Growth Substances 



409 



Some gases have been found to exert strong formative effects on 

 plants and thus deserve to be included among the growth substances, 

 although chemically they are very different from the rest. In studying 

 the effects of illuminating gas and its constituents on greenhouse plants, 

 workers at the Boyce Thompson Institute found that in tomato plants 

 exposed under bell jars to atmospheres containing 1 per cent of carbon 

 monoxide the stems became covered after a few days with an abundant 

 growth of roots. Other gases produced similar effects. Carbon monoxide 

 was found to induce rooting in many other plants (Zimmerman, Crocker, 

 and Hitchcock, 1933 ) . These results led to the investigation of the effects 

 of ethylene, acetylene, and propylene (Zimmerman and Hitchcock, 

 1933). All were found to induce rooting and root-hair formation, leaf 



0.01 



100.0 



0.1 1.0 10.0 



IAA CONCENTRATION: MG./L. 

 Fig. 18-26. Growth of Avena coleoptile (upper curve) induced by various concentra- 

 tions of indoleacetic acid. The degree of inhibition of this growth by an auxin an- 

 tagonist ( 4-chlorophenoxyisobutyric acid) at concentrations of 1, 10, and 50 mg. /liter 

 is shown in the three successively lower curves. ( From Foster, McRae, and Bonner. ) 



epinasty, proliferation of callus-like masses of tissue, and abscission of 

 leaves, flowers, and fruits. These gases, however, do not stimulate growth 

 in the absence of auxin. The relation of ethylene to auxin has been 

 discussed by Michener (1938). 



In Puerto Rico Rodriguez (1932) discovered that ethylene induces 

 flowering in the pineapple, and in Hawaii it was found soon afterward 

 that acetylene will accomplish the same result (Lewcock, 1937). The 

 effect of these gases on pineapple is much like that of the synthetic 

 growth substances which induce flowering (p. 398). 



In addition to the growth substances which have here been discussed- 

 auxin, various other naturally occurring substances, and the synthetic 

 compounds— another is now being actively studied and is assuming an 

 important place in morphogenetic problems. This is gibberellin. 



