270 PLANT GROWTH SUBSTANCES 



Growth of control segments. — The ability of the stem segments to 

 grow on the basal medium is a function of their size and of the vigor of 

 the plants from which they were taken. Pieces 3x2x2 mm. or less in 

 size ordinarily remain alive for several weeks but do not grow appreciably 

 unless they are supplied with lAA. Larger pieces (7x5x2 mm.) which 

 were selected as standard material for these and all subsequent experi- 

 ments form masses of callus at their basipetal ends within two to four 

 weeks in culture. After longer periods (6 to 10 weeks) a few pieces (5 per 

 cent on the average) may form a bud or root or both, but growth is 

 restricted mainly to the formation of callus, which starts from the cambial 

 region of the basipetal surface. This callus may be subcultured on the 

 same medium with added lAA, but, at least in most cases, neither the 

 original stem tissue nor the first transfer of callus will continue to grow 

 on the basic medium without lAA. 



Effects of adenine treatments. — When adenine is added to the medium 

 the formation of callus is less than in controls, but a very striking forma- 

 tion of buds occurs. These buds grow out most abundantly along the 

 lateral edges and especially from the apical halves of the segments with 

 relatively lower auxin content, but also to some extent from the basipetal 

 halves and attached, newly formed, callus. As indicated in Figure 3, 

 Dr. Sterhng has found that they originate from parenchyma formed by 

 the cambium and from both external and internal phloem regions. The 

 number of pieces forming buds in a given treatment and the number of 

 buds per piece both increase as functions of the concentration of adenine 

 in the medium up to about 50 mg./l. Higher concentrations are often 

 toxic. With optimal concentrations, generally between 90 and 100 per 

 cent of the segments form buds. The effects of concentration of adenine 

 and time of culturing are illustrated in Figure 4. In this experiment the 

 highest average number of buds per segment was 2.7 after twenty-eight 

 days. In an experiment with tissues from field-grown plants, however, 

 an average of about 20 buds per segment and extremes of 35 to 40 buds 

 per segment have been obtained. The number of buds obtained is not 

 necessarily the maximum number that can be produced by the tissue, 

 because as one or a few buds start to develop, they produce auxin in 

 sufficient quantities to raise the concentration in the original stem tissues, 

 and thus tend to retard or prevent further bud formation. The buds 

 which form early may develop several leaves, but, unless root formation 

 occurs, the buds are incapable of continued growth. If they form roots 



