96 • Floral Hormones and the Induced State 



well known; in addition, much of the rooting behavior of cuttings 

 can be explained in terms of their auxin content and sensitivity. 

 Yet it is also known that the same compound plays a major role 

 in other developmental processes having nothing to do with root 

 initiation, so that it would be grossly misleading to call it 

 "rhizogen" (root-maker). That development is controlled by the 

 balance of various substances common to many processes is strik- 

 ingly illustrated by the work of Skoog and Tsui (1948) and Miller 

 and Skoog (1953). Tobacco stem segments grown in aseptic culture 

 produce roots if supplied with a particular level of auxin and 

 shoots if supplied with another substance, adenine. Both com- 

 pounds together cause the production of more or less disorganized 

 callus tissue; but increasing the adenine again leads to shoot forma- 

 tion, whereas increasing the auxin leads to root formation. Thus 

 the balance of auxin and adenine controls the production of roots 

 or shoots in this system. Adenine, as a component of the nucleic 

 acids and many respiratory co-enzymes, is probably present in every 

 living cell; the many roles of auxin have already been mentioned 

 (see Audus, 1959). 



A simpler example of control by an unspecific substance was 

 found by Wetmore (1953), who studied the development of young 

 fern apices in aseptic culture. The first few leaves produced by 

 ferns, as by many other plants, may differ considerably from the 

 later ones, being characteristically "juvenile" in some way; the 

 ferns in question (Todea, Adiantum) have juvenile leaves with 

 few or no divisions, whereas the older leaves are deeply lobed. In 

 culture, mere variation of the sucrose content of the medium 

 suffices to bring about almost any degree of "juvenility" or 

 "maturity" in leaf shape, with the lowest sucrose level giving the 

 least lobed leaves. Thus the normal leaf progression, regarded as 

 a fundamental developmental property of the meristem and one 

 of considerable evolutionary significance, is susceptible to regulation 

 by a substance that presumably serves merely as a general energy 

 source. This result may have more than illustrative value here. 

 If, as Philipson (1949) suggests, the reproductive apex simply 

 represents a normal later stage in the ontogeny of the shoot, as 

 does the transition from juvenile to mature foliage, then perhaps 

 a local increase in carbohydrates may play a central role in 

 flowering itself. 



