DEVELOPMENT OF THE HORMONE CONCEPT 19 



p. 852). The same views were developed with respect to 

 geotropism and bud inhibition. The greater part of his 

 results fit in very well with our present knowledge of the 

 role of auxins and can be explained on that basis. The 

 somewhat similar experiments of Appleman (1918) on the 

 development of shoots in pieces of potato tubers led him 

 to postulate both growth-promoting substances, present 

 in the tuber, and growth-inhibiting substances formed in 

 the growing buds. 



Another interesting case of correlation by the transmis- 

 sion of a substance was discovered by Ricca (1916). He 

 found that the well-known transmission of excitation in the 

 sensitive plant. Mimosa, takes place in the vessels by 

 means of a substance, secreted into them upon stimulation 

 and carried principally with the transpiration stream. 

 Snow (1924, 1925) afterwards found that there are three types 

 of stimulus involved, only one of which is due to a diffusible 

 substance. Fitting (1930) investigated the action of pure 

 substances of biological interest in stimulating Mimosa 

 and found a number of compounds, especially a-amino- 

 acids and anthraquinone derivatives, to be active. Their 

 action, however, is not identical with that of the Mimosa 

 leaf extract. The active substance in the leaf extract itself 

 was shown to be heat-stable by Umrath (1927) and at- 

 tempts to purify it have been made by Fitting (1936a) and 

 by Soltys and Umrath (1936). The latter workers have 

 succeeded in obtaining a highly purified extract, from the 

 properties of w^hich they conclude that the active substance 

 must be an oxy-acid with a molecular weight of about 500. 

 They have also partly purified a similar oxy-acid active on the 

 leaves of certain sensitive Papilionaceae (Umrath and Soltys, 

 1936), and this, it appears, may be identical with the sub- 

 stance active in the Wehnelt test. 



Although their hormonal nature in the strict sense is 

 questionable, mention must be made here of those sub- 

 stances causing the onset of protoplasmic streaming in 

 Vallisneria leaf cells. In the first place Fitting has shown 



