208 PLANT PHYSIOLOGY 



the root as it grew and the stem end the one nearest the end of 

 the stem. Sachs (1882) explained this on the basis of stem- and 

 root-forming substances. Materials were formed in the plant, 

 according to this hypothesis, which determined the formation of 

 roots and migrated toward them, so that there was always more 

 of this " root-former" in the root end of a piece than in the stem 

 end. The stem-forming materials formed a similar gradient in 

 the reverse direction, with the result that one end was a root 

 and the other a branch end. He said: "Just as organs differ in 

 form, so there is a corresponding diversity in the materials which 

 compose them. These organ-forming substances act like ferments 

 upon large masses of plastic material although they themselves 

 are present in very small amounts." We now use the term ferment 

 in a very different sense (Chap. XVI), and for the material which 

 Sachs had in mind, Starling (1905) proposed the word hormone. 



Hormones, or chemical messengers, are compounds which are 

 produced in one part of the organism and, when carried to other 

 parts, there bring about very marked and striking changes. The 

 word was first used in animal physiology where much more work 

 on hormones has been done. Here they go under the common 

 name of "internal secretions" and include the products of the 

 ductless glands such as the thyroid, adrenal, pituitary body, and 

 the internal secretions of the gonads. These chemical compounds 

 when carried to the other parts of the body result in normal growth 

 and in the normal development of the secondary male or female 

 characters, or else in gigantism, homunculism, a goitrous condition, 

 etc., depending upon whether they are present in normal or 

 abnormal amounts. As Brown-Sequard has shown, these secre- 

 tions may influence not only the entire physical character of the 

 body but also the psychic life and the mentality of the organism. 



It was previously thought that the chief connection of the parts 

 of the body with each other was through the nervous system, 

 but now we know there are these chemical connections as well. 

 As Biedl (1910) sums up the matter: "The theory of internal se- 

 cretions now plays an important role in nearly every problem of 

 physiology and pathology and is very important in general biolog- 

 ical problems. Schiefferdecker's hypothesis concerning the part 

 played by specific internal secretions in the control of the nervous 

 system shows better than anything else the recent change in our 

 attitude toward the role of these secretions. . . . These concep- 



