Growth Substances 391 



The arrangement of structures in a radially symmetrical pattern doubt- 

 less involves differential distribution of auxin, but little is known 

 about this. When such radially symmetrical structures are placed in a 

 horizontal position they often tend to become dorsiventral, as in certain 

 flowers and stems. Since most such direct effects of gravity seem to be 

 produced by differential distribution of auxin, as in the well-known 

 cases of geotropism, auxin presumably is involved in structural dorsi- 

 ventrality as well. Its role here in young liverwort plants has been 

 described by Kohlenbach ( 1957 ) . 



Of wider morphogenetic interest are the effects of growth substances 

 on the determination of specific structures. From his study of flower 

 production Sachs (1882) suggested that organ-forming substances are 

 operative in plants, especially in the determination of flowers and roots. 

 The growth of knowledge of morphogenetically effective substances 

 revived interest in this hypothesis, and it has stimulated a wide range of 

 experiments which have thrown much light on the mechanisms of de- 

 velopment. Organ-forming substances of many types have been postu- 

 lated in the formation of roots, stems, leaves, flowers, galls, sexual struc- 

 tures, and others. Just how such substances produce their effects is not 

 known, and the actual existence of some of them is not yet proved, but 

 the theoretical, and also the practical, importance of these problems is 

 great. The most obvious way to account for development is to postulate 

 the operation of a series of such substances. The difficulties of this con- 

 ception, however, are obvious, for a very large number of them would be 

 necessary. The tendency today is to assume the activity of a relatively 

 small number and to explain the variety and specificity of their effects 

 through their interactions and in other ways. 



Root Formation. Van der Lek (1925) observed that a piece of stem 

 (used as a cutting), on which there was a bud or young leaf, formed 

 roots at its base, whereas a naked stem piece did so much less readily or 

 not at all. This suggested that there was a substance, formed in buds 

 and leaves, that moved downward and stimulated root production. Went 

 ( 1929 ) demonstrated this by showing that an extract from the leaves of 

 Acalypha, applied to the apex of a cutting, promoted root formation at 

 its base. Several workers soon discovered that auxin and various natural 

 and synthetic substances also have this effect and that cuttings could 

 be made to root by the application of such substances. 



In 1935 Laibach and Fischnich described a technique by which indole- 

 acetic acid in lanolin paste applied to a stem would promote root for- 

 mation. In the same year Zimmerman and Wilcoxon ( 1935 ) reported 

 that several synthetic substances such as indolebutyric, indolepropionic, 

 phenylacetic, and naphthaleneacetic acids had this effect and could 

 be used in horticultural practice to hasten the rooting of cuttings when 



