226 PLANT GROWTH SUBSTANCES 



In discussing the tropical uses of plant hormones, I will attempt to do 

 so by including the underlying principles as far as they are known at 

 present. They could be classified under the following headings: Fruiting, 

 Rooting, and Weeding. 



Fruiting 



Growth-regulating substances are known to affect tropical fruit crops 

 in a variety of ways. They may promote the flower initiation of the plant. 

 This can be done directly, as in the pineapple, or indirectly, as in litchi. 

 The synthetic hormones may also regulate the fruit growth and develop- 

 ment after the flowers have been initiated naturally, as is exemplified by 

 the fig. Finally, after the fruit has been formed by natural processes, 

 hormones may delay its abscission from the tree and improve its keeping 

 quaUties. This is illustrated by the orange. Each of these effects will be 

 dealt with below. 



Crop control in the pineapple. — The pineapple plant produces in its 

 lifetime only one fruit. Flowering, under natural conditions, starts in 

 the fall and continues through the winter (Fig. 2). Approximately six 

 months after flower formation the fruit is ready for harvest. The age 

 at which a pineapple plant begins to flower depends upon its variety and 

 the external conditions. On the average, when a pineapple plant during 

 the flowering season reaches an age of 1 8 months it is capable of producing 

 a marketable fruit. Some varieties Hke the Puerto Rican Cabezona take 

 occasionally as long as 5 years before they flower. Such slow varieties 

 can be brought into earlier production by treatment with synthetic plant 

 hormones (48). These hormones will also cause flowering in very young 

 plants too immature to flower in the natural season. 



In the tropics the growing of plants is not restricted by frost and 

 similar factors which so drastically curtail crop production in our middle 

 latitudes. There is no reason, therefore, why a crop like the pineapple 

 cannot be made to yield beyond its natural season, or even throughout 

 the entire year. In the pineapple industry several methods have been 

 in use for several decades by which growers have succeeded in extending 

 the harvest season. 



It began when it was accidentally discovered in the Azores that smoke 

 will force the plants into early flowering (46). Older growers in Puerto 

 Rico still remember how tents were erected over the rows under which 

 a smoky fire was made during the night. An investigation of the active 



