1893.] The Gases in Living Plants. 101 
nized, but was best demonstrated by von Héhnel in 1879, to 
whom it occurred to cut off stems under mercury. In doing so 
the mercury rose to a considerable height in the vessels of the 
stem, and as mercury is without capillarity, this can only be 
ascribed to the greater pressure of the outside air, or in other 
words, to a partial vacuum in the plant. 
An observation was made by Hales, whom I have mentioned 
so often, which we may use to illustrate how such a negative 
pressure, as it has been called, can be brought about. He cut 
off a branch, fastened an empty tube to the cut end, and 
plunged the other end of the tube into aliquid. He found that 
as evaporation of moisture from the leaves took place, the 
liquid was drawn up into the empty tube. This phenomenon 
can now be explained more satisfactorily than could be done 
at that early day. By evaporation the liquid water inside the 
plant escapes in the form of vapor, and the space it occupied is 
filled by the gases, thus rarifying them. This rarifaction 
may go on in uninjured plants until the internal pressure is 
greatly reduced. But in the experiment, the pressure is equal- 
ized by the rise of the liquid in the tube. A later modifica- 
tion of Hales’ experiment is to use a forked branch, place the 
cut end in water to give a continuous supply of moisture for 
transpiration, and attach the empty tube to one of the side 
forks of the stem, cut away for that purpose. 
PERMEABILITY OF TISSUES. 
It is self-evident that such condensation and rarifaction of 
the gases in the plant could not take place if the cell walls 
were readily permeable to gases. Thus it comes about that 
one of the most important topics in connection with the move- 
ment of gases in the plant, is the permeability of tissue walls 
of various kinds, and especially those constituting the surface 
covering of plants. 
I shail not attempt to conduct you through the tangle of. 
supposition and fact, errors in experiments, correct and incor- 
rect conclusions, and the general confusion which has come 
from the labors of physicists, chemists and botanists for the 
last twenty-five years, during which the subject has received 
