400 PLANT GROWTH SUBSTANCES 



other methods. For example, 2,4-dichlorophenoxyacetic acid and its 

 sodium salt induced negative curvature on stems and leaves of sensitive 

 plants at 1.5 x lo"- grams per liter and modified organs at 3 x io~^ 

 grams per liter (34), while with sunflower tissue cultures 2,4-dichloro- 

 phenoxyacetic acid inhibited growth progressively from the lowest con- 

 centration of I X io~^^ grams per liter to the highest concentration. The 

 sensitivity of this tissue to the other compounds was also greater than 

 that observed with whole plants. The sensitivity of the cultures to some 

 of the compounds compares with that of the Avena coleoptile. The 

 exposure of the cultures to the materials over a six week period may 

 account for the sensitivity to such minute amounts. 



No macroscopic evidence was observed with sunflower and tobacco 

 tissue of differentiation of leaves, stems, or roots. Under certain con- 

 ditions such changes did occur in tobacco tissue in a liquid medium 

 (29,22). The stimulating or inhibiting effects of low and high concentra- 

 tions of growth-regulating substances described here were also reported 

 by Skoog (22) for tobacco hybrid tissue, and by de Ropp (2) for sun- 

 flower gall tissue. The possibihty that histological differences occurred 

 in such tissues with different concentrations of these materials was 

 examined next. 



Histological effects of growth-regulating substances on sunflotver gall 

 tissue. — The influence has been tested of the concentrations of some 

 representative growth-regulating substances on the structure of sun- 

 flower tissue of crown-gall origin (25). Cultures incubated on weak and 

 strong concentrations of four compounds and on control media lacking 

 supplements were studied. Cultures from weak concentrations of indole- 

 3-acetic acid and a-naphthaleneacetic acid (i x io~i^ grams per liter), 

 indolebutyric acid (i x io~^ grams per liter) and /7-chlorophenoxyacetic 

 acid (i X 10"'^ grams per liter) were compared with cultures incubated 

 on the strong concentration (i x io~^ grams per liter) and with the 

 controls. The weak concentrations for the respective compounds were 

 selected because they represented optimal concentrations. The strong 

 concentrations were inhibiting (see Fig. i and Table i). 



The structure of the sunflower tissue was relatively simple. Such tissue 

 consisted of hypertrophic, hyperplastic, and thick-walled scalariform 

 cells. The control tissues contained all three kinds of cells. Very few 

 mitotic divisions appeared perhaps because growth was at a minimum 

 after the six-week incubation period. Cells with scalariform thickenings 



