1gtt] OVERTON—TRANSPIRATION AND SAP-FLOW 55 
fresh turgid leaves of this plant contain about 80 per cent of their 
dry weight of water, while drying leaves on steamed stems directly 
after treatment, that is, 2-3 days, contain only about 50 per cent. 
As further noted in the table, the drying and withering proceed 
gradually, the leaves first drooping at their bases. There can be no 
doubt that steaming a section of stem reduces the water supply to 
the leaves, and that they thus come to contain less water. Whether 
this is the cause of withering, however, is not altogether clear. 
It may well be that there is water enough to maintain turgidity, 
even though the total amount has been reduced. 
Dixon (9-12) concludes from experimental and _ histological 
evidence that the initial stages of fading are caused by a poisoning 
of the mesophyll cells of the leaves, while the final stages are accel- 
erated by a clogging of the walls and a stoppage of the lumina of 
the conducting tubes. Upon microscopical examination of the 
steamed region, no visible disorganization of the cells except in 
the peripheral parenchyma could be found. In these cells the 
protoplasts were collapsed and the chloroplasts were discolored. 
The vascular bundles and internal parenchyma appear normal. 
When such stems are cut off and set in eosin, however, the color 
does not appear alone in the bundles, but diffuses out into the sur- 
rounding tissue. Since in these experiments the stem has been 
confined in a sterile chamber, there is no chance for the action of 
bacteria, and I have failed to find bacteria present in the tissues 
after a period of 10 days. Although no visible disorganization 
in this region can be made out with the microscope, it does not follow 
that disorganization has not occurred, and that certain decom- 
position products may not be exuded into the transpiration stream 
and be carried to other parts of the plant. 
When steamed stems are split lengthwise, even without the aid 
of the microscope numerous dark streaks or lines in the tissues 
may be seen reaching as much as 15-20 cm. above the killed 
portion and often as far as the leaves. When the leaves are held 
up to the light, dark streaks can also be seen along some of the 
veins. I have studied both cross and longitudinal sections of the 
stems both above and below the steamed region. Under the 
microscope these streaks are seen to be vascular bundles, which | 
