RIKER AND THOMAS 4^9 



off a series of events, we may wisely consider the changes in growth that 

 may arise if we have the factors necessary for normal growth in an 

 unbalanced combination. 



As already mentioned a considerable number of physical, chemical, 

 and biological factors may induce gall formation. The relatively non- 

 specific nature of this phenomenon is striking. From the evidence avail- 

 able, the bacteria in culture, bacteria in the tissue, and important growth 

 substances all seem associated with lower oxidation-reduction potentials 

 or reduced oxygen uptake. 



Other critical factors besides respiration may include changes in 

 osmotic pressure and surface tension as well as altered amounts of growth 

 substances, vitamins, enzymes, irritating substances, and food materials. 

 Any living cell, even a resting cell, that would fail to react when the 

 normal balance in such factors is disturbed would seem to be unresponsive 

 indeed. 



With the development of plant-tissue-culture techniques we have 

 improved means for determining the effects of known substances. A 

 trigger mechanism might be touched oft' in various ways. Perhaps of 

 more importance is the kind of a gun the trigger is on, what the source 

 of energy may be, how much is present, and what inhibiting and direc- 

 tional factors may opeiate. Tissue cultures help with such determinations. 



Much progress has already been made as discussed elsewhere in this 

 symposium. The work with mineral salts (6), with sources of nitrogen 

 (17), and with sources of carbon (5) all emphasize the importance not 

 only of the particular substance but also of its concentration to make 

 the proper balance in relation to other items. The encouragement or 

 inhibition by certain amounts of particular substances has sometimes 

 been quite conspicuous. 



Let us summarize. In normal growth a number of factors, including 

 growth substances, apparently operate in suitable balance. However, 

 if these factors are out of balance in one way we can expect pathological 

 growth of a certain kind. If these factors are out of balance in another 

 way we can expect diseased growth of a different kind. Whether it is 

 right or wrong, this point of view suggests many interesting experiments. 



REFERENCES 



I. Berge, T. O., Riker, a. J., and Baldwin, I. L., Phytopathology, 26:86 

 (1936). 



