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1911} OVERTON—TRANSPIRATION AND SAP-FLOW 39 
stoppage of the vessels, which in themselves would influence the 
passage of water even if the killed cells were otherwise capable 
of conduction. Both Jost and CzapEexk doubt whether all changes 
induced in the cells by heat can be seen with the microscope. The 
experiments of WEBER (40) on heated portions of Picea excelsa, 
in which he found that both boundaries of the treated region 
between the dead and living portions were unable to transmit water 
even under pressure, were confirmed and extended by JANSE (16). 
WEBER further detected a transparent substance in the vessels. 
The experiments tend to show that a stoppage of the vessels may — 
occur in heated stems, and also that a partial poisoning of the 
leaves follows the death of the cells of the stem, since withering 
of the leaves begins before stoppage of the flow can be observed, 
as has been shown by Dixon. 
In microscopically investigating the leaves of Populus, Tilia, 
Syringa, Salix, and Acer borne above a treated stretch, Drxon (10) 
found that the mesophyll and the walls of the tracheae of the leaves 
become discolored before wilting occurs, which led him to suspect 
that the leaves are dying from other causes than mere drying. 
RosHArpT contends, however, that the discoloration of the meso- 
phyll and veins is no more an indication of poisoning than it is of 
mere lack of water, and that the discoloration of leaves on a stem 
with a killed section is not nearly so general as Dixon implies. As 
further proof that injurious substances are caused by heating the 
_ stem, Drxon mentions instances of leaves wilting directly below 
the killed stretch. RosHarpr states that in all of the 800 species 
_ which he treated he could not find wilting in any of the leaves — 
__ below the killed portion. That substances can be forced downward 
_ by steaming the stem is shown by my observations on Cyperus. 
I have found that some vessels below the killed portions were often 
_ totally or partially filled with resinous substances, even as far as 
4 the roots. RosHaRpt, on the other hand, failed to observe such 
«4 stoppages either above or below the heated region. As further 
and the stem heated, numerous air bubbles are forced out of the 
cut end by the treatment. 
