Ill] OF HORMONES 263 



minute doses to accelerate the growth of the higher plants*. Some 

 of these "growth-substances" have been extracted from moulds or 

 from bacteria, and one remarkable one, to which the name auxin 

 is especially applied, from seedhng oats. This last is no enzyme 

 but a stable non-nitrogfenous substance, which seems to act by 

 softening the cell- wall and so facihtating the expansion of the cell. 

 Lastly the remarkable discovery has been made that certain indol- 

 compounds, comparatively simple bodies, act to all intents and 

 purposes in the same way as the growth-hormones or natural 

 auxins, and one of these "hetero-auxins." an indol-acetic acidf, 

 is already in common and successful use to promote the growth and 

 rooting of cuttings. 



Growth of duckweed, with and without peat-auodmone 



Without With 



There are kindred matters not less interesting to the morphologist. 

 It has long been known that the pituitary body produces, in its 

 anterior lobe, a substance by which growth is increased and regulated. 

 This is what we now call a "hormone" — a substance produced in 

 one organ or tissue and regulating the functions of another. In this 

 case atrophy of the gland leaves the subject a dwarf, and its hyper- 



* The older literature is summarised by Stark, Ergebn. d. Biologies n, 1906; 

 the later by N. Nielsen, Jh. wiss. Botan. Lxxm, 1930; by Boyson Jensen, Die 

 Wuchsstojftheorie, 1935; by F. W. Went and K. V. Thimann, Phytohormones, New 

 York, 1937, and by H. L. Pearse, Plant hormones and their practical importance. 

 Imp. Bureau of Horticulture, 1939. Cf. Went, Bee. d. Trav. Botan. Neerl. xxv, p. 1, 

 1928; A. N. J. Heyn, ibid, xxviii, p. 113, 1931. 



t Discovered by Kogl and Kostermans, Ztschr. f. physiol. Chem. ccxxxv, p. 201, 

 1934. Cf. {int. al.) P. W. Zimmermann and F. W. Wilcox in Contrib. Boyce- 

 Thompson Instil. 1935. 



