GROWTH AND GROWTH HORMONES IN PLANTS 



397 



light. Cholodny in 1927 proposed that these tropisms of roots were due to 

 growth inhibition by auxin — gravity would cause accumulation on the lower 

 side as in shoots, but here the extra auxin supply would inhibit and the 

 root would therefore curve downward. The auxin would come principally 

 from the root tip. While this has in general been shown to occur, no one 

 has really satisfactorily restored geotropic sensitivity in a decapitated root 

 by applying auxin, and all the attempts leave something to be desired. Some 

 synthetic chemicals apparently prevent geotropic curvature without affecting 

 the rate of elongation, while some compounds chemically related to auxins can 

 reverse the tropism, causing roots to grow upward. 



Less physiological are the striking inhibitions of growth, often culminating 

 in death of tissue or of the whole plant, caused by large overdoses of auxin. 

 Concentrations from 10 to 1000 times that optimal for growth promotion are 

 needed to cause these effects. They can, of course, only be studied with 

 synthetic auxins, and, indeed, they have actually been very little studied 

 in a fundamental way because attention has been so focused on their ap- 

 plication. Many tons of synthetic auxins, especially 2,4-D (2,4-dichlorophen- 

 oxyacetic acid) and its relative 2,4,5-trichlorophenoxyacetic acid (2,4, 5-T): 



CHoCOOH 



I ^ 

 



CHpCOOH 

 



are sprayed annually on weeds to kill them. Sensitivity varies widely from 

 plant to plant. Curiously enough, monocotyledons are in general highly re- 

 sistant, though their seedlings are of course very sensitive, as we have seen 

 above. In general the synthetic auxins are much more effective and safe 

 weed killers than the strong acids or arsenic formerly used, and the different 

 sensitivities often make it possible to kill weeds without affecting the crop 

 plant growing among them. Mustard growing in wheat is a classical example, 

 and the principle has been used in forestry, where hardwoods can be killed 

 without affecting stands of pine or spruce. It is a paradoxical thought that 

 so far the biggest application of these growth substances to practical agricul- 

 ture is their use to kill plants. 



In the preceding pages mention has been made of synthetic auxins without 

 further explanation, and some chemical details should now be given. The 

 commonest auxin of higher plants and also of many fungi was stated above 

 to be indoleacetic acid, and it is on this model that so many compounds have 



