LABORATORY STUDIES 79 
with water has a strong lifting power, cover the end of a thistle 
tube tightly with a piece of bladder or fill the mouth with a 
tightly fitting thin layer of plaster of Paris. Invert the tube 
and fill completely with water that has been boiled to remove 
the air so that bubbles will not be produced in the tube. Invert 
again with one end of the tube in a dish of mercury. Wet the 
bladder or plaster of Paris plug externally. As evaporation 
progresses, the mercury will be drawn up into the tube until a 
point is reached where the pressure of air on the outside of the 
bladder or plaster of Paris is sufficient to force the water 
back out of it so that it is no longer wet. It then permits air 
to pass through rapidly and the mercury soon recedes to its | 
original level. Similarly, it is assumed that the 
evaporation of water from the wet cell walls into the 
intercellular spaces of the leaves exerts a strong lift- 
ing power on the water in the stem of the plant. 
This will be shown by the following experiment. 
(g) Cut a leafy twig and fasten it, without allow- 
ing the cut end to dry out, into a glass tube filled yo. 44. 
with water and with its lowerendinmercury. This dion eet. 
experiment, if successful, will also show a rise of mer- ment (/). 
cury in the glass tube as in the preceding one. 
(h) Place the cut end of a stem (preferably a herbaceous one) 
in a strong aqueous solution of safranin. After an hour or so, 
make cross-sections at various points. The colored solution 
will be found in the tracheary tissue (and after longer standing 
also in some of the immediately surrounding tissues, especially 
in wood fibers). 
(i) Place a branch which has been girdled (i.e. the bark 
removed to but not including any of the wood) with its lower 
end in water, the girdled area being protected from drying out 
by coating with grafting wax or paraffin. Compare with a 
similar branch not girdled. Take a third branch and through 
a small slit in the bark cut off the wood entirely with as little 
injury to the bark as possible. Place it in water like the other 
two. Note the differences in the rapidity of wilting in the 
different cases. 
(j) Take a potted plant, e.g. a geranium or begonia, and 
after watering it well, envelop the pot in a sheet of rubber, 
tying the rubber firmly about the stem of the plant. Instead 
of using the rubber, the outside of the pot and the top of the 
