Chemical induction of growth 



In the first part it will be assumed that the cells are adequately furnished 

 with the media which endow them, to their maximum extent, with the 

 ability to grow by cell division. These media consist either of coco-nut milk 

 or of some equivalent extract obtained from immature corn (-^^fl) or Aescuhis, 

 etc., supplemented as necessary by casein hydrolysate. The investigations 

 will then concern the extent to which still further added substances either 

 stimulate or depress the ability of the tissue to respond to these materials. 

 In this connection the use of the potato tuber tissue is most effective 

 because it does not grow to any appreciable extent in the basal medium, 

 even though supplemented by the coco-nut milk medium, but it requires the 

 further addition of substances like 2:4-D that are synergistically active with, 

 and enable the tissue to respond to, coco-nut milk. In the second part of this 

 more chemical discussion some observations will be made on the known 

 chemical nature of those fractions of the coco-nut milk or similar extracts 

 that have proved susceptible of isolation and, to some extent, of chemical 

 identification. 



Part II: The Chemical Nature of the Growth-Promoting Substances in 

 Coco-nut Milk and Similar Fluids : The Present Position 



SUBSTANCES INVOLVED SYNERGISTICALLY WITH COCO-NUT MILK 

 IN A GROWTH RESPONSE OF POTATO TISSUE 



Much work has been done up to the present time on the effects of 2:4-D or 

 similar compounds in their synergistic effect, with coco-nut milk, on potato 

 tuber tissue. First, to summarize briefly the work that has been published. 

 A number of other substances have been tested and some of these have been 

 found to possess greater ability than 2:4-D to stimulate the growth of potato 

 tissue which is normally induced by the combination of 2:4-D and coco-nut 

 milk. This investigation was carried out jointly with the laboratory of 

 Dr. R. L. Wain. Though maximum activity occurs at different concentra- 

 tions for various substances, it is quite clear that some of these sub,stances are 

 more effective than is 2:4-D at its optimum concentration. This was particu- 

 larly true in the case of 1 :2: 3: 4-tetrahydro-l -naphthoic acid, as shown in 

 Table 1 of the paper on this subject (Shantz, Steward, Smith, and Wain, 

 1955). However, from the standpoint of the chemistry of this phenomenon, 

 the greatest interest attaches to investigations of those substances which are 

 active in this synergistic combination and which are optically active. It was 

 found in two examples of this kind, namely, a-(2-naphthoxy)propionic acid 

 and a-(2:4:5-trichlorophenoxy)propionic acid, that the ( + )-forms of these 

 compounds were active in this growth stimulation, while their respective ( — )- 

 forms were not. Furthermore it was found, in striking confirmation of work 

 previously published from Wain's laboratory (Smith, Wain, and Wightman, 

 1952), that the ( — )-form was not merely inactive but tended to suppress the 

 activity of the ( + )-form in mixtures of the two. The contrast between the 

 role of the optical enantiomorphs of the same substance clearly indicates that 

 these chemically induced growth phenomena require to be explained by 

 some very subtle properties of the causal molecules. 



173 



