1911] OVERTON—TRANSPIRATION AND SAP-FLOW 33 
‘Dixon (10-12) finds that water forced through a steamed por- 
tion of a stem is tinged with a brown substance, while water forced 
through an unsteamed or otherwise heated stem emerges as a clear 
liquid. He also performed experiments on cut branches of Syringa, 
placing some in water and others in a decoction of the same plant. 
The leaves on the stems set in water wilted in 5 days; those set in 
the decoction wilted in 3 days. Ursprunc, performing similar 
experiments with Impatiens and obtaining similar results, does 
not believe that the leaves fade because they are poisoned by the 
decoction, but that they fade because of a stoppage of the vessels 
at the cut end of the branch with a brown mass which interferes 
with the water supply. Among other experiments along this line, 
Drxon killed one arm of a bifurcated Syringa shoot with hot water. 
The leaves were then removed and pure water was supplied through 
it to the living branch, so that the leaves on it received their water 
supply through the main stem and through the dead branch. In 
spite of the ability of the leaves on the living branch to receive 
their water supply from the roots as well as through the dead branch, 
they soon showed signs of wilting, resembling the early stages of 
fading characteristic of the leaves above a heated portion. Drxon 
thinks that the leaves fade because substances are formed during 
the heating of the stem which are carried to the leaves and act 
injuriously upon them. He further concludes from his experi- 
mental evidence that the fading of the leaves on a heated stem is 
not the same as occurs normally when leaves are simply deprived 
of water. In leaves above a killed stretch he observed a contrac- 
tion of the protoplasts of the mesophyll cells and a discoloration 
of the chloroplasts in the areas which were turning color. Drxon 
holds, therefore, with VEsQuE (38, 39) that in one case the leaves 
die because they dry, while in the other case they dry because they 
die, which appears from my observations to be a true statement of 
the facts. 
RosHArpDT (24), proposing to study the problem of sap-flow as 
outlined by Ursprunc, has performed experiments on 131 species 
of 59 families of shrubs and herbaceous seed plants, by killing 
certain stretches of the petioles, stems, and branches with steam, 
ether, xylol, or low temperature, and determining what effect the ~ 
