2IO 



MOVEMENTS AND EXCHANGES OF FLUIDS 



the same external diameter by means of a short section of rubber 

 tubing secured by wound and twisted wire. Invert the tube and 

 fill the tube with water which has been boiled, and use other pre- 

 cautions to exclude air. Place the finger over the end of the tube 

 and set upright in a small dish of mercury. As the shoot uses 

 water it will withdraw it from the tube and raise a column of mer- 

 cury. Note the height of the column at intervals of 12 hours 

 for a day or two. The lifting of the mercury will continue until 



Fig. 106. Potometer. A, base. B, reservoir for water. C, calibrated tube. D, 

 separatory funnel for water supply. E, fitting of plant and tube. 



its weight is sufficient to pull air down through the stomata and in- 

 tercellular spaces of the cortex into the tube, where it will gather at 

 the lower end of the branch and prevent absorption. It is thus to 

 be seen that the experiment does not measure the full lifting power 

 of transpiration, which often exceeds 6 to 8 atmospheres. The 

 greatest lifting power may be demonstrated in species furnished 

 most sparingly with intercellular air spaces. The action will be 

 found most rapid in Eucalyptus and similar forms, and most en- 

 during in stems furnished with cladodes, and phyllodes instead of 



