OCCURRENCE AND FORMA TION OF GROWTH SUBSTANCES 61 



nodes, displacement of the axis from its usual vertical position 

 apparently brought about a renewed production of the growth- 

 promoting hormones. 



Boysen Jensen investigated the growth substance of potato 

 tubers, extracting it in alcohol. From his results it seems clear 

 that dormancy in potatoes is not conditioned by a lack of such 

 substances. 



© """^ -^ 



Before _ 

 flowering 



_ Flower 

 formation 



_Flowering and_ 

 fruiting 



20 



25 



5 10 , 15 



Age of plant in weeks 

 Fig. 19.^Presence of growth substance in Helianthus at different periods in 

 the life cycle. The growth hormone is reported to be abundant during seed 

 germination, relatively scarce during the period of vigorous vegetative growth, 

 and high again at the time of seed formation. The height of the plants at the 

 difTerent stages was as follows: 1) seeds, 2) 2 cm., 3) 10 cm., 4) 25-35 cm., 5) 

 40 cm., 6) 70 cm., 7) 150-180 cm., 8) to 16) about 200 cm. (unpub. data). 

 (After Laibach and Meyer, 1935.) 



Roots. — A great many investigations on the occurrence of 

 growth substance in root tips have been carried out without much 

 success. Cholodny (19336) showed that Zea mays root tips 

 produced a growth effect about equal to that of Avena coleoptile 

 tips. Growth substance can be extracted from root tips of Zea 

 curagua and Vicia Faba when agar containing 10 per cent 

 dextrose is used instead of pure agar (Boysen Jensen, 19336). 

 This subject is treated at greater length elsewhere (see p. 25). 



Pollen. — According to the investigations of Laibach (1932, 

 1933a), Maschmann and Laibach (1932), and Laibach and 

 Maschmann (1933), the pollen hormone, which Fitting (1909) 



