8o 



Journal of Agricultural Research 



Vol. VI, No. 2 



been observed in serial paraffin sections. Within three to five days after 

 inoculation the disease is evident in the form of oily or watery spots. 

 Within another week with favorable weather conditions and on young 

 leaves the epidermis will have ruptured on one or both surfaces and open 

 cankers will have formed. At this stage, before the exposed cells have 

 become desiccated, the greatest danger of spreading the infection exists. 

 Young tender tissues seem to be more susceptible to infection at this time 

 than mature tissues. The disease progresses more 

 rapidly, too, in young tissues than in older parts. 



RELATION BETWEEN PARASITE AND HOST 



No Study has been made other than the prelimi- 

 nary account of Hasse (6) of the effects of Psevdo- 

 monas citri on Citrus tissues. She states (p. 98) 

 that— 



There is a rapid development of cells, and the tension re- 

 sulting from the abnormal growth quickly ruptures the 

 epidermis. The cells are found to be filled with short rod 

 bacteria. All the cells exhibit more or less enlargement. 

 In later stages in the development of the canker some of the 

 cells disintegrate, and lesions are formed. The organism 

 appears to act more vigorously on the cell contents than on 

 the cell walls, and in due time the cell contents are exhausted. 

 The cell walls which remain become suberized. 



This problem was first attacked by making a 

 histological study of the diseased tissues. For this 

 purpose cankers in various stages of development 

 on fruits and leaves were cut out so as to include 

 some of the surrounding healthy tissue. Cankers 

 which had developed under conditions of very high 

 relative humidity (PI. IX, fig. 3, and PI. X, fig. 4) 

 and which were consequently of the spongy type and 

 white in color yielded especially interesting results. 

 This white color is due to the presence of air between the cells and can be 

 made to disappear if the cankers are immersed in water. These excised 

 cankers were then killed in strong alcohol, embedded in paraffin, sec- 

 tioned, and stained with carbol fuchsin. This stain renders the bacteria 

 bright red, making it easily possible to determine their position within 

 the tissues. 



Contrary to Hasse's observation, the bacteria teem around and 

 between the host cells, being present in especially large numbers in 

 the intercellular spaces (fig. 5). When the organism occurs within the 



Fig. 4. — Early stage of Cit- 

 rus canker in cross section 

 on a young leaf of seedling 

 grapefruit. The leaf was 

 inoculated by immersion 

 in a suspension of Pseu- 

 dorrwnas citri from pure 

 culture. The material was 

 collected 72 hours after in- 

 oculation. It was then 

 killed in strong alcohol, 

 embedded in paraffin, sec- 

 tioned, and stained with 

 carbol fuchsin. The or- 

 ganism entered the leaf 

 through the stoma, multi- 

 plied in the substomatal 

 chamber, and spread to 

 adjacent intercellular 

 spaces. Drawing made 

 with a camera lucida. 

 X600. 



