Plant Qrowth'Suhstances 



regulating substances of the most practical importance at 

 the present moment. Before discussing the property of 

 root-initiation the author desires to draw the attention of 

 the reader to the fact that the adjective growth-promoting 

 has been twice used in explaining bending tests, but that 

 the term growth-regulating has been reverted to in speaking 

 of root-production. This is because at the cut surface of a 

 seedling coleoptile the initiation of cell-elongation is promo- 

 tion of growth. On the other hand, initiation of root-forma- 

 tion on a mature plant, while leading to the differentiation of 

 new tissue in some places, is often accompanied by diminished 

 growth in other parts, and the plant may even become dwarfed 

 (Hitchcock and Zimmerman, 1935 ; Pearse, 1936-7). Hence it 

 would be illogical to apply the term "growth-promoting" 

 indiscriminately in alluding to substances other than the 

 nutrients in fertilizers. There seems a need to distinguish, 

 as zoologists have done in their own sphere, between the 

 promotion of cell growth and the initiation of tissue dif- 

 ferentiation in plants. 



The adjective "growth-promoting" is sometimes applied 

 by physiologists to those amino-acids, without which animal 

 growth (in size) cannot take place. It is doubtful whether the 

 adjective is valid in that connexion, which is mainly foreign 

 to the purpose of this book. Some of the work of R. W. 

 Jackson, and of Berg, and others, had its origin in a study of 

 the necessity of certain amino-acids in animal nutrition. 



A method of actually observing the formation of roots 

 was reported by F. W. Went (1934), who found that the 

 number of roots formed on cuttings of sweet pea was approxi- 

 mately proportional to the concentration of rhizocaline 

 ("the root-forming substance"). 



Hitchcock (1935), of the Boyce Thompson Institute at 

 Yonkers, New York State, caused initiation of roots, epinasty 

 of leaves, and bending and swelling of stems of intact tomato, 

 tobacco, African marigold, and other plants by making local 

 applications of synthetic indole-acetic and -propionic acids. 

 He made the interesting prophecy that "The fact that one 



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