20 .PHYTOHORMONES 



that, just as is the case with auxins, a number of different 

 substances can bring about the effect (Fitting, 1927, 1929, 

 1932, 1933, 19366). In the second place, as with the activity 

 of auxins on Avena, some of them are active in extremely 

 high dilution, and the activity of different substances varies 

 widely. Thus, N-methyl-^histidine stimulates streaming 

 at 1 part in 10^, or about 10^^ mol.; aspartic acid, asparagine, 

 histidine, and some other amino-acids are active down to 

 10-^ mol.; alanine, serine, and phenyl-alanine down to 

 10~^mol., while glycine, proline, and glutamine have very low 

 activity. Some, e.g. tryptophane, are completely inactive. 

 A number of non-nitrogenous acids, including galacturonic 

 acid, are active down to 10^^ to 10"^ mol. In the amino- 

 acids activity decreases with increasing distance between 

 the carboxyl and amino-groups. Neither peptides nor 

 amines have much activity, so that, presumably, amino- 

 groups only confer activity if they are accompanied by 

 an acid radical. 



Small changes in the molecule greatly affect the activity; 

 thus, N-methyl-/-histidine is five times as active as ^histidine 

 itself. These differences may, of course, be caused by dif- 

 ferent rates of penetration into the cells, and not by true 

 differences in ''sensitivity" of the protoplasm for these 

 compounds, as is probably the case for the different activity 

 of auxins (see VIII G). 



