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 wiU 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 Uft- 

 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 fiUed fig. 41. 

 with water and with its lower end in mercury. This — Evapora- 



■ . p tion expen- 



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, ^nd 

 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 



