562 The American Naturalist. [July, 
pass around the clog in the spirals by way of the unfilled pit- 
ted vessels and to enter the spirals once more farther up. 
Were this not so, i. e., were pitted vessels filled as readily, as 
quickly, and as fully as the spirals, we should have not the 
gradual wilt of leaf after leaf up and down the stem, but the 
sudden collapse of all the leaves beyond the original point of 
attack. This is exactly what does happen in watermelon 
vines attacked by Fusarium niveum, (for a brief account of this 
parasite see Proc. Am. Asso. Adv. Sci., Vol. 48, 1894, p. 289, and 
Ibid, Vol. 44, 1895, p. —_-) where the pitted vessels appear to 
fill with the fungus as soon, if not sooner, than the spirals. 
These two diseases of cucurbits are very interesting from 
a physiological standpoint, and both parasites lend themselves 
readily to infection experiments, their slightly different be- 
havior being, perhaps, accounted for by the fact that the fun- 
gus is strictly ærobic, while the bacillus is facultative anærobic. 
Whatever be thought of butter or gelatine, it certainly cannot 
be maintained that the mere presence of these parasites in 
the lumina of the vessels destroys the carrying capacity of the 
uninjured walls, and yet they act quite as effectually as gela- 
tine, paraffin, or cocoa butter plugs, causing, when they fill the 
vessels only incompletely, a flabbiness of the foliage, which is 
proportionate to the extent of the plugging and to the activity 
of the transpiration, and which may give place to complete 
turgor in periods when the transpiration is small (night, early 
morning, or damp days), and producing, when they com- 
pletely fill the lumina of the vessels, an entire collapse of the 
foliage, from which there is no recovery. In case of the cu- 
cumber this collapse takes place as soon as the spiral vessels 
leading into any petiole are filled by the bacillus. 
