxin, showing positive curvature. C, after exposure to weak auxin, 

 showing stem-reference method of reading curvature. 



Reading 



After the peas have been in the test solutions for 6-24 hours, read the curva- 

 tures obtained. Inward curvature is due to growth. Read curvature as the angle 

 formed between (a) the tangent at the point where inward curvature commences, 

 and (b) the tangent at the point where inward curvature ceases (i.e. the apical 

 tip) as shown in figure 15 B. Read the curvature on both halves of each stem. 

 This will give you 10 to 12 readings for each solution tested. 



