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MECHANISM OF OVERGROWTH IN PLANTS. 

 By ERWIN F. smith. 



(Read April 13, 19 17.) 

 I. Introductory. 



For 12 years I have been an eager student of overgrowths in 

 plants, partly on account of agricultural phases of the problem 

 which are of economic importance but chiefly because they have 

 seemed to me to offer a clue which might lead to the solution of 

 the greater and very obscure problem of the origin of malignant 

 human and animal tumors. 



For a long time I have believed that the direct cause of these 

 plant tumors (of all malignant tumors for that matter) must be 

 chemical substances liberated in the tissues by parasites. It is not 

 a far cry to such a view, especially where parasites are known to 

 cause the overgrowth, and no doubt many other persons have held 

 the same view and have stated it more or less definitely. I ex- 

 pressed it clearly in 191 1 in our first crown gall bulletin (U. S. 

 Dept. of Agric, B. P. I., Bui. 213, p. 175). 



The difificulty has been to determine the nature of these chemical 

 substances. This is still unsolved so far as relates to the products 

 of gall-forming larvae of all kinds, and apparently must so remain 

 until they can be grown in quantity in pure culture so as to give to 

 the chemist an abundance of material for his studies. The chemist 

 is very greedy of material and without a great abundance he can 

 seldom accomplish much. Various gall-forming fungi and bacteria 

 offer easier problems because they can be cultivated in flasks on 

 simple culture media in any desired quantity and their products 

 determined with a minimum of labor. 



This, rather than the analysis of tumors, is, I am satisfied, the 

 proper method of procedure, because the cells of a tumor are only 

 the cells of a plant or animal grown under an abnormal stimulus, 



437 



PROC. AMER. PHIL. SOC, VOL. LVI, CC, AUGUST 5, I9I7. 



