68 • Floral Hormones and the Induced State 



The idea that the formation of flowers, and of other organs as 

 well, is controlled by hormones specific for each type of organ- 

 " organ-forming substances"-was favored in the nineteenth century 

 by Julius Sachs, the so-called "father of plant physiology." Evidence 

 at the time was almost nonexistent; more recent evidence, at least 

 for flowering hormones, will be considered below. First, however, 

 it is useful to describe briefly a different and better known class of 

 plant hormones, the auxins. Research on these substances, starting 

 in the 1920's, has had a strong influence on the less successful 

 investigations on possible flowering hormones; in addition, auxins 

 may play at least a minor role in the control of flowering. 



If the tip of a growing shoot is removed, the elongation of the 

 remaining stump generally ceases rapidly. If the tip is replaced, the 

 stump may resume and continue elongating for some time, although 

 not necessarily as fast as in the intact plant. This effect of the tip 

 may even occur if it is separated from the stump by a thin layer of 

 agar or gelatin. In such cases, elongation can be brought about 

 simply by placing on the stump a piece of gelatin or agar on which 

 the cut surface of the tip, or several similar tips, have rested for 

 some time. Such results indicate that a substance or substances that 

 can move out of the tip and into or through gelatin are required for 

 the continued elongation of the tissue below. Such substances are 

 termed auxins. It is now known that low concentrations of many 

 substances, both natural and synthetic, can promote the elongation 

 of shoot tissue deprived of its natural auxin sources. Most of them 

 are relatively simple organic compounds, such as indole-3-acetic 

 acid; those occurring naturally are clearly plant hormones since 

 they are produced in shoot tips (or other young, actively growing 

 regions) and affect tissues elsewhere. The action of auxins is not 

 confined to causing the elongation of shoot cells, however; depend- 

 ing on the concentration, they may either promote or inhibit 

 many plant processes, including root initiation, leaf abscission, and 

 cell division. Space forbids further discussion of auxins as such, 

 but they will figure in a number of the topics to be considered. 

 For additional information on the general topic of auxin physi- 

 ology, which has a voluminous literature, see Audus (1959), 

 Leopold (1955), or the recent volume, Plant Growth Regulation 

 (1961). 



