156 PLANT GROWTH SUBSTANCES 



In the treated stems of Coleus Fischnich (14) found that the cells of the 

 parenchyma enlarged, the cells in the vicinity of the cambium pro- 

 hferated, the cambium became active, new vascular bundles appeared 

 between the old ones, and adventitious roots arose principally in the 

 borders of the vascular bundles. Czaja (13) studied the effects of indole- 

 acetic acid-lanolin paste on Helianthus hypocotyls and described some 

 of the tissue responses. Snow (35) showed that auxin-a, indoleacetic 

 acid, and extracts of urine stimulated cambial activity in Helianthus 

 seedlings. 



Relatively little detailed description of the histological responses of 

 plants to any growth-regulating substance had appeared, however, prior 

 to the publication of the paper by Kraus, Brown, and Hamner in 

 1936 (23), dealing with the gross and histological reactions of the red 

 kidney bean to applications of indoleacetic acid. This paper stimulated 

 tremendous interest and gave direction for numerous papers which 

 followed, deahng not only with the responses induced by indoleacetic 

 acid but with a number of other chemical substances. Because of the 

 accurate and detailed observations presented, as well as of the influence 

 exerted on subsequent histological studies following the application of 

 indoleacetic acid and other growth-regulating substances, it is deemed 

 advisable to point out some of the most significant of their findings. 



Responses of Young Decapitated Stems to Indoleacetic Acid 



The indoleacetic acid was apphed as a 3 per cent mixture in lanohn 

 to the cut surface of the second internode of young bean plants which 

 had been cut off squarely about i mm. below the base of the petiole 

 of the first compound leaf. In some instances axillary shoots were used. 

 Essentially the same responses were obtained in both cases. 



Following decapitation and application of the lanohn mixture, little 

 gross response of the stem was noticeable within 1 8 hours. Swelling of the 

 stem for i to 2 mm. down from the cut surface then began, and by the 

 end of 48 hours the topmost portion of the stem had become distinctly 

 enlarged. At 72 hours the end of the stem attained a diameter nearly 

 twice that at 5 mm. below the cut surface, and by the end of 1 10 to 120 

 hours glistening tips of root primordia were visible about the periphery 

 of the tumor. At 144 to 168 hours the roots were more evident and some 

 of them emerged from the surface of the tumor. Such tumors may con- 

 tinue to grow for periods longer than six months and frequently attained 



