Apr. lo, 1916 



Citrus Canker 



81 



cells, one is led to conclude, since they appear to be confined to such cells, 

 that entrance was effected after some mechanical rupture of the host 

 cells. 



A microscopic examination of sections of young spongy cankers in 

 which there has been no desiccation from contact with the air shows that 

 the host cells are not killed at first. Instead, they are considerably 

 hypertrophied and become lightly attached to each other, as shown in 

 figures 6 and 7. In fact, if fresh cankers are cut off with a sharp razor 

 and mounted on a sHde in a drop of water, some of the host cells separate 

 intact and of their own accord from the mass of cankerous tissue. Little 

 if any hyperplasia is believed to occur. It is highly improbable that cell 

 division would occur in cells in which such profound changes Vv^ere taking 

 place. It is evident from figures 6 and 7 that the enlargement of cells 

 already present would account for the production of the cankerous tissues. 

 The same is believed to be true in Plate X, figure i , illustrating Hasse's 

 observations. It is 

 not clear, however, 

 from her explanatory 

 statement that 

 "there is a rapid de- 

 velopment of cells" 

 whether hypertrophy 

 or hyperplasia is 

 meant. Death of 

 cells in the later 

 stages of develop- 

 ment of canker is 

 probably caused by 

 drying (PI. IX, fig. 

 5). The dried canker- 

 ous tissues gradually 

 become suberized. 



To explain the enlargement of the cells and their separation from each 

 other, two hypotheses are advanced : First, the middle lamellae are dis- 

 solved by an enzym, pectinase, secreted by the bacteria; second, osmotic 

 pressure of the colloidal cell contents is modified so that the cells have a 

 greater affinity for water. Evidence in support of both hypotheses has 

 been secured which in part, at least, explains these interesting phenomena. 



An attempt was made to demonstrate the secretion of pectinase by 

 Pseudomonas citri by the following method: Six flasks of bouillon were 

 inoculated with pure cultures of the organism. It was realized that the 

 production of enzyms is largely dependent on the nature of the culture 

 medium and that pectinase might be formed only within the host tissues. 

 For this reason grapefruit leaves were placed in three of these flasks of 

 bouillon prior to their sterilization and inoculation. After the organism 



Fig. s. — Pseudomonas citri: (a). In the mesophyll tissue and (6) in the 

 palisade parenchyma. This material was fixed in strong alcohol, infil- 

 trated with paraflBn, sectioned, and stained with carbol fuchsin. Out- 

 lined with a camera lucida. 



