GROWTH AND GROWTH HORMONES IN PLANTS 399 



one, as in indolebutyric acid, which is highly active (though perhaps also 

 after conversion to indoleacetic acid in situ), but there cannot be less than 

 one ; in other words, the compound in which the acid group is attached directly 

 to the nucleus, indolecarboxylic acid, is inactive: 



COOH 



H 



Until recently no compounds of this pattern, no matter what the ring struc- 

 ture, had been found active, but now we have a few, including some halo- 

 genated benzoic acids, showing real auxin activity; so far 2,3,6-trichloro- 

 benzoic acid is the most active. Evidently, then, it is not a particular group 

 nor even a simple spatial pattern which confers activity; it is rather some 

 property of the whole molecule, some arrangement of atoms and charges 

 which enables it to fit on to a receptive surface and thus "activate" it. There 

 has been too much discussion of the exact nature of this spatial arrangement 

 to summarize here. Compounds exist whose auxin activity provides an ex- 

 ception to most or all of the theories, and just recently an active compound 

 which does not even contain a ring was described : 



CH2COOCH3 



S 



I 



CHo CH3 



Its activity was not very strong, but seems to be real. The matter is made 

 harder, too, by the wide range of active compounds, which is in striking con- 

 trast to the very limited variation in formula which has been found possible 

 with the animal hormones. It is also made difficult by the absence of any 

 in vitro or cell-free system on which auxins show satisfactory activity. Many 

 isolated enzymes have been tested at different times, under the influence of 

 one or another plausible theory, but none is activated by auxin. Curiously 

 enough, tissues treated with auxin may afterward show marked changes in 

 these same enzymes, but that is another matter. It begins to look as though 

 the whole cell were necessary to auxin activity. 



The discovery that auxin promotes not only cell elongation but also cell 

 division in some tissues, which was first made for cambium by Snow, has 

 had far-reaching implications both theoretical and practical. The initiation 

 and downward spread of cambial activity in trees in the spring is due mainly 



