98 Journal of Agricultural Research voi. iv, no. i 



necessary, as infections may be obtained without this procedure. As 

 soon as the plants were inoculated they were placed under bell jars and 

 kept at a temperature of about 86° F. Under these conditions the 

 organism takes a vigorous hold on its host, and in three or four days evi- 

 dences of infection can be noted. At the end of a week definite, well- 

 defined cankers which penetrate the tissue of the leaf have been formed. 

 Owing to the stimulating influence which the organism has upon the 

 infected leaf tissue, there is a rapid development of cells, and the tension 

 resulting from the abnormal growth quickly ruptures the epidermis and 

 exposes the soft, spongy, underlying canker tissue, which is distinctly 

 visible on both sides of the leaf. The cankers produced by artificial 

 inoculation present a characteristic appearance and closely resemble 

 natural cankers in macroscopic as well as in microscopic features. They 

 penetrate the tissue of the leaf and are more or less raised on both the 

 upper and the lower surface. The outline is circular, and there is a sharp, 

 distinct demarkation between the canker and the surrounding normal 

 leaf tissue. Young cankers have a soft, spongy structure and at first 

 show a light-green color, which later turns red-brown. The cells in the 

 canker tissue become suberized and produce a corky growth, which is a 

 symptom of the disease. This open, spongy type of canker is the result 

 of rapid growth due to favorable conditions of temperature and moisture. 



The identity of natural and artificial cankers is shown in Plate X. 

 Sections of cankers about 2 weeks old show the pathological and histo- 

 logical features obser\-ed in young natural infections. (See PI. IX, fig. i .) 

 The cells are found to be filled with short rod bacteria, and the stimulus 

 exerted by the organism on the infected tissue is distinctly visible. The 

 natural differentiation of palisade and parenchyma tissue has been 

 obliterated, and all the cells exhibit more or less enlargement and dis- 

 tortion, which is due to the activity of the invading organism. As a 

 result the diseased tissue of the canker is raised above the normal leaf 

 surface. 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 and constitute the corky cankerous growth which is a 

 characteristic symptom of this disease. Numerous cankers obtained 

 from pure-culture inoculations upon grapefruit seedlings are shown in 

 Plate IX, figures 3, 4, 5, 6. 



While the canker is still soft and young, the organism is in a very 

 active condition and can be isolated very readily. Upon teasing out a 

 small piece of canker tissue in a drop of sterile water, motile bacteria in 

 great numbers ooze out and give the water a milky, turbid appearance. 

 The motility of the organism can be most satisfactorily observed by 

 means of dark-field illumination. The organism was reisolated from 



