Expanding Concepts 19 



We have become convinced that growth regulators are involved 

 in the flowering of plants even though their chemical nature still 

 eludes us. The sex hormones of the water mold Achlya first reported 

 by Raper (15) must also be admitted to the growing list of plant 

 growth regulators, together with the antheridium-inducing factor 

 from fern prothallia recently investigated by Niif (10). I would in- 

 clude also among the plant growth regulators, acrasin, the autogenic 

 chemical substance involved in the organization of some of the slime 

 molds (16). 



To refer to all the plant growth regulators, that is, all the specific 

 chemical substances which in minute quantities affect, even deter- 

 mine, the rate and pattern of plant growth and also those for which 

 there is convincing evidence, though they have not yet been isolated 

 and identified, would make a long list indeed and one which is in- 

 creasing rapidly. This is not the place to undertake such a review. 



How delicate the dynamic equilibrium of the metabolic systems of 

 a living organism is. What profound effects on the rate, amount and 

 character of growth can be produced by minute amounts of a specific 

 organic compound. A few molecules of vitamin Bjo determine 

 whether Euglena will grow and how fast. p-Chlorophenoxyacetic 

 acid causes the apex of Kalanchoe to develop into a spathe-like organ 

 which can be cut off and rooted. The ortho- and mefa-chlorophenoxy- 

 acetic acids are inactive (Zimmerman and Hitchcock, 27). 



At the same time, how stable the systems are: like the treasures 

 in a safe-deposit vault the course of metabolism which results in nor- 

 mal growth and normal form is protected against all the agents which 

 impinge upon it, except for those which have the right configuration, 

 which are the keys capable of unlocking the vault door. 



This is the area which concerned Percy W. Zimmerman most. It 

 is true that his botanical interests were broad. He developed hardy 

 strains of Camellia japonica and of the American and English holly. 

 He was a pioneer in the study of the effects of air pollution on plants, 

 and because of long experience and extensive researches he was recog- 

 nized as an authority in this field. But his great and abiding interest 

 was the search for compounds which modified the normal growth 

 pattern of a plant. 



Dr. Zimmerman was not a locksmith. He cared less for the mecha- 

 nism of the lock and how the key turned the tumblers than he did 

 for the key and the treasures in the vault. What keys he found, and 

 what treasures were revealed! 



In cooperation with his close associate, Dr. A. E. Hitchcock, he 

 investigated the properties of more than 500 compounds as growth 

 regulators. (3-Indole-3-butyric acid, 1-naphthaleneacetic acid, substi- 



