v] NATURE OF PLANT-ANIMALS 135 



automatic process of the production of pancreatic 

 juice is independent of the nervous system. It occurs 

 in the absence of all nervous connections between 

 the intestines and pancreas. Now Starling has 

 shown that the stimulus which induces secretion in 

 the pancreas is due to a definite, chemical sub- 

 stance (secretin). This substance is produced in the 

 small intestine as the result of the passage of food 

 into that organ. It passes from the intestine into 

 the blood-stream, is carried to the pancreas and gives 

 the signal to that organ to commence its secretive 

 activity. Such specialised, chemical stimulators 

 Starling calls "hormones," and it is not to be doubted 

 that they play an important part in inducing large 

 numbers of normal processes which, as we know, 

 arise as the consequences of antecedent processes. 

 In plants in particular, it would seem that we must 

 look to hormones, or chemical stimulators to pro- 

 vide us with an understanding of many phenomena 

 which are at present ignored, or ascribed vaguely to 

 nervous action. For example, the living tissue in the 

 stem of plants, known as cambium, which is re- 

 sponsible, by continued growth and division, for the 

 increase in thickness of the stem, occurs in young 

 plants as definite, localised patches or sheets lying 

 between the vascular-bundles. After the plant has 

 reached a certain stage, the non-dividing cells of the 

 cortex which are coterminous with the cambium 



