442 SMITH— MECHANISM OF OVERGROWTH IN PLANTS. 



as soon as the stimulus is exhausted. (See Mechanism of Tumor 

 Growth in Crown Gall, in Jour. Agric. Research, Jan. 29, 1917.) 



I have been asked in what way these overgrowths differ from the 

 ordinary healing of wounds. The growth while excessive is prob- 

 ably not fundamentally different from a wound reaction, but then, 

 for that matter, we may regard all tumors as so many efforts at 

 healing which come to naught because they are continually modified 

 and frustrated by the presence of a parasite, or in animal cancers, 

 let us say, since we do not know their cause, by an abnormal and 

 oft repeated stimulus of some sort, most easily explained in the 

 absence of exact data by the hypothesis of a parasite, especially 

 since the same phenomenon in plants can now be referred to a 

 definite microorganism. 



IV. The Kind of Tumor Depends on the Type of Cells 



Stimulated. 



The first crown galls I studied seemed to me to be overgrowths 

 of the conjunctive tissues and most of our many inoculations up 

 to the end of 1915 produced that type of tumor which corresponds, 

 I believe, to overgrowths of the connective tissue of animals and 

 which I have called plant sarcomas. 



We had found indeed, as early as 1908-9, and had produced 

 by bacterial inoculation, plant tumors bearing roots, but the full 

 meaning of this discovery, as related to cancer, did not occur to me 

 until early in 1916, when I found crown-gall tumors bearing leafy 

 shoots on some of our inoculated hothouse geraniums. Beginning 

 with this discovery I made numerous inoculations in the leaf axils 

 of various plants which resulted in the production of leafy tumors, 

 and subsequently I produced them freely on leaves and on cut 

 internodes where no buds occur normally. Tumors bearing roots 

 have also been produced by us on the top of plants, and in one cut 

 internode of tobacco I succeeded in producing a tumor which bore 

 flower buds. These perishable root-bearing and shoot-bearing 

 tumors I regard as plant embryomas and have so described them.^ 



These experiments render it probable that every growing organ 



^Journ. Cancer Research, April, 1916, p. 241. 



