HORMONES, VITAMINS, AUXIMONES 211 



cells which seem to be secretory, thus giving a histological basis 

 to the hypothesis of Went. 



Haberlandt interprets the growth of cells near a wound as due 

 to a wound hormone that diffuses out from the injured cells. If 

 the surfaces are washed after cutting or wounding, fewer meristem 

 cells form than when pulp of crushed cells is spread over the 

 surface. 



It must be remembered that some writers (Child and others) 

 consider that, since hormones have not been isolated, these phe- 

 nomena in plants can be explained best by the conduction of stim- 

 uli rather than by the transference of definite, tangible, chemical 

 compounds. Furthermore, cooling a petiole to 3° C. results in 

 physiological isolation, which points to a stimulus rather than a 

 hormone, according to Child; but the transfer of materials in 

 plants is also checked by a decrease in temperature and so this 

 argument does not seem to be extremely weighty. Since only 

 small quantities are necessary and since hormones are known to 

 exist in animals, it seems only reasonable to assume their existence 

 here in light of the phenomena described. True, there is not the 

 means for rapid distribution in the plant as in the animal, but 

 there is the means for a slow distribution and the responses in 

 plants are in general not so rapid as in animals. The transfer of a 

 stimulus should take place much more rapidly than the responses 

 cited above. Thus the time factor would favor the hormonic 

 rather than the " nervous" theory in the cases cited, even though, 

 as will be stated later (Chap. XXVIII), there are cases in plants 

 where the response is thought to be due to what may be called 

 an elementary nervous stimulus. As to the criticism that hor- 

 mones have not been isolated in plants, it is claimed by some 

 authors that many of the more common substances such as the 

 glucosides and alkaloids have an hormonic effect in stimulating 

 growth and development. 



The hormones are thought to stimulate the action of enzymes, 

 which in turn bring about the metabolic changes we find in plants. 

 The hormone is thus the " trigger" which excites enzyme ac- 

 tivity and causes the enzymes to be secreted. Similar results in 

 the formation of galls and other excrescences are produced by 

 the injection of materials when the plant is stung by gall insects, 

 and thyroid extract has been found to promote enormously the 

 growth of hyacinth bulbs and potato tubers. How the hormones 



