72 



BACTERIA IN RELATION TO PLANT DISEASES. 



channels occur in the bark has not been determined. The channels in the wood, which 

 probably begin with an occlusion of some of the spiral vessels, generally occupy but a very 

 small portion of the stem, although they are easily visible on cross-section even to the 

 naked eye as small brown specks and on longitudinal section as a dark line bordering the 

 pith and connecting the two tumors. 



In the Paris daisy attacked by Bacterium tumefaciens I have been able to obtain leaf- 

 tumors in a considerable portion of my inoculations by making a single needle-prick into 

 the soft young stem below the leaf, the infected needle being thrust into one of the three 

 leaf-traces. A primary tumor results at the point of inoculation on the stem, and some weeks 

 later a secondary one develops in the inner tissues and subsequently bursts through the 

 upper surface of the leaf, i. e., through the petiole or midrib. Sometimes there are a series 



of such internal secondary tumors (fig. 24). 

 In young daisy plants inoculated January 

 13, 191 1, upon the stems, secondary (inter- 

 nal) tumors appeared upon the leaves very 

 promptly and after 16 days were visible 7 to 

 10 cm. away from the primary tumor. The 

 movement in this case was upward and the 

 little tumors had not yet burst through the 

 midrib. Sections were cut and the internal 

 tumor tissues studied in these midribs, and 

 motile bacteria-like rods in small numbers 

 were seen inside the cells in the gall tissue. 

 The channel of infection in this case is 

 not so easily traced. There is no plain disor- 

 ganization of tissues, and no brown stain 

 such as occurs in the olive, and although I 

 have studied a good many sections I do not 

 yet know through what tissues the channel 

 of infection passes or whether there is any 

 definite channel. It is more likely that the 

 tumor tissue itself carries along the organism 

 within the rapidly dividing cells, there being 

 a chain of tumor cells all the way from the 

 primary to the secondary tumor. 

 Since the above paragraph was written additional observations have been made con- 

 firming this hypothesis. A strand of tumor tissue has been discovered connecting the 

 primary tumor with the secondary tumors. This is usually in the inner wood or at the 

 junction of pith and wood. Moreover, when the primary tumor is in the stem and the 

 secondary tumor is in the leaf', the latter tumor does not have the dorsi-ventral structure 

 of the leaf in which it is growing but rather the structure of an imperfect stem. In cross- 

 section it is circular or nearly so, and is composed of a pscudopith of tumor tissue beyond 

 which is xylem surrounded by cambium and what I take to be phloem. Often remnants of 

 the petiole or midrib adhere to the surface of the tumor as a shell or as wing margins, in 

 which one finds the unchanged dorsi-ventral structure of the leaf. The strand or wedge 

 of tumor tissue proceeding from the primary tumor grows through the inner tissues of the 

 stem, petiole, and leaf -ribs much like a foreign body, developing the secondary tumors from 

 its substance in places of least resistance, or where the food supply is most abundant. These 



Fig. 23 : 



*Fig. 23. Metastatic small tubercles (nearly erumpent) on midrib of olive leaves at xx. Inoculations were made 

 Nov. 26, 1907, on stems about 0.5 inch below nodes, and growths developed at these points: Subsequently, these leaf- 

 tubercles developed from wiUiin. Inoculated plants Nos. 424a and 412. Photographed Feb. 29, 1908. 



