GROWTH SUBSTANCES FOR OTHER PHENOMENA 121 



were thought to arise as secondarily induced phenomena follow- 

 ing the artificial addition of substances causing cell enlarge- 

 ment. In another paper, Czaja (1935a) reported the results of 

 further investigations dealing with the effects of growth substance 

 over a considerable range of concentration when applied to 

 Helianthus, Avena, and other plants (Fig. 37 E). Additional 

 evidence was obtained for the way in which cells increase their 







20;u. 



20/J. 



A _ B 



Fig. 38. — Longitudinal sections of Helianthus hypocotyls, showing effects 

 of the application of 3-indole acetic acid on polarized growth. A, the normal 

 cortex is comprised of cells markedly elongated in the direction of the long axis 

 of the organ. B, cells in a hypocotyl treated with growth-hormone paste become 

 nearly isodiametric, and have a much greater volume than those in the normal 

 hypocotyl. (After Czaja, 1935a.) 



dimensions under the influence of growth-substance supply. 

 The lateral application of relatively high concentrations brought 

 about retardations of growth in length and increase in thickness 

 of roots, stems, etc. (Fig. 38). It was concluded that the direc- 

 tion of growth by cell enlargement is controlled by the direction 

 of transport of growth substance. 



Recently LaRue (1935) has investigated the role of the auxins 

 in the development of intumescences on poplar leaves. The 

 injection of heteroauxin into twigs or its application in lanolin 

 directly upon the leaves brought about proliferation of tissue. 

 Cell outgrowths from the mesophyll of Mitchella repcns were 

 produced by the feces of insect larvae or applied droplets of 

 0.0005 per cent 3-indole acetic acid. 



