PHTSIOLOGT 25 



in constant motion, flowing spirally up the cell 

 in one direction and down in the other, so that 

 the motion is opposite on the two sides. 



By cutting one end of the cell and applying 

 gentle pressure the sap can be squeezed out, free 

 from protoplasm or chloroplasts. In making 

 experiments the cell is placed in a solution of 

 the substance whose penetration we are studying, 

 removed after a suitable exposure, rinsed, and 

 dried on the surface by means of filter paper. 

 The sap may then be squeezed out without 

 danger of contamination, since it is not allowed 

 to come in contact with the surface except for an 

 instant in the immediate neighborhood of the 

 cut, and even this contact may be avoided by 

 piercing the cell with a fine glass capillary and 

 collecting the sap by means of gentle suction or 

 pressure. 



Analysis of the sap reveals a most interesting 

 series of facts. Nearly all of the substances con- 

 tained in it exist at a much higher concentration 

 than is found outside. There must be a mechan- 

 ism which traps these substances^ and causes them 

 to accumulate. In order to discover what this 



