THE SOIL. 71 



the passage of the molecules of liquids and dissolved solids 

 through membranes, and before any such movement can 

 go on it is necessary that the two liquids or solutions 

 concerned be able to mix with one another, and that the 

 membrane be permeable to at least one of the liquids. 

 And now to apply this to the case of the root-hair in 

 the soil. 



Under normal conditions the cell -sap is a stronger solu- 

 tion than the soil water, and so more water enters the cell 

 than leaves it ; that is, there is an inward flow. As the 

 substances dissolved in the soil water are on the whole 

 readily diffusible, these pass in with the water and so the 

 plant obtains the mineral matters necessary for its growth. 

 The water and dissolved matters so absorbed are passed 

 up through the plant to the actively growing points, and 

 what water is not built up into the body of the plant 

 finally leaves it as transpiration water. As the plant is 

 steadily losing water in this way, and the substances dis- 

 solved in the cell -sap do not readily diffuse out into the 

 soil, it follows that the cell- sap will continue to be a 

 stronger solution than the soil water and the inward flow 

 of water will be maintained. 



Many of the soluble salts passing into the plant are 

 there changed into other forms which do not readily 

 diffuse out again, and so the original substances continue 

 to enter. Should, however, a substance be taken up and 

 remain unchanged, then after a time its accumulation in 

 the cell-sap would result in the se'tting up of an outward 

 diffusion which would eventually balance the inward one. 



Although, as already stated, the dissolved matters of 



