Last Work. Final Honors 583 



closest in bactcria-frcc ci\)\\n-L;all tissue, "' and in L. M. Black's 

 " virus-tumour tissue." The *" virus wound tumour " i^Anwrican 

 Jourfhtl of Botiiny 32: 408, 19-1'S) is described as follows: 



Susceptible plants can be alfcctcJ with the wound tumour virus cither 

 throii'Ji ihc ar.cncy oi acalliaii leal hoppers or by bein^i; _L;r.'.lted to iiile. fed 

 plants. Infection is systematic and symptoms include enlargement of the 

 veins as well as the formation of tumours on roots and stems in many 

 species of infected plants. Most of these tumours appear to arise at points 

 where root or stem has been wounded. Black {Nature 158: 56, 19 16) 

 showed that the tumour tissue from sorrel could be cultured for as long as 

 twenty months on an artificial medium without the production of roots, 

 stems or leaves. 0n being grafted back to healthy plants, tissues from 

 these cultures induced a systemic infection in the stocks, showing that the 

 virus was still present. Such a tumour-producing virus has obvious resem- 

 blances to entities such as the Rous sarcoma or the Bittner milk factor. 



De Ropp's discussion of the work on bacteria-free crown gall 

 tissue included that of many workers since Smith's time: his own, 

 Braun and White, Riker, T. Laskaris, E. M. Hildebrand, A. C. 

 Hildebrandt, and others. The explanation of a tumefacient factor 

 and a growth hormone seems indicated from most of the studies. 

 However, it is said: 



If a virus is responsible for the formation of induced tumours, it is 

 probably of the type transmissible only by grafting, as extracts of tumour 

 tissue are without effect on normal tissue [White and Braun, Cd/zf^r 

 Research, op. cit.\ de Ropp, Bull. Torrey Bot. Club 75(1) : 45-50, 1948]. 

 The existence of such a bacteria-borne tumefacient factor has still not been 

 conclusively demonstrated. 



At the 1949 meeting of the American Association for the 

 Advancement of Science, Dr. Braun presented a paper on the 

 development and proliferation of plant cancers caused by crown 

 gall bacteria. In this he described experiments, the results of 

 which pointed to the conclusion that "it is before and during 

 early stages of active wound healing that normal cells are con- 

 verted into tumor cells." Two factors (among the other important 

 conclusions reached) were elaborated: first, a w^ound by which 

 the bacteria gain entrance to the plant and, second, a time factor 

 during which the " tumor-inducing principle," probably a hor- 

 mone, transforms the " normal plant cells to tumor cells." Because 

 of his studies " so well designed that they chart a basic course 



