J. VAN OVERBEEK 24I 



are other soils in which the 2,4-D concentration front moves downward 

 to a considerable extent with only a slight amount of rain. Between these 

 extremes a number of intermediate soil types exist. It is obvious that 

 when conditions exist which will bring the 2,4-D front within the zone 

 of growing sugar-cane roots, the latter are killed and the crop is lost. 

 Such conditions may occur after heavy rains in soils which fix 2,4-D 

 rather well; or again, they may occur after light rains in soils which hold 

 2,4-D only poorly. Ideal conditions for pre-emergence use of 2,4-D are 

 those in which the herbicide remains fixed to the upper soil layers, thus 

 preventing weed growth without interference with the deeper cane 

 roots. 



Relatively little definite knowledge is available on why 2,4-D is such 

 an effective herbicide; neither do we know upon which properties its 

 action as a selective herbicide rest. It is clear, however, that 2,4-D is 

 an auxin and as such is translocated within the plant. It can reach the 

 sensitive meristematic regions of the plant through normal channels 

 because during the time of transport it is not toxic to the plant and for 

 this reason does not interfere with its own translocation as is so often 

 the case with some of the more violent plant poisons. After it arrives at 

 the site of action, 2,4-D is not easily inactivated like the native auxin, 

 indoleacetic acid. This persistence inside the plant is also in all probability 

 a contributing factor to the high efficiency of 2,4-D as an herbicide. 

 The depletion of carbohydrate reserves associated with the effect of 

 2,4-D on plants is not believed to be the primary cause of its phyto- 

 toxicity (33,38). Interference with the aerobic respiration seems not 

 unlikely as a primary cause. Germinating seeds (26) as well as bacteria 

 C56) which require free oxygen for their respiration are smothered by 

 2,4-D, while those organisms capable of anaerobic respiration are not 

 affected to any significant degree by it. For a more extensive discussion 

 on tropical weed control the reader is referred to (52) ; for tropical weed 

 species, to (58) ; and for the reasons for the physiological activity of 2,4-D 

 as a herbicide, to (57). 



Concluding Remarks 



In conclusion one can say that the outstanding uses of synthetic 

 plant hormones in the tropics are: the use of naphthaleneacetic acid 

 in the pineapple industry, making crop regulation possible to an extent 

 unequaled in other fruit crops; the use of indolebutyric acid in the whole- 



