I CYTOPLASM 209 



In this context the group of auxins amongst the phytohormones 

 will be discussed briefly as a further example of compounds having 

 a specific effect (Went and Thimann, 1937). The auxins admittedly 

 are not very specific, as they initiate all kinds of different reactions of 

 growth : elongation growth of meristematic cells, division growth of 



C-CH,-COOH C^C-CH^-COOH 



f^H I Indolyl-^- acetic odd ^^^^CH2 ^ lnden-3-acfiic acid 



C-CH2-CH2-COOH |^V~li^ 



iE" Cumoryl-a- acetic acid 



^ .CH \^ X-CH2 -COOH 



/^/^ HI Indolyl-p- propionic acid 



N—C-CHp-CHNH.-COOH CH2 



II II ' \\ 



CH CH CH2 



NH Y Histidine ^ Ethylene 



Fig. 120. Molecular structure of plant growth and stimulant substances. 



parenchyma and cambium cells, epinastic curvature of leaves, initi- 

 ation of callus and root formation in cuttings, inhibition of extension 

 of axillary buds, etc. The experience that chemically different com- 

 pounds stimulate the same, or at least similar, growth created a still 

 greater sensation than this diversity of positive or negative reactions 

 caused by the growth substances appUed. The nearly identical, though 

 quantitatively different effects of indolyl, inden and cumaryl com- 

 pounds (Fig. 120) are well-known. For this reason it has often been 

 suggested that in the case of these auxins there is rather a universal 

 stimulation of the metaboHsm than a specific hormonal effectivity. 



However, a comparison of the 4 structural formulae of the com- 

 pounds I-IV in Fig. 120 (Thimann, 1936), all of which are stimulants 

 of growth (ahhough the compounds II-IV are active to considerably 

 less extent), shows that they have morphological characteristics in 

 common : all of them contain a five-membered ring with at least one 

 double bond. Six-membered rings (naphthyl derivatives) are also 

 active (Thimann and Bonner, 1938). It appears to be immaterial 

 whether this ring is homo- or heterocyclic and what side chains are 

 substituted in it. A further characteristic is that all four substances 

 are monobasic acids, in which, however, the COOH-groups must be 

 separated from the ring by at least one C-atom (Koepfli, Thimann 

 and Went, 1938; exception: 2,4,6-trichloro benzoic acid). The mor- 



