INTRODUCTION 3 



tion and transport come the tropisms, particularly the reac- 

 tions of plants to gra\dty and light, insofar as they are 

 caused by unequal growth (Chapter X). 



Further, these same active substances play an important 

 role in a number of other correlations in plants, particularly 

 in the formation of roots (Chapter XI), in bud inhibition 

 (Chapter XII), and in the stimulation of cambial activity 

 (Chapter XIII). There are also a number of other phenom- 

 ena not so well understood. 



Finally the findings and general conclusions will be com- 

 pared with those from other fields, bringing the work on 

 plant hormones into the realm of general physiology 

 (Chapter XIV). 



B. Definitions 



The definition of hormone which we propose to apply is 

 this: a hormone is a substance which, being produced in any 

 one part of the organism, is transferred to another part and 

 there influences a specific physiological process. This is essen- 

 tially the definition of Bayliss and Starling (1904): "the 

 peculiarity of these substances (hormones) is that they are 

 produced in one organ and carried by the blood current to 

 another organ, on which their effect is manifested" (Bay- 

 Hss, 1927, p. 712). There is, of course, no blood stream in 

 plants, but, as Bayliss emphasizes, ''these hormones are 

 characterized by the property of serving as chemical messen- 

 gers, by which the acti\'ity of certain organs is coordinated 

 with that of others." ^ There is no strict necessity for the 

 production of the hormone in speciaHzed organs, since even 

 if all the cells of the plant should produce it, the phenomenon 

 of polarity would bring about its specific distribution. In 

 general, however, the points of production and response 

 are spatially separated. 



To avoid the possibility of confusion with animal mech- 

 anisms the term phytohormones has been introduced for 



* The conception of hormones has recently been somewhat broadened (see 

 Huxley, 1935). 



