130 GROWTH HORMONES IN PLANTS 



Under the proper circumstances, growth substance may bring 

 about the formation of tumors, the building of callus tissue, and 

 the initiation of new roots. The substances present in the 

 coleoptiles of maize, orchid pollen, urine, and also pure hetero- 

 auxin have been shown to bring about an increase in the bulk 

 of the tissues near the site of their application in both roots and 

 stems. Such hypertrophy may be brought about either through 

 an increase in the number of cells or by a shift in the direction 

 of growth. Proliferation of cells has been demonstrated, too, by 

 the application of heteroauxin to mesophyll tissue. 



The role of the auxins in the neoformation of roots has been 

 demonstrated in numerous instances, as has the fact that these 

 same substances also inhibit the growth in length of roots. That 

 the auxins are not unique in their ability to produce new roots 

 has been shown by the effectiveness of many other substances in 

 this respect, such as, a-naphthalene acetic, indole butyric acid, 

 etc. The mechanism of action of these different compounds, 

 whether direct or through some influence upon the auxins or in 

 still other ways, is not known. 



The differential growth on the two sides of bilaterally sym- 

 metrical organs leading to nastic movements has been shown to 

 be influenced by the local application of auxins and divers 

 other substances. For example, heteroauxin caused marked 

 nastic responses in the leaves of such plants as Nicotiana, 

 Mimosa, etc., and a number of other substances have been shown 

 to bring about epinasty in Nicotiana and Lycopersicon. Pro- 

 longation of the growth period and prevention of petiole abscis- 

 sion by auxins, also, have been demonstrated in several species. 

 Future investigations may provide an explanation of the funda- 

 mental mechanism of growth-substance activity in connection 

 with these diverse manifestations of hormone-controlled growth. 



