Plant Qrowth'Suhstances 



nor anyone else can give, and it is not likely that the ideal will 

 be even approached for a long time. 



The book now contains sketchy outlines of some physio- 

 logical aspects. These have been introduced not as a con- 

 cession to the over-weening specialists, but partly because one 

 paradox was not properly explained in the first edition (though 

 no reviewer noticed that), and partly because it is becoming 

 possible to account to some extent for actions which pre- 

 viously were brought about purely empirically. 



The difference between just using the synthetic growth- 

 substances to produce a result (such as rooting of cuttings) and 

 understanding how growth occurs is like the difference 

 between pharmacy and physiology. A year or two ago there 

 was practically no knowledge about how the artificial growth- 

 substances produced their results: they were applied like a 

 drug from the outside, produced a certain or uncertain 

 effect, and tliat was all. There was an increasing under- 

 standing of the way normal growth was regulated inside 

 plants by their own natural hormones, but a discussion of these 

 belongs to books on plant physiology. 



This book, then, is about drugs that produce, control, or 

 regulate growth in plants. Being foreign to the plant, such 

 applied substances are not hormones, though they are fre- 

 quently miscalled "plant hormones" or "phytohormones". 

 Both these latter terms connote, and should be restricted to 

 mean, the (natural) hormones of plants, which are the pro- 

 vince of physiology and are not discussed in this book, 

 except briefly on pages 53 and 93, and elsewhere incidentally 

 in relation to the synthetic substances. 



Since the first edition was written there have been very few 

 changes of significance to the pure chemist, as far as the 

 phenyl- and indolyl-acetic group of substances is concerned. 

 No important new synthetic growth-substance of that type has 

 been prepared; no new method of preparation has been 

 published ; and no known substance, belonging to the group, 

 but previously unused, has received any extensive application. 

 The truth of this last can be realized from the fact that the 

 only fresh development worth mentioning from the chemical 



20 



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