24 CELLULAR 



takes place to an increasing degree through the 

 protoplasm and this increase can be measured. 



In order to determine what substances enter 

 under normal conditions we require accurate de- 

 terminations of actual penetration by direct 

 analysis of the contents of the cell. I have been 

 so fortunate as to find plants in which this is 

 possible. These have large multinucleate cells 

 from which the sap can be squeezed out without 

 danger of contamination and without any of the 

 handicaps which rendered previous attempts, 

 made with cells of ordinary size, unsatisfactory. 



The fresh water plant Nitella produces 

 multinucleate cells up to six inches in length and 

 a thirty-second of an inch in diameter. Within 

 the cellulose wall is a delicate layer of proto- 

 plasm containing nuclei and chloroplasts. The 

 chloroplasts have a regular arrangement which 

 is disturbed if the cell is slightly injured 3 at the 

 same time the chloroplasts become more opaque. 

 These are valuable criteria of normal condition. 



Within this layer of protoplasm is the large 

 central vacuole filled with sap in which float 

 globules of protein, the whole being as a rule 



