PLANT HORMONES 157 



Stance. If the terminal bud is removed, the inhibition of the 

 remaining top bud will disappear. The same type of inhibi- 

 tion can be shown for sprouting potatoes. If a tuber is cut 

 into cross section pieces with a bud on each piece, they will 

 all grow if placed in a favorable condition for sprouting. 



Hormones are being used for commercial purposes in such 

 cases as the spraying of fruit trees to delay the falling of fruit. 

 It is not known whether the falling is due to the presence or 

 to the absence of a natural plant hormone. At present it 

 appears that a very small amount of spray solution of naph- 

 thalene acetic acid or naphthalene acetamide on the stem of 

 an apple will prevent the formation of the plate of cork cells 

 across the stem (called the abscission layer) , the weak point 

 at which the fruit breaks from the tree. The whole tree may 

 be sprayed but the stems of the fruit must be covered. It 

 retards the fall of the leaves for a shorter time than the fall 

 of the fruit. 



Other applications have been attempted with success. 

 It is not too much to expect that we may learn to use hor- 

 mones in many ways, among which might be, to grow short 

 stems longer, to prevent long stems from growing so long, 

 to prevent the drop of flower buds, to inhibit the growth of 

 certain buds, and to stimulate the growth of others. 



REFERENCES 



Boysen- Jensen, P., Growth Hormones in Plants, Translated by G. S. Avery and P. R. 



Burkholder, McGraw-Hill Book Co., 1936. 

 Hitchcock, A. E., and P. W. Zimmerman, "Effect of Growth Substances on the Rooting 



Response of Cuttings," Contributions from the Boyce Thompson Institute, vol. 8, 



pp. 63-80, 1936. 

 Nicol, Hugh, Plant Growth Substances, Leonard Hill, London, 1938. 

 Went, F. W., "Auxin, the Plant Growth Hormone," The Botanical Review, vol. 1, pp. 



162-182, 1935. 

 Went, F. W. and K. W. Thimann, Phytohormones, Macmillan Co., 1937. 



