WILLIAM J. ROBBINS 469 



are derived appears to be associated with an increase in the efficiency of 

 nitrogen metaboHsm (46). 



Some Unsolved Problems. — Our knowledge of the relation of vitamins 

 to higher plants is still far from complete. I have called attention to the 

 substantial number of species for which the conditions necessary for 

 unlimited growth of excised roots have not yet been defined. The same 

 comment applies to excised stem tissues. 



The evidence for the effects of vitamins on intact higher plants is 

 confused and in most instances unconvincing. However, the growth 

 requirements of such parasites as dodder or saprophytes like Indian pipe 

 have not been defined. The effects of the additions of natural organic 

 supplements on the growth of the Lemnaceae (50), the relations of 

 mycorhiza to some higher plants, the increased growth frequently 

 associated with polyploidy, the phenomenon of hybrid vigor, genetical 

 dwarfs, flowering and photoperiodism, all deserve further investigation 

 from the standpoint of partial deficiencies of essential metabolites in- 

 cluding vitamins. 



We know very little of the relations of higher plants to pteroylglutamic 

 acid or to vitamin B12. One green plant, Eiiglena, is heterotrophic for 

 vitamin B12 (22). The presence of this vitamin in higher plants has not 

 been demonstrated, and no instance of its importance for higher plants 

 has been reported. 



The difference in vitamin requirements of tissues derived from stems 

 or storage organs and those of excised roots deserves further investiga- 

 tion. For the former, an external supply of lAA is the critical factor; for 

 the latter, thiamin, pyridoxine, or niacin include the essential supple- 

 ments. This distinction applies even when the excised roots and the 

 excised stem tissue are derived from the same species. It exists also when 

 the requirements of tissue obtained from a storage root are compared 

 with those of a seedling or fibrous root of the same species. Bonner (5) 

 obtained unlimited growth of seedling carrot roots in a sugar-mineral 

 salt solution supplemented with thiamin and pyridoxine; both vitamins 

 were essential. Tissue from the storage root of carrot requires for con- 

 tinuous and vigorous growth only lAA or its equivalent; other supple- 

 ments are not necessary. Nobecourt (36) observed that fibrous roots 

 which develop from the tissue isolated from the storage root do not 

 grow in the sugar-mineral salt medium supplemented with lAA when 



