WILT OF CUCURBITS. 



251 



green cotyledon separated from the pricked leaf by a distance of 2 cm., was also wilted. At 4 p.m. 

 of the same day the vine was brought into the laboratory and photographed along with a healthy 

 vine of the same age (Vol. I, fig. 8). The first leaf below the pricked one was examined for bacilli 

 in the veins of the blade. They were present but not yet numerous. The leaf -blade had been wilted 

 rather more than 25 hours (samples were put into alcohol for the microtome). The blade of the 

 first leaf up which was normal at 3 p. m. the previous day but had wilted 

 either the morning of the seventh day or the previous night, was also 

 examined : The spiral vessels at the apex of the petiole contained numerous 

 bacilli. The veins of the blade were not examined but samples were put 

 into alcohol for sections. Bacteria were also found in the vessels of the 

 uppermost small leaf at the junction of blade and petiole. The petioles of 

 these leaves (pricked one included) were rigid and neither these nor the 

 stern had changed color. For the appearance of a single infected bundle 

 in cross-section see fig. 69. For appearance of a whole petiole in cross- 

 section with low magnification consult Vol. I, pi. 3. 



(151.) Cucumber var. White wonder (Cucumis sativus). Many pricks 

 were made in a basal lobe of a leaf about 2.25 inches across. The morning 

 of the fifth day the first slight trace of the wilt appeared in the pricked part 

 of the leaf. At 2 p. m. the wilted part was duller green and covered an area 

 of nearly i sq. cm. in the middle of the pricked part. The following day 

 (3 p. m.) there was only a small increase of the wilt: Not one-twentieth 

 of the leaf -blade was involved. The eighth day about one-fourth of the 

 pricked leaf was flabby and the middle of the diseased area was now brown 

 and dry. The following day the pricked side of the leaf was dry-shriveled 

 and the rest of the blade had wilted. The petiole and the other leaves were 

 turgid. The ninth day the first leaf down (3 cm. below) wilted and the 

 following day the first leaf up (2 cm.) drooped its blade. The second leaf 

 up was then turgid (9 a. m.), but at i p. m. its blade was drooping. The 

 petioles were still rigid except that of the first leaf down which was a very 

 slender one. The fifteenth day after inoculation all the leaf-blades were 

 wilted but the stern and all the petioles were green and turgid. Thin sec- 

 tions of the blade of the second leaf up were now examined under the micro- 

 scope and the bacilli were found to be plentiful in the spirals of the leaf- 

 blade and forming cavities around them. They were apparently not in the 

 green parenchyma-cells. Portions of the petiole of the pricked leaf were 

 put for 2 minutes into boiling absolute alcohol containing i per cent picric 

 acid and were then transferred to 75 per cent alcohol which was repeatedly 

 renewed, i. e., until all the picric acid was removed. Other portions were 

 fixed directly in 75 per cent alcohol. The first mentioned fixative gave the 

 best results. The petiole of the pricked leaf was also examined microscopi- 

 cally. The vessels were gorged with bacilli and the primary vessel-paren- 

 chyma was broken down. 



(152.) Winter Squash var. Sibley's or Pikes Peak (Cucurbita sp.). 

 Many pricks were made on a leaf 3 inches broad, on one side about midway 

 from the base to the apex of the blade. The ninth day there was no trace 

 of the wilt, nor did it appear later. 



( I 53-) Winter Squash var. Sibley's or Pikes Peak. Many pricks were 

 made on the apical part of a blade about 2 inches broad. 



There was no result from the inoculation. 



(154.) Winter Squash (same variety). Many pricks were made on one 

 side of the blade of a leaf about 2.5 inches broad. 



No result. 



m 



Fig. 68.* 



(155.) Winter Squash (same variety). This was growing in the same pot as 154. 

 were made on the apex of the blade of a leaf about 2.25 inches broad. 

 There was no result from the inoculation. 



Many pricks 



*Fic. 68. B, cross-section of a squash-leaf wilted by Bacillus tracheiphilus, showing that wilt of parenchyma is due 

 to cutting off water-supply rather than to actual occupation of parenchymatic tissues by the bacteria. At base is 

 a bundle destroyed by the bacteria. Beyond this is a long wilted area in which no bacteria occur. Only a portion of 

 this wilted area could be shown in the picture, the whole length being shown in fig. C; the portion represented in the 

 drawing corresponds to the black part of C. A, neighboring uncollapsed portion of the same leaf, the bacteria in this 

 being confined to a portion only of the vessels of the bundle. Slide 362-1, lower row, last section but one at the right. 

 Drawn with a Zeiss 8 mm. apochromatic objective, No. 12 eye-piece, and Abbe camera. 



