2 26 BACTERIA IN RELATION TO PLANT DISEASES. 



gradually and on this date (December 10) its last tiny leaf was found shriveled and also the upper 

 2 or 3 inches of the stem. Thin sections from the hypocotyl, and first, third, fourth, and fifth 

 internodes, the last one of which was flabby, were examined under the microscope but there was not 

 a trace of the bacillus. 



(22.) Cucumber (check). Plant about 6 inches high, with two big leaves, two small immature 

 leaves, and two green cotyledons. The three largest leaves were sprayed on the under surface and 

 the bell-jar at once placed over the plant. The eighth day the vine was healthy and free from ants 

 and aphides. It had grown about 3 inches since inoculation. Two days later it was nearly as tall 

 again as when put under the bell-jar. The temperature in the hothouse that day (November 4th) 

 was 83 F. The twenty-third day the bell-jar was removed. The vine was somewhat spindling but 

 free from the disease. It was very warm in the hothouse when the bell-jar was removed, and part 

 of the leaves wilted, the rest soon following in spite of careful watering. As soon as the wilting became 

 noticeable the vine was repotted (November 18th) in a 4-inch pot and this probably hastened its 

 collapse. On the twenty-ninth day the leaves were all shriveled and the stem was beginning to 

 shrivel also. The latter was now cut at various heights, where it was still partly turgid, and exam- 

 ined microscopically. The vessels were found to be full of bacilli which varied greatly in size. 



(23.) Cucumber (colony of aphides). Plant about 6 inches high, bearing two good leaves (blades 

 about 2x2 inches), and two undeveloped leaves. The under surface of one of the larger leaves on 

 which 20 aphides had been colonized October 24 was sprayed with the bacterial fluid until it was wet 

 with mist and tiny drops. Fifty aphides, from a neighboring watermelon plant, were then placed on 

 the wet surface, along with those already there, and the bell-jar replaced. Four hours later many tiny 

 drops remained on the sprayed surface, one-fifth to one-sixth of its surface was still covered by these 

 drops. Two of the aphides had moved to the upper (dry) surface, and 5 had crawled to the under 

 surface of another leaf. The eighth day this vine was less healthy than the others. It had made 

 very little growth owing to the aphides which were clustered on the terminal portion, causing it to 

 be twisted and stunted. After removing the jar, to clean off the aphides and give fresh air, the plant 

 had a drooping aspect. The sprayed leaf especially and one other, lacked turgidity and on close 

 inspection I found that large areas of the leaves were pale green. Two days later, however, there was 

 no sign of the flabbiness. The top was badly bent, twisted and stunted by the punctures of the 

 aphides and the plant had elongated not more than an inch. The leaves, however, were turgid. 

 The lack of turgor the eighth day was due probably to leaving the plant uncovered too long, the 

 atmosphere under the bell-jar being nearly or quite saturated, while that of the hothouse was com- 

 paratively dry. The twenty-third day the bell-jar was removed permanently. The plant was 

 spindling. Six days later (November 23), the vine was in a very bad condition although it was im- 

 possible to tell by inspection whether or not this was due to the multiplication of the bacteria in its 

 vascular system. Three days later all the leaves on the growing tip had dried up and nothing was 

 living and turgid except the pale green stem. The final slow death of the leaves was probably attribu- 

 table to the direct effect of the aphides as the plant was badly dwarfed by their presence and had 

 made but little growth. No signs of the bacterial wilt appeared, and on examination of thin sections 

 I could find no bacilli in the vessels. They were entirely free from obstructions. 



Remarks. The manner of performing this experiment seemed to give every oppor- 

 tunity for infection through the ordinary stomata, since the under surface of the sprayed 

 leaves remained wet over night. The results, however, do not bear out this hypothesis. 

 This experiment also lends no support to the hypothesis that the bacterial wilt of Cucurbits 

 may be spread by aphides. The entire lower surface of one leaf was sprayed thoroughly 

 (wetted) with an infectious culture and the aphides which were colonized on it soon made 

 numerous punctures but the plant did not become affected. Only one of the four plants 

 contracted the disease and that was a check. The bacteria probably entered the latter 

 through some fissure or other injury rather than through the stomata. Otherwise it is 

 difficult to explain the immunity of the other three vines which were equally exposed to 

 stomatal infection. It is, however, not known positively that infection never takes place 

 by way of the stomata. This remains to be determined. Further experiments should be 

 made also with aphides. 



Inoculations of October 25, 1894. 



Ten old vines of ( 'ucitmis sativus, were inoculated in the hothouse from a potato-broth- 

 culture (tube 8, October 23) of the gray-white, wet-shining, motile Bacillus tracheiphilus. 

 The pricks were made with a sharp steel needle, in most instances on a single leaf-blade, but 



