Levine: Studies on Plant Cancers 



7 



We may turn now to consider the cases in which no evidence 

 of injurious effects of the gall in tissue above and below it in the 

 stem have yet appeared. In all, I have observed two such cases 

 as mentioned above against ten in which death of the region 

 above and below the gall or both occurred. 



It is natural to suspect in view of the statements of the authors 

 quoted that the injurious effects I have observed may be due to 

 the presence of some additional infection or to some special 

 direct physical effect of the crown gall on the rubber tree. I am 

 however convinced that this is not the case. 



As in the case of Bryophyllum (Levine 1919), Bacterium 

 tumefaciens does not cause the formation of embryomata when 

 inoculated into F. elastica in a region where embryonic cells are 

 to be expected. At the time this photograph was made, twelve 

 months after inoculation, the upper gall was still active although 

 parts of it were beginning to distintegrate. The stem above the 

 gall appears as noted to be entirely unaffected and in good 

 physiological condition. A cross section of the stem made at the 

 level indicated by the line " AA " shows complete disorganiza- 

 tion of more than one half of the wood cylinder. The remaining 

 half is not unlike the apparently healthy portion of the wood 

 shown in figure 4. A photograph of the cut end of the stem at 

 the level of " BB " (Fig. 8) is shown in figure 9. Here little of 

 the vascular cylinder appears to be invaded by the crown gall 

 tissue. At this level the great mass of the crown gall seems to 

 have developed from the cortical layer of the stem only and has 

 not, at this time, affected the central cylinder. 



The gall from which the branches " Y" and " Z " appear (Fig. 

 9) is unlike all other crown galls so far described in that almost 

 its entire surface is smooth and not tubercular; it is covered with 

 small brown corky patches. The lower left side of the gall in 

 the picture shows the typical crown gall convolutions. 



A section still higher up on the stem made at the level indi- 

 cated by the line " CC " cuts through this smooth gall at a point 

 near the origin of the branches "Y" and "Z" (see Fig. 10). 

 An abundance of milk comes from the entire surface above the 

 dark area of the crown gall shown in this figure. No invasion 



