62 Elementary Plant Physiology. 



millimeters internal caliber. Carefully heat in colorless Bunsen 

 flame,- and bend in the form of two engaging U- tubes, as in Fig. 

 37. Close one end of the tube by fusing in the flame. Invert 

 and fill with mercury so that it will fill half of the closed arm and 

 stand at exactly the same level in the other arm of the U-tube. 

 Now fill the remainder of the free arm with distilled water, 

 and attach to the stump of the plant to be tested in the 

 manner described above. If the apparatus is properly set up, 

 no water will gain access to the air above the mercury in the 

 closed arm of the manometer, and this air will be under normal 

 atmospheric pressure. The exudation pressure of the plant will 

 be registered by the decreased volume of the enclosed air, and 

 may be calculated by Boyle's law, as above. 



54. Imbibition pressure. The energy of the surface 

 tension existing between the particles of a wall, a starch 

 granule, or other similar substance, and water, causes the liquid 

 to penetrate the particles of the solid substance, pushing them 

 apart and giving rise to the phenomena of swelling. The ex- 

 pansion of the solid is accompanied by a display of enormous 

 force, which may not be easily computed. It will be profit- 

 able, however, to note the results obtainable by some common 

 forms of apparatus. Secure a fruit jar with a screw top, and 

 make a hole in the center of the top sufficient to receive a 

 glass tube from which a manometer may be made. Bend the 

 glass tube into a closed-arm manometer, as in 53, and 

 thrust the open end through the hole in the lid. Fill the ma- 

 nometer with mercury and water, as in the preceding experi- 

 ment, using all the necessary precautions, and then thrust the 

 end of the manometer tube into a rubber bulb of a capacity of 

 about 100 cubic centimeters, which has been filled with water. 

 Bind this fitting tightly with copper wire twisted in place with 

 a pair of pliers. Hold the bulb in the center of the glass jar, 

 and pour peas (Pisum) or soja beans around it until the jar is 

 completely full, when the lid should be screwed on. A second 



