RnsHARCH ON Pi ANT Tumors 499 



bacterial infLxtion. The jMant tuinDi of Bacterim)! tuniejac'tetis 

 was more than the wilt of cucurbits (^Bitcillus Irdcheil^hiliis) and 

 the softrot of potato {Biicillns phytophthorus) . A physical- 

 chemical action was necessary to explain tumors, and Smith 

 believed 



that any soluble substance whatsoever, except .1 killini;, a plasmolyzing, or 

 an oxygen-absorbing substance, if continually liberated in excess locally in 

 tissues would be competent to induce tumor formation. . . . [D]ilute 

 ammonia causes intumescences and [it also seemed] probable that ammonia 

 liberated within the cell in small quantities by the imprisoned bacteria must 

 be one of the causes of the excessive and abnormal cell proliferation in 

 crowngall. Probably amin compounds also help to determine it. Since an 

 acid and an alcohol are likewise produced by the crowngall bacteria and 

 this alcohol and this acid (as well as many other acids) in pure dilution 

 and also in combination with ammonia caused galls or intumescences in 

 my experiments; the acid (or acids), the alkalies, and the alcohol must, 

 I believe, act together in producing the tumors, and osniotically rather than 

 cheiuically. . . . [Cjhanges in stimulus can produce changes in struc- 

 ture. . . . (pp. 177, 183) 



Smith's study of " Mechanism of Tumor Growth in Crowngall " 

 must have been one of his papers of this period which prompted 

 Dr. Mayo to say to the American Medical Association: 



Tlie work of Erwin F. Smith on plant diseases is monumental, especially 

 his discoveries as to the cause of certain plant tumors which show again 

 that our bacteria and insect chemi[cals] are the prime offenders through 

 the development of their varying acids, which may be stimulating or des- 

 tructive to other cell life, causing tumors or decay. We should appreciate 

 and aid the work of these departments. 



Dr. MacCarty had kept Smith's work before the laboratory staff 

 members of the Mayo Clinic. In a letter of July 4, I916, he told 

 Smith he had had lantern slides made of Smith's photographic 

 illustrations. The impression made had been " very good indeed. 

 What you are doing with plants, I have been doing with animal 

 tissues," MacCarty wrote, " and I feel that we are presenting a 

 correct, modern and scientific conception of neoplasmata and 

 their biological position. ... I feel that we are on the right 

 track so far as the histogenesis of neoplasmata is concerned. I also 

 feel that you have made great strides in the field of etiology." 



The year 1916 had been a banner year for Smith in his research 

 work on crown gall. He apprised Thaxter of this in a letter of 

 October 21: 



