232 PHYTOHORMONES 



ure 33), Tradescantia (Uyldert, 1931), and Bambusa (u); 

 probably the shoots of all trees and shrubs with periodic 

 growth behave in this way. 



The correlation between the amount of auxin which dif- 

 fuses from buds of different plants and the rates of growth 

 of the shoots below them (Zimmermann, 1936) may also be 

 mentioned. 



All such observations support the view that quantitative 

 relations between different parts of the plant are expressions 

 of the quantitative relation between auxin and its growth effects. 

 This generalization can now supersede the older view that 

 such growth relations are determined by the amounts of 

 food material present. 



B. Comparison with Animal Hormones 



The progress of the work on phytohormones compares very 

 favorably with that which has been made in other fields. 

 This may be attributed to the relative simplicity of the 

 relation between the plant and its hormones, as contrasted 

 with the apparent complexity in animals. The hormonal 

 correlation between different parts of an organism will be the 

 more complex the larger the number of parts; the plant, 

 with few organs, is thus a relatively favorable organism 

 for study. One important complexity in animals is that the 

 action of hormones is so often indirect, so that the hormonal 

 activity of one gland may be expressed through its effect 

 on the activity of another gland. The pituitary exerts an 

 effect on the gonads, vitamin D may act through the 

 parathyroid, and so on. Where there are introduced in this 

 way additional links in the chain of action the mechanism 

 of the process is correspondingly harder to elucidate. This 

 does not necessarily mean that in plants the fundamental 

 process is itself any simpler, but it does mean that it is 

 more open to attack, as is shown by the fact that it has been 

 possible to separate the primary processes in which auxin 

 takes part from the secondary ones which prepare for its 

 action. 



