212 INJURY, RECOVERY, AND DEATH 



water. Janse 24 found that filaments of Spirogyra which 

 had been kept for a time in a solution of KN0 3 , gave a 

 test for N0 3 after being rinsed and caused to burst in 

 a solution of diphenylamine. It is evident that in his 

 method, there is serious risk of contamination by the 

 substance in and upon the cell wall (or between the cell 

 wall and the protoplasm). 



In order to avoid this difficulty, the writer has 

 employed the cells of a species of Nitella, some 

 of which reach a length of four or five inches and a diam- 

 eter of a thirty- second of an inch. 



Within the cell wall the protoplasm forms a thin 

 layer in which are imbedded the chlorophyll bodies. 

 Inside this layer of protoplasm is the large central 

 vacuole filled with cell sap. It is possible to obtain the 

 cell sap without contamination in various ways. The 

 writer has made use of the following methods: The 

 cells are placed for the desired length of time in a solu- 

 tion containing the substance whose penetration is to be 

 tested. They are removed, washed in running tap water 

 (followed in many cases by distilled water) and dried by 

 means of filter paper. The cells are so large and turgid, 

 that this manipulation presents no difficulty. A cell is 

 then placed on a piece of glass or filter paper and pierced 

 with the point of a capillary tube (which has been drawn 

 out to a fine tip). The cell sap is drawn up into the tube 

 (by capillary action) quite free from protoplasm or chlo- 

 roplasts. 25 Another method, which is preferable in many 

 cases, is to suspend the cell by a pair of forceps attached 

 to the upper end, cut off the lower end and bring it in 

 contact with a glass slide, and then grasp the upper end 



24 Janse (1887). 



K During the manipulation care should be taken to prevent the sap 

 from running out of the cell and coming in contact with its outer surface. 



