WILLIAM J. ROBBINS 465 



thiamin to be the essential factor in the dried yeast (47). This confirmed 

 the earher suggestion by Robbins (40,41) that the failure of an excised 

 root to continue growth when repeated transfers of the root tip are 

 made is because the seedling root contains some materials derived from 

 the seed other than water, mineral salts, sugar, and free oxygen, which 

 are necessary for continued growth and cannot be synthesized in solution 

 cultures from the materials supphed. 



Later Robbins and Schmidt (48) demonstrated that tomato roots 

 have a partial deficiency for pyridoxine, and Bonner (5) found some 

 clones to have complete deficiencies for thiamin and for pyridoxine 

 and partial deficiencies for niacin. All three vitamins show a high degree 

 of specificity. 



The roots of a substantial number of species have been cultivated in 

 excised condition, and for many of them potentially unlimited growth 

 has been obtained. Among these are Acacia melanoxylon\ alfalfa {Medi- 

 cago sativa) ; aster {Callistephus chinensis) ; buckwheat {Fagopyrum escu- 

 lentum)\ carrot {Daiicus carota); celery {Apium graveolens)\ chicory 

 {Cichorium sp.); red clover {Trifolium pratense); white clover {Trifolium 

 repens); sweet clover {Melilotus alba); cotton {Gossypium hirsutum)', 

 Crepis rubra; Jimson-weed {Datura stramoniuni); mustard {Brassica 

 nigra); pea {Visum sativum); Petunia violacea; radish {Raphanus sativus); 

 soybean {Glycine soja); tobacco {Nicotiana Tabacum, N. langsdorjii); 

 tomato {Lycopersicon sp.); and vetch {Vicia sp.) (6,10,60). 



A number of the above have been cultivated only in a sugar-mineral 

 salt medium supplemented with yeast extract and their exact vitamin 

 requirements have not been defined. For the others, thiamin, pyridoxine, 

 and niacin are the only vitamins which have been demonstrated to be 

 of importance. The excised roots of one species, flax, make limited growth 

 through an indefinite number of transfers in the absence of any added 

 growth substance. The addition of thiamin, however, increases the 

 growth substantially. White clover is able to grow indefinitely in the 

 absence of thiamin but requires niacin. Of 12 additional species, all 

 require thiamin. Four species require thiamin and pyridoxine, but their 

 growth is increased by the further addition of niacin. The results to 

 date emphasize the importance of thiamin as a vitamin requirement for 

 the growth of excised roots, but, depending upon the clone or species 

 used, all possible combinations of complete and partial deficiencies for 

 the three vitamins occur. 



