36 



PHYTOHORMONES 



TABLE III 



C. 4. Technical Modifications 



A number of modifications of this technique have been 

 used at various times and for different purposes. Only a 

 few will be mentioned. The original method was to decap- 

 itate only once, at 5 mm. from the tip, and apply the agar 

 blocks after waiting 40 minutes. The sensitivity of such 

 plants is lower by 2 to 3 times than that of plants treated 

 as above. Heyn (1935) recommends 3 decapitations, spaced 

 2 hours apart, the total length removed being still about 

 5 mm. The sensitivity of such plants is slightly increased. 

 Van der Weij (1931) was the first to prove that increasing 

 the time between first decapitation and putting on of the 

 agar blocks increased the sensitivity of test plants. Subse- 

 quently this was also found by Brecht (1936) and in our 

 own experiments {u ^). Skoog (1937) has introduced the 

 use of de-seeded plants, at about 30-36 hours after planting. 

 The plants, no longer held in place by the socket, are held 

 in the guides by wrapping cotton around the young coleop- 

 tiles (see Figure 16). The curvatures produced in such 

 plants continue to increase for at least 6 hours without the 

 regression due to regeneration, and this method is therefore 

 able to detect much smaller amounts of growth hormone. 

 The sensitivity of this method appears to be about the 

 same as that of Soding's Cephalaria test method (see below). 



Laibach and Kornmann (1933, 1933a) have applied auxin 

 in agar blocks to the outside of intact Avena coleoptiles and 



1 The symbol u, which will be used throughout the text, refers to unpublished 

 experiments. 



