Chemical induction of growth 



best concentration of approximately 1-5 p. p.m., is shown in such a way that 

 the structure may be related both to the increment of cell number in the 

 presence of coco-nut milk and also to the change in average cell size, obtained 

 by dividing the weight of the tissue by the number of cells it contained. 

 The comparisons between structure of the added substance and growth are 

 most easily made as follows. 



By comparison with the initial tissue, or the controls in a medium free of 

 the added substance, the relative increase due to the substance in either cell 

 number or cell size may be simply calculated. The first important conclusion, 

 evident from an inspection o^ Figure 4, is that the differing structures of the 

 substituted phenoxyacetic acids markedly determine whether the substance 

 acts predominantly on cell number, or on cell size. This is an important 

 result because it immediately establishes the usefulness of this general tech- 

 nique in further work of this sort. It is obviously advantageous to be able to 

 distinguish between the substances that stimvdate cells to grow mainly because 

 they stimulate division, and the substances which stimulate cells to grow 

 mainly by causing them to enlarge. Furthermore, some substances, for 

 example 2:4-D itself, have inherent in their molecules the ability to stimulate, 

 in balanced degree, both of these processes. 



Figure 4 shows clearly that to a greater or lesser degree all of the substances 

 stimulated growth by cell enlargement when added to potato explants 

 growing in basal medium plus coco-nut milk. However, the ability to cause 

 the tissue to respond by cell division was conspicuously lacking in the case of 

 that compound in which the number 4 ring-position remained unoccupied. 

 Scanning these data, preliminary as they may well appear to be, leads to the 

 following general conclusions : 



1 . For maximum ability to stimulate the growth of potato cells, which 

 require the presence in the medium of the growth factors present in coco-nut 

 milk, the aryloxyacetic acid used as a synergist shoidd have the number 4 

 position occupied by chlorine. This being so, other groups may be added in 

 the ring but cell division will still be stimulated. While there may be effects 

 due to the proximity of other groups in the 3 or 5 position, the main effect is 

 clear; namely, that it is the number 4 position that seems to hold the key 

 to the stimulation of growth by cell division in this system. 



2. In the absence of the 4 substituent, marked stimulation of growth by cell 

 enlargement may occur, but in all these cases the position that seems to hold 

 the key to the stimulation of growth by cell enlargement is the number 2 

 position. If the number 2 position is occupied, other groups may be inserted 

 in the nucleus, leaving number 4 unoccupied, and growth by enlargement 

 will still occur. 



3. Clearly then, the prevalent role of 2:4-D as a growth-stimulating 

 substance is susceptible to the following interpretation: One may assume 

 that the tissue has inherent factors essential for growth by cell division or 

 indeed cell enlargement, factors which are represented by the chemical 

 substances present in coco-nut milk. The action of such a substance as 

 2:4-D can be explained through its ability to accentuate markedly the 

 tendency to cell enlargement by virtue of the substituent in the 2-position and 

 it may also accentuate cell division, to which the tissue may itself be some- 

 what prone, by virtue of substitution in the 4-position. These results are 



175 



