240 PHYTOHORMONES 



We can now attempt some comparisons between the processes 

 in which these substances are involved, in plants and animals. 



The analysis which we have made in the case of root 

 formation enables us to consider organ formation in more 

 general terms, and especially organ formation on isolated 

 parts — the so-called ''regeneration." 



It is clear that the substances and processes acting to 

 form roots are doing so continuously in the normal intact 

 plant. In a cutting these factors will continue to operate 

 in the same way as they would have done in the intact plant, 

 the place of accumulation being now, of course, different. 

 Thus the auxin which normally would have moved from the 

 buds to the base of the shoot, or even into the roots, will 

 now move merely to the base of the cutting. It follows that 

 the roots so formed are not new formations (''regenerates"), 

 nor is their formation in any way a response to the loss of 

 the others. On the contrary it is merely the normal genera- 

 tion going on in an atypical — that is, an artificially condi- 

 tioned — place, and thus has nothing to do with any "tend- 

 ency towards completion (Ganzheit)." 



In animals, if a part is removed, there is a tendency to 

 replace it, and this is very pronounced in the Coelenterates, 

 which furnish the best comparison with root formation. 

 These animals show apical growth, and, what is important 

 from our point of view, they have no true circulation of 

 food. Correspondingly, their growth is not diffuse, as in 

 higher animals, but is typically polar. This must be due 

 to the polar transport of one or more growth substances, 

 which in higher animals would be carried in the blood stream 

 instead. In view of the neglect of this subject in recent years, 

 it would be of great interest now to transfer our knowledge 

 of "regeneration" in plants back to such animals, with 

 special reference to the role of growth substances and of 

 their probable polar transport. 



A well-analyzed case of regeneration in a very interesting 

 object is the work of Hammerling (1934, 1935, 1936) on the 

 alga Acetabularia. This bears out all the points pre\4ously 



