386 PLANT GROWTH SUBSTANCES 



highly tumorous and nontumorous strains of carrots, as has been done 

 with mice. 



The temperature at which the tissues are held after inoculation with 

 crown-gall bacteria has an effect on tumor formation. Riker (32) found 

 that galls developed poorly on tomato plants kept at temperatures 

 between 28° and 30°C. and that none developed above 32°C., a finding 

 which was later confirmed by Braun (7). The tumefacient process 

 appears to take place within 36 to 48 hours from the time of introduction 

 of the bacteria (6). 



That plant hormones have an effect on the tumefacient process was 

 demonstrated by Riker (34) whose observations were further extended 

 by Braun and Laskaris (8). Tomato stems inoculated with an attenuated 

 strain of crown-gall organisms were found to generate galls if treated 

 at the same time with lAA in lanolin. This finding caused Braun and 

 Laskaris to suggest that tumefaction by crown-gall bacteria takes place 

 in two stages, the host cells being converted to tumor cells in the first 

 stage and stimulated to continued multiplication by a growth substance 

 in the second. 



Some of the most important studies on the physiology of crown-gall 

 tumor tissue were initiated in 191 8 by C. O. Jensen (24), who showed 

 that tumors from red beet could be transplanted to sugar beet or mangel 

 and that, under these conditions, they continued to grow as tumors. This 

 suggested a close analogy between the behavior of animal cancer tissue 

 and crown-gall tissue and gave support to E. F. Smith's contention 

 that crown gall is a plant cancer. The most significant part of Jensen's 

 work was overlooked at the time. It consisted in the observation that 

 crown-gall bacteria could not be isolated from tumors thus transplanted, 

 from which he concluded that the cells of the beet tissue, under the 

 influence of the bacteria, had developed abnormally increased pro- 

 liferated power which persisted independently of continued stimulation. 

 Smith had frequently observed that crown-gall bacteria were difficult 

 or impossible to isolate from galls on composites, but it was not until 

 1943 that Braun and White (5) proved that up to 96 per cent of the 

 secondary galls which form on infected sunflowers are devoid of crown- 

 gall bacteria. 



This discovery led to the isolation of bacteria-free crown-gall tumor 

 tissue from secondary galls on sunflower (45). This crown-gall tissue 

 grew indefinitely on a medium containing 2 per cent sucrose, mineral 



