402 PLANT GROWTH SUBSTANCES 



Both hypertrophic and hyperplastic cells were larger than those of 

 tissues incubated on weak concentrations or on control media. Accord- 

 ingly the number of cells per unit area on each of the four compounds 

 was fewer with the strong concentration. Also the number of tracheal 

 elements was fewer. 



It appeared therefore that the strong concentration of growth-regulat- 

 ing substance tested stimulated a swelling of the cells. This swelling was 

 greater between the strong and weak concentrations than between the 

 weak concentration and the control. The greatest number of scalariform 

 vessels was found on media containing the weak concentrations. The 

 greater differences in the number occurred between the strong and weak 

 concentrations than between the weak concentrations and the control. 



These results were comparable to certain of those reported by de Ropp 

 (2) for sunflower and periwinkle tissue of crown-gall origin. He reported 

 that different concentrations of growth-regulating substances had no 

 effect externally and internally except for differences in weight after a 

 six-week culture period. The studies described here indicated however 

 that internally the structure of sunflower tissue of crown-gall origin 

 was considerably modified. The reasons for these differences are not clear. 



The influence of growth-regulating substances on the respiration of 

 cultures of crown-gall origin may provide some of the answers, but this 

 approach to the problem has hardly been touched because of certain 

 technical difficulties. Mitchell, Burris, and Riker (17) found that 0.002 

 M. indole-3-acetic acid inhibited respiration of sunflower gall tissue 

 by sixty-eight per cent. This reduction in respiration with indole-3- 

 acetic acid was comparable to that observed in stems, roots, petioles, and 

 gall tissues from a number of sources. 



Summing up then, growth-regulating substances induced striking 

 effects on plant tissue cultures just as they did on whole plants. At 

 least three general types of responses were observed. Tissue cultures 

 from normal plants were especially responsive to these materials. With 

 tissues from a number of species these materials were indispensable. 

 If the growth-regulating substance was not added to the medium the 

 piece of tissue originally isolated from the plant failed to form callus, 

 and therefore cultures capable of unlimited growth were not estabhshed. 

 Furthermore, cultures of tissue from normal plants of certain species 

 required added growth-regulating substance in the medium to maintain 

 indefinite growth. Concentration of added growth-regulating substance 



