Growth Substances and Plant Tissue Cultures 681 



most responsive. Growth of eight strains from this family was re- 

 duced; bush bean cotyledon tissue by as much as 50 per cent of the 

 control growth. 



The four fastest-growing cultures were all inhibited. These are 

 holly root, pole bean hypocotyl, pinto bean root, and bush bean coty- 

 ledon. Of the slowest-growing cultures, sweet clover virus stem tumor 

 was stimulated, while sweet clover root crown gall, the nonpigmented 

 cactus stem crown gall, and Stapelia leaf tissue were slightly inhibited. 



Over the last several years, we have found a few cultures that 

 change somewhat in appearance. These "variants" have been segre- 

 gated and maintained as separate cultures. Since they are morpho- 

 logically distinct from the parent culture, they were tested to deter- 

 mine their growth response to added gibberellin. Melilotus root crown 

 gall (white and green) and Opuntia stem crown gall (red and white) 

 show that the variants respond differently to gibberellin from the 

 parent cultures. Whether these "variants" are true somatic mutations, 

 merely segregants from mixed cell populations, or represent other 

 phenomena is not known. 



Because of the possibility that the auxin-gibberellin relationship 

 in specific tissues might play a part in their response to gibberellin, 

 two types of experiments were set up. In the first type, two tissues 

 were selected that grow on a synthetic medium (LP) with no added 

 auxin. One tissue (tobacco stem) is stimulated by added gibberellin, 

 whereas the other (sunflower petiole) is inhibited. Both tissues are 

 crown galls. In the second type, two tissues were used that were 

 maintained on a medium supplemented with 2,4-D and coconut 

 milk. The growth of one of these was promoted by added gibberellin 

 (broad bean cotyledon), the other was killed (avocado cotyledon). 

 These two tissues are both normal. 



Before discussing the results of these experiments, it is well to 

 point out the effect on growth of the removal of 2,4-D from the 

 medium. The growth of avocado is increased by removal of 2,4-D 

 (Figure IB). 'Pontiac' potato tuber tissue, on the other hand, dies 

 when 2,4-D is removed from the medium, even on the first subcul- 

 ture (Figure IC). Similar results are obtained with holly and yam 

 tissues. 



The results with the two crown-gall tissues are shown in Table 

 4. Tobacco, which is stimulated by gibberellin at 10 p. p.m., is in- 

 hibited by 2,4-D at 1 p. p.m. The addition of both substances causes 

 an intermediate response. Sunflower, which is inhibited by gibber- 

 ellin at 10 p.p.m., is also inhibited by 2,4-D, but to a greater extent. 

 Both substances together give the same effect as 2,4-D alone. 



The responses of the two normal tissues to gibberellin and 2,4-D 



