IV.] VARIOUS THEORIES, &c. 127 



The process may continue until the pressure inside 

 causes water to filter out through the wall. 



But there are two possible ways by which this ex- 

 filtration of the water under pressure might occur, 

 (i) By increasing the force which presses the water 

 out ; or (2) by diminishing the attractive forces which 

 retain the water, and if we can assume a regular 

 periodicity of either one of these, we can explain 

 root-pressure. 



As a first possibility, suppose a parenchyma-cell in 

 contact with the water of the soil on the one side, and 

 on the other (with the intermediation of other similarly 

 disposed cells) with a tracheide or vessel. Water is 

 absorbed, and the cell in question 'becomes turgid. 

 Then suppose one or both the following changes to 

 occur in the cell, due to forces liberated by respiration : 

 (i) the protoplasm as a whole contracts, and (2) the 

 particles nearest the vessel undergo some alteration 

 of position, of such a nature as to permit of filtration. 

 The consequence would be that after the tension due 

 to turgescence had reached a certain stage, the proto- 

 plasm by violent contraction drives the water for- 

 wards : restitution of the turgid condition would then 

 follow, and the process be repeated, and so on. 



As a second possibility, let us suppose that respira- 

 tion brings about changes in the substances which 



