ALBERT C. HILDEBRANDT 399 



with varying concentrations, respectively, of /j-chlorophenoxyacetic 

 acid and a-naphthaleneacetic acid expressed as percentage of wet weights 

 varied from 4.6 to 6.8. Similar experiments with a-naphthaleneacetic 

 acid and indole-3-acetic acid, respectively, and tobacco tissue provided 

 dry weights that ranged from 5.3 to 9.9 per cent of the wet weights. 

 When the average wet and dry weights were plotted on a logarithmic 

 scale the resulting curves had similar trends. The dry weight expressed 

 as the per cent wet weight decreased as the average wet weight increased. 

 (The lowest per cent dry weights were obtained with concentrations 



TABLE 1 

 Comparative Inhibiting Concentrations of Some Growth-Regulating Sub- 

 stances on Sunflower and Tobacco Tissue Growing in vitro 



Inhibiting dilution 

 Growth-regulating substance Sunflower Tobacco 



tissue tissue 



2,4-dichlorophenoxyacetic acid 

 2,4-dichlorophenoxy butyric acid 

 sodium 2,4-clichlorophenoxyacetate 

 /?-chlorophenoxyacetic acid 

 indole-3-acetic acid 

 /3-naphthoxyacetic acid 

 a-naphthaleneacetic acid 

 indolebutyric acid 

 a-naphthaleneacetamide 

 cysteine hydrochloride 



providing the greatest wet weight). This suggested that stimulation of the 

 growth-regulating substances was associated with a swelling of the 

 tissues. 



The inhibiting effect of the growth-regulating substances was more 

 striking with sunflower tissue than with tobacco tissue. This was perhaps 

 because the sunflower grew more rapidly. Sunflower tissue on control 

 media increased in wet weight during six weeks to an average of 450 

 mg. per tissue piece as compared to only 200 mg. for tobacco tissues. 

 The sunflower tissue also had a greater water content. 



The sensitivity of these tissue cultures to extremely low concentrations 

 of growth-regulating substances suggested some possibilities for assay 

 ot those compounds that have been difficult or impossible to assay by 



