62 



PHYTOHORMONES 



none. When the plant is about 40 cm, in height, production 

 by its terminal bud ceases. The dormant axillary buds 

 produce practically no auxin until, following decapitation, 

 they start to develop; their auxin production then rapidly 



increases. In the dark, auxin produc- 

 tion soon ceases but its utilization 

 continues, with the result that the 

 auxin, which at first is present in con- 

 siderable quantities in the stems, rap- 

 idly becomes exhausted from them. 

 The production of auxin by the leaves 

 of Nicotiana was studied by Avery 

 (1935), who showed that the maximum 

 amount was produced by young leaves 

 of about 20 sq. cm. area, the produc- 

 tion rate thereafter steadily decreasing 

 with age. Production takes place in 

 the blade only in light, and the auxin 

 is accumulated in the veins. In the 

 leaves of Ipomoea Koning (1933) found 

 the largest auxin production to coincide 

 with the period of most rapid growth, 

 and the same is true for the developing 

 buds of Aesculus and Malus (Avery, 

 Burkholder, and Creighton, 1937). 



Laibach and Meyer (1935) have 

 published data on the total auxin con- 

 tent of Zea Mays and Helianthus plants 

 during their whole life cycle. The re- 

 sults, which were obtained by alcohol 

 extraction, are open to objection on 

 account of the probable loss of auxin 

 by enzymic inactivation. They seem to show that auxin is 

 absent from Zea Mays, and almost absent from Helianthus, 

 during the period of most rapid development. 



The presence of auxin in the pollinia of several orchids 

 and the pollen of Hibiscus was shown by Laibach (1932a); 



Fig. 28. Auxin produc- 

 tion in Vicia Faba. Young 

 plants, grown in light. 

 The figures represent the 

 amount of auxin diffusing 

 from the various leaves 

 and buds in AE per hour. 

 (Data of Thimann and 

 Skoog, 1934.) 



