PLANT HORMONES 



223 



group brings about activity for cell elongation when substituted in the 

 meta position of the benzene ring, and the NH2 group in the para position ; 

 chlorine activates the molecule in the ortho, meta, or para positions. To 

 date it has not been possible to predict physiological activity by study of 

 the molecular configuration alone — biological assay is necessary. The 

 study of configuration in relation to activity, however, has helped to locate 

 a large number of active compounds. Table 25 shows a long list of phenoxy 

 acids in relation to activity and inactivity for cell elongation and formative 

 influences. Table 27 ^'' shows a similar list of xylenoxy acids, and Table 28 

 derivatives of benzoic acid. 



Table 28. Active and Inactive Derivatives of Benzoic Acid 



The formative influence of derivatives of benzoic acid ^^ is illustrated in 

 Fig. 83. Some of these compounds modify the flowering habit and change 

 the correlation of organs. 



In addition to the nature of the molecule, the constitution of the receptor 

 tissue in the plant is important. First, the genetic constitution of the tissue 

 plays an important part, and secondly, the location in the organ and the 

 age of the tissue are determining factors. 



Though in the same family group, tomato and potato tissue do not 

 respond alike to a given substance, due perhaps to the difference in their 

 genetic constitution. Apple and lilac stem cuttings can be induced through 

 chemical treatment to produce adventitious roots in the spring of the year, 

 but not in autumn or winter. Though the tissue is receptive at an early 

 age, the capacity to respond to the chemicals is soon lost. With still other 

 species young tissue does not respond to chemical treatment, whereas older 

 tissue is susceptible. Many other illustrations could be given to indicate 

 that there are complex internal and external influences playing upon the 

 living protoplasm and that the sum total of these regulates the gro^\'th and 

 development of the plant. 



