28o BACTERIA IN RELATION TO PLANT DISEASES. 



which did not develop secondary signs were still living (Nos. 357, 368 and 372) ; one was dry 

 (374) but bore a green healthy fruit, and of the fifth (370) no record was made later than 

 August 28. The first case (No. 368) appeared on July 20, at 2 p.m., i. e., at the end of 

 the fourth day. Eight cases developed at the end of the fifth day (Nos. 355, 357, 359, 366, 

 369, 370, 374 and 375). One case (372) appeared after about 5I days.. Bight cases developed 

 at the end of the seventh day (Nos. 356, 358, 361, 362, 373, 376, 377 and 378). Four cases 

 appeared at the end of the ninth day (Nos. 360, 363, 365 and 371), and one plant (364) 

 came down the tenth or eleventh day. The last plant to become diseased was No. 367. 

 This did not show any signs until the thirteenth day. With the exception of the stems of 

 355 and 366, which were beginning to shrivel, all of the stems were turgid and sound 

 externally until after August 8. Throughout, the plants were free from insects. 



The plants were otherwise very healthy, were watered regularly and properly cared 

 for, and the signs obtained were characteristic of the disease. They could be ascribed only 

 to the initial bacterial infection of July 16. In many cases the removal of the inoculated 

 leaf was delayed so long after the first signs appeared (several days) that we can not tell 

 therefrom how short a number of days is requisite for the general infection of the plant when 

 the bacteria are introduced into the blades of the leaves. A few, however, give us some 

 basis for judgment. In vine 355, at the end of 7 days, the bacteria were 7 inches in advance 

 of the signs of wilt, i. e., they had already traversed the vessels a distance of 9 inches from 

 the point of inoculation (how much farther we can not tell) and had entered the stem, as 

 shown by the subsequent behavior of the plant. In 356, at the end of 1 1 days, the bacteria 

 were at least 9 inches in advance of the signs of wilt and had passed through the spiral 

 vessels of the leaf a distance of 10 inches (or more) into the stem, as shown by the subsequent 

 behavior of the plant. In 359, at the end of 5 days, as shown by subsequent signs, the 

 bacteria had entered the stem, having passed through 8 inches of vascular system and 

 being that much at least in advance of the signs of wilt. In 369, in 5 days, as shown by 

 subsequent signs, the bacteria passed through 9 inches of tissue (vascular system) and entered 

 the stem. In 372, on the contrary, at the end of 7 days, the bacteria had not yet entered 

 the stem (9.5 inches distant), although at the time the leaf was cut away there were 15 sq. 

 cm. of wilted tissue, reaching nearly to the middle of the blade, and signs had been present 

 for at least 26 hours. From these results we may conclude that under favorable circum- 

 stances infection of the main axis in the cucumber is comparatively prompt, the bacteria 

 being able to pass down through the vessels of the leaf at the rate of about 0.75-inch to 

 2 inches (2 to 5 cm.) a day. 



Weather conditions have much to do with the rate of progress of this disease. Cool 

 weather retards it, warm weather hastens it, extremely hot weather if long continued 

 checks it altogether. 



Following these inoculations, daily weather records do not appear to have been kept, 

 at least on the sheets bearing my pathological memoranda, but there are occasional refer- 

 ences to the weather. It was .cool on July 23, and cool with heavy rains on July 24. This 

 weather temporarily checked the progress of the disease. It was hot on July 25, and on 

 July 29 and 30. It was cooler on July 31, but windy so that transpiration would be rapid. 

 It was very hot on August 5 to 10. On the unrecorded days there was probably the ordinary 

 summer weather. 



Between August 5 and 8 numerous freshly wilting leaves (secondary wilt) were cut 

 from these plants and fixed in strong alcohol to determine whether the bacteria are actually 

 in the vessels of the leaf at the time the secondary wilt appears or whether this wilt is due 

 simply to the plugging of the vessels of the stem. These leaves were well grown and sound 

 externally. Thin microtome sections were made from the basal part of the petiole of 66 of 

 these leaves after infiltration with paraffin. These sections were fixed on clean slides and 

 carefully examined after removal of the paraffin and staining in carbol fuchsin. Bacteria 



