464 PLANT GROWTH SUBSTANCES 



to be acquainted with those of other groups, both plants and animals, 

 and to inckide in their investigations not only vitamins as such but 

 amino acids, hormones, and other substances which have been demon- 

 strated to play a significant role in plant and animal metabolism. 



Intact Plants. — Whether we consider vitamins in the narrower sense, 

 that is, the B vitamins, vitamin A, C, D, E, and so on, or in the broader 

 sense, intact higher plants have no vitamin requirements. This is demon- 

 strated by their ability to grow in a mineral medium with no organic 

 supplements. The evidence at hand indicates also that with some possible 

 exceptions and under special circumstances, higher plants have no partial 

 deficiencies for vitamins, that is, their growth is not improved by the 

 addition of vitamins to the medium. They appear to synthesize adequate 

 quantities of all the vitamins they need. Bonner and Bonner (10) in a 

 recent review of this subject say that thiamin is not a limiting factor 

 In the growth of most species of higher plants and can become a limiting 

 factor only for a few species or under particular environmental circum- 

 stances. The same conclusion appears to hold for the effects of niacin, 

 pyridoxine, adenine, pantothenic acid, riboflavin, and other similar sub- 

 stances for which growth increases have been reported by one investigator 

 or another. 



Excised Parts. — Although the intact higher plant is self-sufficient as 

 far as vitamins are concerned, isolated parts are not. They may evidence 

 heterotrophism for various vitamins. 



Excised roots of a number of species have been demonstrated to have 

 complete or partial, deficiencies for thiamin, pyridoxine, and niacin. 

 Stem tissues and the tissues of storage organs require indoleacetic acid 

 (lAA) or its equivalent, and in some instances (willow and hawthorne) 

 pantothenic acid and biotin. Thiamin and cysteine are not essential but 

 may improve growth (18), The contrast between the relation of intact 

 plants to vitamins and that of isolated organs or tissues emphasizes the 

 interdependence of the parts of a higher plant and has led to the thesis 

 that the B-vitamins may be considered to be plant hormones (10). 



Excised Roots. — We know more perhaps about the vitamin relations of 

 excised tomato roots than about those of the roots of any other plant. 

 White (58) first succeeded in obtaining unlimited growth of tomato roots 

 in a solution of mineral salts, sugar, and dried yeast. Robbins and Bartley 

 (43) demonstrated that excised tomato roots would not grow unless the 

 yeast extract was included in the sugar-mineral salt solution. They found 



