GENERAL CONCLUSIONS 



233 



There are important contrasts between the hormones of 

 the plant and animal kingdoms. In animals, of course, the 

 foods and hormones all travel along the same path, namely 

 that of the blood stream. This has the effect that all cells 

 receive the same hormonal stimulus and the result will 

 depend upon their ability to respond to it. Plants, on the 

 other hand, have no true circulation and the movement of 

 hormones in them is mainly unidirectional, independent of 

 the mass movements of water and foods. Thus hormones 

 and foods move by different paths and hence every cell is 

 not in a position to respond; this results in local growth 

 zones and phenomena such as apical growth. 



However, the fact that the term ''hormone," first coined 

 for animal physiology, has been used for the auxins shows 

 that there are important parallelisms. The fundamental 

 function of hormones, namely that of chemical messengers, 

 is the same in plants as in animals; — the phytohormones 

 are true hormones. Further, the acti\'ity is in both cases 

 exerted in concentrations too low to allow of their making 

 up an appreciable part of the cell (cf. VHI E). Table XVI 

 summarizes very approximately the minimal active doses 

 per gram of fresh weight of the test organism, for some differ- 

 ent hormones. These figures, however, are not really 

 comparable, not only because the molecular weights are 

 different, but also because we do not know the actual 

 concentrations at the place of action. 



It is interesting to compare the specificity of the animal 



TABLE XVI 



