238 PHYTOHORMONES 



increase being proportional to the amount added. The 

 activity of any one factor alone is zero, the action of sugar 

 alone being ascribable to the presence of small amounts of 

 auxin in the plant. We thus have a complete parallel to the 

 interaction of bios I, II, and III on yeast growth, or that of 

 the Wuchsstoff B and Co-Wuchsstoff on Aspergillus growth; 

 or, to go still further, to the interaction of enzymes and co- 

 enzymes. Such interlocking systems of limiting factors are 

 widely distributed in nature and probably will be encoun- 

 tered in any process which is sufficiently analyzed. 



While bios, a growth substance for fungi, plays a part 

 in the growth of higher plants, there is no evidence that 

 auxins have any action on fungi or other lower organisms. 

 Boysen Jensen (1932), Nielsen and Hartelius (1932), Bonner 

 (1932), and Biinning (1934) all found that the auxin produced 

 by fungi has no effect upon their own growth. Ronsdorf 

 (1935) failed to find any effect of the addition of auxin a 

 to the culture medium of various fungi; the auxin, however, 

 was apparently destroyed in the medium. Similar destruc- 

 tion of indole-acetic acid in Rhizopus cultures was also 

 noted by Thimann and Dolk (1933) after the time of max- 

 imum auxin production had been passed. The above experi- 

 ments make it highly improbable that the action of plant 

 exudates on growth of Phytophthora can be due to auxins, 

 as supposed by Leonian (1935). 



The report of Popoff (1933) that the excysting action of 

 plant extracts on Euglena is due to auxin is probably equally 

 unfounded. For the excystment of Colpoda Thimann and 

 Barker (1934) found that at least two substances are in- 

 volved; one is present in impure auxin preparations but is 

 not identical with auxin, the other is completely unrelated 

 to auxin. Pure indole-acetic acid and other auxins have no 

 excysting action on Colpoda or some other protozoa (u). 



D. Regeneration 



We have made comparisons between the active substances 

 of higher plants and those of lower plants and of animals. 



