276 PLANT GROWTH SUBSTANCES 



even though they may be hmiting factors for the formation of buds and 

 roots respectively under normal conditions as well as under a given set 

 of experimental conditions, it is obvious that different conditions could 

 be selected, where organ formation would be limited by other factors. 

 Effects of inorganic nutrients. — While White's nutrient medium was 

 originally devised for continued culture of excised roots, it contains all 

 necessary ingredients required for continuous growth of tobacco callus 

 cultures, but certain quantitative modifications in mineral components 

 result in more rapid growth rates of the tissues. It is a question, therefore, 



TABLE 4 



Effect of adenine sulfate added to the nutrient medium at different /?H values 

 on the formation of buds in stem segments of tobacco. Exp. 39. Started 8/5/48 



to what extent changes in mineral composition might also affect the 

 capacity of the tissues to form organs. So far only a few of the many 

 possible variations in nutrient composition have been tested in tobacco 

 stem segments. Thus, a marked effect on pYi on bud formation in response 

 to added adenine has been established. As indicated in Table 4, adenine 

 is effective in inducing bud formation in the range from pW 4 to pW 7 

 which was tested, but acid pW is highly favorable. Since acid pW also 

 favors bud formation of callus tissue in submerged cultures without added 

 adenine, it may be assumed that the pW effect is not entirely, if at all, 

 on the adenine uptake by the tissues. 



Similarly, increases in phosphate supplies in the medium increase the 

 effectiveness of the adenine treatments. As shown in Figure 8, a threefold 

 increase in phosphate content (from 12.5 mg./l. to 37.5 mg./l.) causes 

 about twice the number of buds to be formed per segment treated with 

 40 mg./l. adenine. However, a ninefold increase in phosphate is evidently 



