470 PLANT GROWTH SUBSTANCES 



detached from the tissue. They require thiamin.* This suggests that the 

 growth requirements for cambial meristem are different from those of 

 the apical root meristem. 



When we include a consideration of the vitamin requirements of the 

 meristems of such tissues as plant tumors, it seems likely that the 

 metabolism of various meristems in or from the same plant is not 

 identical. This might be assumed from the differences in the cellular 

 elements and tissues formed by the various types of meristem. However, 

 the determination of the growth requirements of these meristems defines 

 some of the specific differences in their metabolism. How these dif- 

 ferences come about, what elements in the cells are responsible for them, 

 their implications for differentiation or morphogenesis and for abnormal 

 growth, are questions for further research. 



Methods of Investigation. — Methods of investigation in this field are 

 exacting and the interpretation of results requires an appreciation of 

 certain general concepts. Some considerations which should be borne 

 in mind are as follows: 



(i) Species differ in their requirements. Clones of the same species 

 and even meristems from the same plant may exhibit different responses. 



(2) Vitamin deficiencies may be complete or partial, single or multiple, 

 absolute or conditioned, permanent or temporary. 



(3) Because of the relation of microorganisms to vitamins, experiments 

 under nonsterile conditions must be reviewed with caution. When vita- 

 mins are added to soil or sand cultures, effects noted on higher plants 

 may be the result of their action on the microflora. 



(4) Reserves of vitamin in the seed or excised portion of a plant may 

 compensate for a deficiency in the medium. Successive passages in a 

 vitamin-free medium are advisable. White (60) has suggested ten passages 

 as a criterion for determining potentially unlimited growth. 



(5) In experiments involving vitamins, the basal medium should be 

 adequate in all other respects and approach as nearly as possible that 

 most suitable for the organism in question. 



(6) Environmental conditions, including hydrion concentration, tem- 

 perature, and salt concentration, may affect vitamin requirements. 



(7) Because of the minute amounts of vitamins which are effective 

 and because of their wide distribution in products of natural origin, 



*The difference in the requirements of excised fibrous carrot roots as observed 

 by Bonner and Nobecourt may be due to tlie use of different varieties of carrot. 



