82 



Journal of Agricultural Research 



Vol. VI. No. a 



had grown in the flasks for four weeks, the bouillon was filtered through 

 a Chamberland filter. This filtrate contained no living organisms, as 

 demonstrated by transfers of platinum loopfuls to agar plates, with no 

 growth on these plates after three days. When at the end of three days it 

 was known that the filtrate was sterile, fresh grapefruit leaves were intro- 

 duced into the filtrate. These leaves were sterilized, prior to their intro- 

 duction, by immersion for half a minute in i to i ,000 bichlorid of mercury 

 and by rinsing them subsequently in three changes of boiled tap water. 

 Negative evidence of the presence of living organisms in the filtrate con- 

 taining the grapefruit leaves was secured by agar plates made one week 

 after the introduction of the leaves into the filtrate. An examination 

 of the leaf tissues at the end of two weeks showed no evidence of dissolu- 

 tion of the middle lamellae. This was true in the case of the filtrate 

 obtained from both sets of the six original flasks. 



In another experiment Irish potatoes were cut into slices and placed 

 in moist chambers on moist filter paper. Pseudomonas citri was then 



Fig. 6. — Drawing of a stained section of a natural canker on grapefruit. 



transferred to these cut surfaces. Within a week hemispherical areas in 

 which the cells were easily separable one from the other had been formed 

 immediately beneath the colonies. That P. citri alone had caused this 

 condition was shown by the reisolation in pure culture of this organism 

 from the softened potato tissues. Because of this result, together with 

 the fact, previously indicated, that the cells of cankerous tissues are so 

 easily separable, and in spite of the negative evidence of enzym secretion 

 in bouillon culture, it is believed that pectinase is secreted by the parasite. 

 The fact of the increased size of cells of cankerous tissue in itself sup- 

 ports the hypothesis that there has been an increased osmotic pressure 

 within affected cells. Several facts contribute toward solving the question 

 of how this increased pressure is brought about. In the first place the cell 

 contents must manifestly be modified by the dissolution of the middle 

 lamellae, since there would be a tendency toward the establishment of equi- 

 librium between the solution between the cells and the cell sap. Again, 

 the growth of the organism between the cells with the consequent passage 



