ROOTS AND THEIR FUNCTIONS 151 



that it is caused by protoplasmic activity of some sort is afforded by the 

 fact that it occurs only in cells supplied with oxygen and sugar (fuel) 

 and that it varies with the rate of oxidation in the cell. The physical 

 mechanism responsible for active absorption is not understood, but ob- 

 viously it involves the activities of the cell membrane, which, as one 

 authority says, "must be of a very peculiar nature." 



