CHEMICAL CORRELATION 171 



ing tip of a branch is injured, some bud along the stem, 

 which may have been long dormant and which would 

 ordinarily continue dormant, will start into activity and 

 carry on the growth of the stem as a whole. If a cutting 

 of geranium is thrust into suitable soil, roots will develop 

 on it at spots which ordinarily would never produce such 

 structures. All these are examples of chemical regula- 

 tion. Nothing is known as to the nature or source of the 

 chemical substances w^hich actually serve as stimuli for 

 bringing about the changes. 



Chemical Correlation in Animals. — In animals 

 all the machinery of adjustment is more highly organized 

 than in plants, and so we find in them a definite mechan- 

 ism for chemical correlation. This mechanism consists 

 of the production of special chemical substances which 

 are poured out into the tissue fluids and which exert 

 definite effects on metabolism. These substances have 

 come into prominence in recent years and bid fair to as- 

 sume even greater importance in the public eye as the 

 years go on. It is worth while, therefore, to become fam- 

 ihar with the technical name which has been applied to 

 them. They are known as hormones, from a Greek word 

 meaning literally " to excite " and referring to their prop- 

 erty of functioning as chemical stimuli. 



Hormone-manufacturing Glands. — All of the higher 

 animals have in their bodies cell masses whose functional 

 metabolism consists in the manufacture of particular hor- 

 mones. These cell masses are similar to the glands al- 

 ready described, by which digestive juices are manufac- 

 tured; the only important difference between them and 

 the more familiar glands is that the materials which they 

 produce are poured out directly into the blood stream to 

 be circulated about the body, whereas the secretions of 

 the glands of the other type, such as the salivary glands, 

 are passed by means of special tubes or ducts into the di- 

 gestive tract, or in the case of the sweat glands, on to the 

 surface of the skin. Because the glands that are now be- 



