43 



transpiring freely and insufficiently supplied with water, althouyli at 

 the same time there is an abundance of moisture in the soil. 



The uniformity with which this fungus first seeks out the vessels of 

 the plant is very striking (PI. 1, 10, PI. IV, and PI. V, (i). This explains 

 the sudden wilt of tlie foliage. The water ducts are clogged to such an 

 extent that they can not function. That this wilt is attributable to 

 lack of transpiration water, brought about by partial or complete clog- 

 ging of the vessels, is shown by the fact that large plants which have 

 begun to wilt frequently recover for a day or two if a rain sets in and 

 the air continues moist. During such weather the progress of the dis- 

 ease in old vines is almost at a standstill, but it recommences when the 

 sun shines out and the moisture of the air is dissipated. The mechan- 

 ical nature of this obstruction is also shown by the fact that collapsed 

 leaves frequently recover their turgor on cutting stems above the fun- 

 gous plugs and plunging them into water, e. g., on cutting across the 

 second internode when the plugs are confined to the hypocotyl and tap- 

 root. It is also shown by the tiict that the terminal portion of shoots 

 which have collapsed and the basal vessels of which are plugged by the 

 fungus (as shown by the death of all the lower leaves and of all the 

 other branches) may survive for several days, if the weather is not too 

 hot and dry, provided they are attached to a large melon from which 

 they can draw a certain amount of water (PI. VII, -5, right-hand branch). 

 Finally, it is much easier for the fungus to plug all of the vessels of a 

 small plant than of a large one, since in the taproot it has to make but 

 a little growth through tender tissues to accomplish this. This ex- 

 plains why yoiiufi vines freiiuently wilt even during rains or when the soil 

 and air are very moist. The leaves of the cowpea usually unjoint and 

 fall off, leaving the green stems bare. Often some of them become 

 yellow and fall without first showing signs of wilt, as Professor Atkin- 

 son has recorded in case of the cotton disease. The watermelon leaves 

 do not become yellow or detach, but wilt suddenly in large numbers 

 and shrivel, so that a large, healthy-looking vine may lose all of its 

 foliage in twenty-four to forty-eight hours (contrast PI. VI and PI. VII). 

 The cotton plant is less susceptible, contains, as a rule, less of the fun- 

 gus, and often recovers partially, so as to produce some fruit. In such 

 instances the fruit-bearing stalks push out of the base of the stem and 

 finally hide more or less completely the main shoot which has been 

 killed by the wilt. 



The xylem of the diseased plants always becomes brown (PI. 1, 10 a, b), 

 and in case of the translucent stem of the cowpea this stain shows 

 through the green bark, giving an unusually dark appearance to the 

 still living stem. This browning of the xylem appears to be common 

 to all plants attacked by parasitic soil Fusaria, e. g., cotton, okra, cow- 

 pea, beans, watermelon, squash, potato, tomato, eggplant, red pepper, 

 sweet potato, cabbage, carnation, asparagus, pineapple, and others. 

 This browning begins in the walls of the larger vessels, and often it is 

 confined quite exclusively to the xylem for a long time, the pith, bark, 



