SMITH— MECHANISM OF OVERGROWTH IN PLANTS. 441 



teins split, and very marked osmotic disturbances set up within 

 the mechanism of the dehcately balanced colloids of the cell upon 

 introduction of these dilute, non-plasmolyzing bacterial acids, alkalis 

 and other products, must be apparent to anyone who is at all familiar 

 with the colloidal chemistry of the cell ; and later, by means of phys- 

 ical chemistry, we ought to be able to determine at least some of the 

 physical-chemical steps in the process of the abnormal cell division 

 brought about by these disturbing substances. 



For the present I interpret the growth in crown gall as due 

 primarily to a physical cause, viz., to an increase in the osmotic 

 pressure due to the heaping up locally of various soluble substances 

 excreted by the bacteria as a result of their metabolism. This 

 would lead to a movement of equalization. Water containing dis- 

 solved food stuffs would move toward the tumor and the stimulating 

 acids and alkalies would move outward so that theoretically the 

 strongest tendency to overgrowth should occur in the periphery of 

 the tumor where, as a matter of fact, it does occur. Also in malig- 

 nant human tumors the growth is peripheral. IVJiy is it peripheral? 

 If this hypothesis is correct we ought to be able to detect at least 

 a slight difference between the concentration of salts in fluids on the 

 periphery of a tumor and in the normal tissues just beyond it. This, 

 I believe, could be determined best electrically, although, if the 

 difference is considerable, the coarser method of extraction of 

 the juice of tumors and of adjacent sound tissues and determina- 

 tion whether there is any depression of the freezing point in the 

 former might yield interesting results. One test made for me by 

 Mr. Rodney B. Harvey indicated that there is a concentration of 

 substances in the juice of daisy tumors, i. c, there was a lowering of 

 the freezing point, but no thorough study has been made. This I 

 contemplate taking up in conjunction with physicists of the Depart- 

 ment of Agriculture. 



The reason I have for thinking the phenomena of plant over- 

 growth is primarily physical is the fact that it can be obtained by a 

 great variety of substances not the products of parasites, anything 

 in fact, which disturbs tissue equilibriums without destroying cells, 

 seems to be capable of causing overgrowths, which cease, of course, 



