60 Elementary Plant Physiology. 



by the exuding fluid at any time. Secure a section of glass 

 tubing of about the same diameter as that of the stem of a plant 

 to be tested. Cut off a piece by means of a blowpipe flame 

 about eight to ten centimeters in length, drawing out the tip to 

 a tapering point with a small opening. Now attach the large 

 end of the tube to the stump of some plant cut off a few cen- 

 timeters above the soil. The test may be made with any of the 

 species named in the previous experiment. The joints must be 

 made proof against a possible pressure of more than an atmos- 

 phere. To accomplish this, first fit a short section of rubber 

 tubing to the stump, drawing it down over it to a distance of 

 about two centimeters. Wrap a short length of copper wire 

 around the tube and stump, bring the ends together, and twist 

 tightly with a pair of small pliers. Thrust the glass tube down 

 into the rubber tube until it comes in contact with the stump. 

 Wire this fitting also. Seal the end of the glass tube by the 

 sudden application of a blowpipe flame to the thin tip. Ascer- 

 tain the exact length of the column of air in the closed tube 

 above the mark. As exudation proceeds, the air will be com- 

 pressed, and the amount of pressure may be found by comparing 

 the volume of the compressed air with its original volume, in 

 accordance with Boyle's law. The pressure varies inversely 

 with the volume of the air. The length of the tube may be taken 

 as an index of the volume. Thus if the air occupied a length 

 of tube equal to 8 cm. at the beginning of the test, and is 

 compressed to 6 cm., the pressure will be eight-sixths of an 

 atmosphere, or one and one-third atmospheres. If compression 

 continues until the air occupies a half of its former volume, the 

 pressure would be eight-fourths of an atmosphere, or two atmos- 

 pheres, a limit which will probably not be reached in the tests. 

 The above method is subject to many errors, one of which lies 

 in the presence of watery vapor in the enclosed air. 



An exact calibration of the pressure may be made by the fol- 

 lowing method : secure a long glass tube of about three or four 



