146 PHYTOHORMONES 



nutritive. Van Raalte further showed that on dextrose 

 agar the production of auxin by the root tip is actually 

 greater in the second hour than in the first, and if such tips 

 were then extracted with ether the amount of auxin ex- 

 tractable had increased. There is therefore little doubt that 

 auxin is produced for a short time in the isolated root tip, 

 when it is fed with dextrose. 



Fiedler (1936), however, found that neither by diffusion 

 into dextrose agar nor by extraction with chloroform could 

 auxin be obtained from isolated root tips of Zea Mays and 

 Vicia Faba in culture solutions after about 24 hours; even 

 after 8 hours in the medium 80 to 90 per cent of the auxin 

 originally present had disappeared. This disappearance of 

 auxin could be largely prevented by coating the cut surface 

 with gelatin or lanoline and is therefore probably due to 

 enzymic oxidation (cf. IV A). Fiedler concludes that root 

 tips in culture produce no auxin and adopts the view of 

 Thimann (1934) that the auxin in root tips comes there 

 from the seed or shoot. 



Since there are a number of chemically different auxins, 

 which all have the same effect on root growth, it is of special 

 interest to know which one is present in the root tip. This 

 question has been studied by Heyn (1935), who found, by 

 the determination of the diffusion coefficient, that the auxin 

 of the root tip of Vicia Faba has a molecular weight of 

 about 370, which is just the same as that for the Avena 

 coleoptile tip ; it is therefore probably auxin a or b. 



The transport of auxin in the root has been the subject 

 of conflicting experiments and views. Went (1932) suggested 

 that the polarity of the shoot was continuous with that of 

 the root, so that auxin transport would be towards the root 

 tip. Gorter (1932) investigated transport directly by using 

 Zea Mays root cyhnders in the same way as van der Weij 

 with coleoptiles (Chapter VI C), and found that transport 

 took place in both directions about equally (see also Heidt, 

 1931). The auxin appeared to be rapidly destroyed by the 

 root. The same thing was found for Vicia Faba by Faber 



