22 GROWTH HORMONES IN PLANTS 



a unilateral incision is made (Stark and Drechsel, 1922) 2 to 

 3 mm. from the tip with a sharp scalpel or razor blade; the tip 

 then is removed by a slight jerk with forceps or the thumb and 

 forefinger. Went (1928a) removed 5 to 8 mm. of the tip when 

 decapitating. Various sorts of instruments have been made 

 (Went, 1928a; van der Weij, 1931; duBuy, 1933) to aid in 

 decapitation. The primary leaf protrudes from the cut surface 

 of the coleoptile stump after decapitation. It may be pulled 

 loose and carefully drawn out with a pair of forceps until only the 

 basal 5 mm. of it remain inside the coleoptile; the protruding 

 portion then is severed about 5 mm. above the tip of the coleop- 

 tile stump. 



When the coleoptile has been prepared as above, the object to 

 be tested, that is, a small plant organ, portion of an organ, or agar 

 block, may be applied unilaterally as in Fig. 1 (Paal and Stark) 

 and Fig. SB. If the object contains substances that influence 

 growth, they migrate down one side of the coleoptile and cause a 

 curvature. 



The actual procedure, from the time of decapitation on, varies 

 with different workers: Immediately after decapitation (of 

 15 to 25 mm. coleoptiles), Boysen Jensen applies unilaterally 

 the object that is being tested for the presence of growth sub- 

 stance. He cautions that while curvature is taking place the 

 humidity must not be so high that the plants guttate and disturb 

 the object being tested or so low that the object dries up; plants 

 grown in soil are best placed under bell jars which are partly 

 lined on the inside with moist paper. The rapidity with which 

 curvature takes place depends upon the temperature; in Boysen 

 Jensen's laboratory (Copenhagen) the work is carried out at 

 21.5°C. At this temperature the maximum curvature occurs 

 after 2^2 to 3 hours. The use of a longer experimental period is 

 not recommended, since "physiological regeneration" of the tip 

 can influence the reaction. 



In the Utrecht laboratory the coleoptiles are decapitated when 

 40 to 60 mm. long (Went, 1928a) and allowed to stand 40 min- 

 utes; at the end of this time, all coleoptiles that are not perfectly 

 straight are eliminated. Any guttation fluid that may appear at 

 the tip of the decapitated coleoptile is removed by "blotting" 

 with a small piece of filter paper. The object to be tested is 

 unilaterally applied and allowed to remain for 120 minutes, at 



