74 PLANT GROWTH SUBSTANCES 



wholly exerted by auxin; that this same auxin was the agent which caused 

 apical dominance in stems, preventing the lower lateral buds from grow- 

 ing out. Auxins were intimately tied up with root initiation, fruit devel- 

 opment, leaf and fruit abscission, and many other phenomena. Morpho- 

 logical polarity in organ formation was apparently largely due to polar 

 auxin transport. On the cellular level auxin influenced a number of 

 protoplasmic properties and activities, such as permeability, viscosity, 

 water uptake, cyclosis, etc. Although none of these phenomena were 

 completely and exhaustively studied, the evidence was so strong that the 

 participation of auxin hardly seemed a problem any more. Yet if we 

 consider in detail the physiological role of auxin in particular cases, we 

 find that there are more discrepancies than we suspect when we view 

 the problem as a whole. As a particular case I might mention the role of 

 auxin in phototropism. After the first main objections against the 

 Cholodny-Went theory were allayed through the work of van Overbeek, 

 who bridged the chasm between this theory and Blaauw's by showing 

 that both principles did coexist in a single plant, the problem seemed 

 almost solved, especially when the carotene activation of auxin-a destruc- 

 tion by light was found. But now in view of the stability of indoleacetic 

 acid towards small amounts of light and many other conflicting facts 

 it seems necessary to reconsider the problem of phototropism from an 

 auxin angle. 



The work on the chemical nature of auxin in the plant, so brilliantly 

 Initiated by Kogl and Haagen-Smit (12) in Holland, and Thimann (23) 

 in this country, led to developments along two difl^erent lines. In the 

 first place the discovery of the activity of indoleacetic acid and many 

 related substances as auxins, by Kogl and Haagen-Smit (13), and its 

 extension in this country by Zimmerman and Hitchcock (30) and many 

 others, notably Norman and collaborators (25), made it possible to 

 apply growth-promoting substances, and later, related growth-inhibiting 

 and herbicidal substances, in concentrations far beyond what the plant 

 tissues are normally subjected to. This led to what has often been 

 referred to as plant pharmacology and made practical applications 

 possible. 



The other development which grew out of the chemical work is the 

 biochemical study of indoleacetic acid inside the plant. Since Haagen- 

 Smit and collaborators, and Avery and Berger, isolated and chemically 

 identified indoleacetic acid in extracts from plants it has become bio- 



