THE TRANSPORT OF GROWTH SUBSTANCES 75 



Avena. More recent studies (Crocker, Hitchcock, and Zimmer- 

 man, 1935) suggest that ethylene itself may be a plant hormone, 

 though the experiments of Michener (1935) have failed to show 

 any growth-promoting effect of ethylene applied in a series of 

 concentrations to Avena coleoptiles. 



Growth-substance Transport in Shoots and Roots. — Studies 

 on the translocation of growth substance in the stems of seedlings 

 and mature plants have brought some new facts to light in recent 

 years. 



Van Overbeek (1933) observed transport of growth substance 

 in the hypocotyls of Raphanus sativus. He found that movement 

 was basipetal and was not influenced by general illumination. 

 The work of Thimann and Skoog (1934) would suggest that auxin 

 can travel from the terminal bud downward in the stem where it 

 may inhibit lateral bud development. Furthermore, Snow 

 (1929a) found that the downward movement of this bud- 

 inhibiting substance takes place chiefly in living cells. The 

 production of growth-promoting substance in the young leaves of 

 Nicotiana and its accumulation and polar movement (in a 

 basipetal direction) in the veins (Avery, 1935) is of considerable 

 interest from the standpoint of the role of growth hormones 

 in normal plant growth. 



Studies on the direction of transport in Coleus hyhridus (Mai, 

 1934) gave the following results: in young, still growing petioles, 

 movement took place only in a basipetal direction; in fully grown 

 petioles, movement occurred in either direction; and in old and 

 falling petioles, there was little or no movement. In stems 

 of Coleus, Vicia, and Phaseolus, and in hypocotyls of Vicia, 

 Phaseolus, and Lupinus, Mai found that (in all but one 

 instance) transport occurred in a basipetal direction only. 



Experiments carried out recently at the Boyce Thompson 

 Institute have shown that growth substances may be taken up 

 from the soil through the roots (Hitchcock and Zimmerman, 

 1935). Following their entry and upward^assage into different 

 parts of the plant, their effects upon growth, root formation, 

 etc., are very striking. These observations appear to contradict 

 previous conclusions as to the polar movement of growth sub- 

 stances. Inasmuch as the upward migration was greater than 

 47 cm. per hour under optimum conditions, it is probable that 

 translocation of the substances in this instance was in the trans- 



