192 GLANDS IN HEALTH AND DISEASE 



an essential acid is that the body needs it for the 

 manufacture of thyroxin; though so far as I know 

 no one has as yet shown that an increase of trypto- 

 phane in our diet increases the quantity of thyroxin 

 in the body. 



Now as to the next question: are there plant 

 hormones? And if so, do they play a part in the 

 plant kingdom analogous to that played by hor- 

 mones in the animal kingdom? Let us quote Pro- 

 fessor Bayliss on this point: 



"Although there is no such effective way of 

 chemical interchange in plants as there is in the 

 circulating blood of animals, there is distinct evi- 

 dence that chemical products of one part are able 

 to influence the activities of other parts. The lat- 

 eral roots, which normally grow horizontally, can 

 be made to grow vertically downwards if the main 

 root is removed. Errera investigated, in pines, the 

 corresponding change of direction of growth of a 

 branch into a vertical stem when the apical bud 

 of the main stem is removed. He suggested that 

 the apical bud of the main stem forms some kind 

 of internal secretion, which prevents the upward 

 growth of the lateral shoots as long as this apical 

 bud is present. 



"Keeble considers that such 'chemical stimu- 

 lators' play a part in the transfer of activity of 

 localized cambium cells to others in their neighbor- 



