13-4 PLANT-ANIMALS [CH. 



which grows on the wall, awaits in vain the signal 

 of high light intensity which is required to call forth 

 the development of its flowers. Many plants exhibit 

 transient or permanent youth-forms ; that is, they 

 pass through or remain in juvenile stages of de- 

 velopment. A like phenomenon, with respect to the 

 organism as a whole, or with respect to single 

 organs is exhibited by animals and by man himself. 

 In the great number of cases, it is to be supposed 

 that the signal which is given or not given is of 

 internal origin. It is very probable that such signals 

 for development are of a chemical nature. The 

 important work of Starling (1906) has supplied 

 physiologists with a new method, capable of precise 

 application, in their work of analysing nervous 

 responses in animals and in plants. Thus, he has 

 demonstrated that a secretion may be the result, not 

 of a nervous stimulus, but of the arrival at the 

 secreting organ of a definite chemical substance. 

 To give but one illustration of Starling's discoveries : 

 some time after food has been swallowed, the pancreas 

 begins to discharge pancreatic juice into the small 

 intestine. Hence, the food, partially digested by the 

 stomach, is met, soon after its arrival in the small 

 intestines, by the pancreatic juice and acted on 

 in such a way that the materials it contains are 

 rendered soluble and diffusible and so capable 

 of passing into the blood-stream. This purposeful, 



