R. S. DE ROPP 



385 



which plant and microorganism are brought together. That strains of 

 crown-gall bacteria differ in tumefacient power has been known since the 

 time of Smith (39). Some strains exist which have lost their tumefacient 

 power completely, others generate small galls on susceptible plants, and 

 still others cause the formation of large rapidly growing galls. The work of 

 Riker and others (33,28) has shown that the tumefacient power of the 

 crown-gall organism can be eliminated by culturing it on a medium 

 containing certain amino acids, notably glycine. Bacteria grown on a 

 medium containing this substance lost their tumefacient power in from 

 10 to 20 transfers. The effect was reversible. 



Crown-gall bacteria, however virulent, will produce no overgrowths 

 unless brought into direct contact with wounded tissue. Riker (31) 

 showed that the size of the gall produced on tomato stems was propor- 

 tional to the area of waterlogged tissue resulting from the wound. He 

 found that tissue ceased to respond with tumor production five days 

 after it had been wounded. De Ropp (15) found that fragments of 

 sunflower stem cultured iti vitro also lost their capacity to respond 

 in about this time. E. M. Hildebrandt (22), using micrurgical methods, 

 showed that single bacteria would induce gall formation on stems of 

 tomato plants. The size of the gall was related to the depth of the wound 

 rather than to the number of bacteria introduced. When injected into 

 the living surface cells of the tomato stem no galls were initiated by the 

 bacteria, which seemed unable to survive in the intracellular environ- 

 ment. The work of Riker (31) and of Robinson and Walkden (36) sug- 

 gests that the organisms are inter- rather than intracellular parisites. 



Even when tissues are wounded they may still be unsusceptible to the 

 tumefacient effect of crown-gall organism. In a series of experiments 

 using slices of carrot roots inoculated with crown-gall bacteria, the 

 writer (r6) has shown that tumor formation occurs primarily along 

 the line of the cambium. The secondary xylem which forms the core of 

 the root is capable only of very bmited tumor production. The secondary 

 phloem outside the xylem responds rather more readily. The periderm 

 does not respond at all. It was also shown that, in any given batch of 

 carrot roots, grown under the same conditions and belonging to the same 

 variety, as many as 10 per cent are generally immune to the tumefacient 

 action of the crown-gall organism and a higher proportion are only 

 slightly susceptible. These variations in susceptibility may be due to 

 hereditary factors. It would probably be possible by selection to develop 



