GROWTH SUBSTANCES FOR NORMAL GROWTH 103 



cultures with 3-indole acetic acid (heteroauxin) (1 part: 

 20,000,000). 



Growth Substance in Relation to Cell Division. — The role of 

 hormones in promoting growth by cell enlargement has been 

 established by numerous investigations upon diverse plant 

 materials. A limited number of observations have led to the 

 suggestion that growth substances may also influence cell division 

 in plants. 



Laibach, Mai, and Mliller (1934) obtained an ether-soluble, 

 thermostable extract from orchid pollen and urine which when 

 applied to the stems of Coleus and Tradescantia brought about 

 increased frequency of cell division leading to callus formation. 

 Further work (Laibach and Fischnich, 1935a) has shown that 

 3-indole acetic acid stimulates cell di^^sion in the epicotyls of 

 Vicia Faba. More recently still, it has been found that applica- 

 tion of purified auxin a and 3-indole acetic acid to the upper ends 

 of decapitated Helianthus seedlings caused growth in thickness by 

 cambial division (Snow, 19356). In a report of new experiments 

 and a review of the literature, Jost (19356) has pointed out the 

 stimulating effect of relatively high concentrations of various 

 substances upon cell division in the pith of Vicia Faba, the main 

 roots of Lupinus, etc. Popoff (1933) studied the influence of 

 growth-substance extracts which were obtained from the coleop- 

 tile and other parts of Zea seedlings and added to cultures of 

 Euglena gracilis. It was reported that oxidation processes, cell 

 division, and germination of the cysts were promoted by dilute 

 concentrations of the extracts. 



Other workers have postulated the existence of other special 

 hormones for cell division. The role of certain substances 

 {e.g., bios) which apparently do not belong in the same category 

 with the auxins has been described by many investigators 

 (Wildiers, 1901; Miller, 1930; Schopfer, 1935; Dagys, 1934; 

 Kogl, 1935, Mitt. XIV; etc.).^ The precise way in which the 

 auxins and these other substances may regulate growth by means 

 of their influence upon cell division has not yet been satisfac- 

 torily explained. 



THE MECHANISM OF ACTION OF GROWTH SUBSTANCES 

 In the foregoing survey of the discoveries concerning the effect 

 1 See supplementary bibliography on p. 247. 



