PHTSIOLOGT 33 



These striking differences between two species 

 of Valonia raise the question whether it is justi- 

 fiable to assume, as is often done, that close 

 relationship involves similarity in chemical 

 composition and metabolic processes. The impli- 

 cations of this question are far-reaching. If it is 

 possible for nearly allied forms to differ so 

 profoundly, it is evident that we have as yet no 

 satisfactory conception of the nature of the 

 variables involved in differentiation or in the 

 development of specific characters. 



These differences raise another question of 

 considerable importance, whether sodium or 

 potassium predominates in the sap of plant or 

 animal cells. For the study of this question it is 

 desirable to employ cells whose sap can. be ob- 

 tained without alteration, as in the case of Va- 

 lonia and Nkella. Nitella from Woods Hole 

 shows approximately equal amounts of sodium 

 and potassium j in a species collected near Cam- 

 bridge there is a little more potassium than 

 sodium. Hoagland and Davis state that in Ni- 

 tella clavatay taken from pond water, the pro- 

 portion of potassium to sodium in the sap is as 



