232 PLANT GROWTH SUBSTANCES 



will bring about flower initiation. Yet, on the other hand, an increase 

 in the auxin level per se does not seem to be necessary for flower forma- 

 tion in the pineapple. 



One might therefore conclude that it is conceivable that ethylene 

 treatment increases the response of the tissues to auxin, thereby bringing 

 about an increase in the physiological activity of the auxin in the plant. 

 This assumption does not seem unlikely when viewed in the light of a 

 recent discovery on the role of auxin in cambial growth. In the January, 

 1949, issue of the Va\blad voor Biologen C. Reinders-Gouwentak (34) 

 discloses that when auxin is applied to dormant branches of Fraxinus 

 growth of the cambium is promoted along the entire length of the 

 branch, provided, however, that this branch has been treated previously 

 by ethylenechlorohydrin. Auxin by itself had only a slight effect on 

 cambium growth in the immediate vicinity of a cut surface, while 

 ethylenechlorohydrin by itself was entirely ineffective in this respect. 

 Apparently the ethylenechlorohydrin changed the metabolism of the 

 branch, making it highly responsive to auxin. Similarly, ethylene might 

 bring about metabolic changes in the vegetative apex of the pineapple, 

 making the tissue more responsive to the auxin it contains, thereby 

 bringing about flower formation. 



Indirect flower induction in litchi. — In the Hawaiian Islands litchi trees 

 flower and fruit so irregularly that it is often believed that this crop has 

 few economic possibilities. It was found, however, that in litchi too, 

 flowering may be induced by treatment with growth-regulating chemi- 

 cals. In contrast to the pineapple where auxins promote flower formation 

 directly, in the litchi flower formation is due to the suppression of lush 

 young shoots by auxins. A concentration of 50 ppm. of NAA applied 

 before October immediately stops vegetative development (37). As a 

 result of such treatments 88 per cent of the trees flowered, as compared 

 to only 4 per cent in the controls. This indirect type of flower induction 

 in litchi is probably due to a building up of nutrients and growth factors 

 which normally would have been translocated away to the rapidly grow- 

 ing vegetative branches. This eflect is comparable to flower induction 

 by girdling of juvenile citrus trees (19), and of the rotenone-producing 

 Peruvian Lonchocarpus (14). Here as well as in litchi trees flower induc- 

 tion apparently is also the result of the accumulation of nutrients and 

 growth factors. 



Shortening of the ripening period of the fig. — Although most commercial 



