446 BOTANICAL GAZETTE [| DECEMBER 
tion the changes eventually leading to the fall of the leaf were 
not checked at all. 
It was thought that possibly the fungus by its action might 
have produced chemical changes in the cell contents and that these 
changes alone were sufficient to produce the effects described. It 
was reasoned, however, that if there was anything in the nature 
of a ferment present it should be capable of propagating itself 
when brought in contact with the cells of healthy pine leaves. 
To obtain a definite answer to this question a series of experi- 
ments were made, which need not be entered upon in detail here. 
Suffice it to say that after more than two hundred trials with 
juices from diseased leaves rubbed on broken and unbroken tis- 
sues of healthy foliage and brought into contact with the healthy 
cells in other ways, not a single case of leaf casting or leaf yel- 
lowing occurred. 
It is unnecessary to go over the ground which led to the 
belief that the changes, as already described, were due to exces- 
sive loss of water. The large rifts in the cortical and mesophyll 
regions, the uninjured conductive tissue, and the fact that no 
cork layer, cutting off the injured portion of the leaf, was formed, 
all pointed to a derangement of the water supply. 
To obtain information upon this matter a number of experi- 
ments were made, some of which will now be described. The 
first experiments were with cobalt paper** to determine the rela- 
' tive evaporation of water from the parts affected by the fungus 
and the uninjured portions. The tests were made before and 
after the fungus had broken through, the usual method being 1° 
fasten strips of dry paper to the leaves by means of glass slides. 
It was found that before the fungus broke through the diseased 
areas lost less water than the healthy portion of the leaf. Ths 
was due to the permanent closing of the stomata over the dis- 
eased areas, owing to the action of the fungus on the adjacent 
tissue. The fact that the diseased spots lost less water than 
healthy portions of the leaf explains why such spots remain 
™ For a discussion of the cobalt method, see STAHL, Bot. Zeit 52: 117-145+ Bott 
reviewed in Bor. Gaz, a1 : 26-33. 1895. 
