240 Plant Biology 



substances naturally produced in minute quantities by plants^, stored in 

 certain regions, and later transported to other regions to produce regu- 

 latory effects on the development and growth of that organism. The 

 term hormone (Gr. hormao, excite) ineans "to arouse to activity." Hor- 

 mones in plants are normally produced in very minute quantities but 

 apparently are sufficient to perform their specific functions. Much of 

 the experimental evidence of the past years concerning the activity of 

 plant growth hormones has helped to explain normal growth, tropisms 

 (responses) to gravity, tropisms to light, and similar phenomena. Plant 

 hormones, like animal hormones, are produced in one part of the plant 



Characteristics of Plant Growth Hormones 



(usually young, vigorously growing parts) and transported to another 

 part of the organism where they actively control specific phenomena, 

 depending on the type of hormone in question. The tropic responses of 

 plants to two of the most important environmental stimuli, gravity and 

 light, are associated definitely with the movement of plant hormones 

 ("auxins") from one region of a stimulated plant organ to another. 

 This phenomenon is known as the growth hormone explanation of 

 tropisms. Several different plant hormones have been found naturally 

 present: (1) auxin A, (2) auxin B, and (3) heteroauxin, which seems 

 to be the most widely distributed of the present hormones. Another 

 plant hormone, traumatin (Gr. trauma, wound) seems to initiate and 

 influence healing of plant wounds. 



In young plant tissues the hormones move only in a morphologically 

 basipetal direction ("polar transportation"), but in older tissues they 

 move in either direction. In very old inactive tissues there is probably 



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