CAUSES OF CONTINUED ENTRANCE in 



You will remember that we spoke of certain solutes 

 which tend to move out of the roots and into the soil water. 

 Evidently such solutes are more abundant in the plant 

 than they are outside of it. Evidently also they are the 

 products of such changes, caused by the protoplasm, as we 

 have just been considering. They are substances manu- 

 factured inside the plant. These outward-moving solutes 

 do not amount to much in volume, but they may amount 

 to a great deal in affecting the fertility of the soil. 



Think, then, of the plant at work. All its cells contain 

 water. A plant is composed of water far more than of 

 any other substance; much over half the weight of com- 

 mon plants is the weight of the water they contain. The 

 movement of substances from one part of the plant to 

 another is absolutely necessary to plant life, and it is the 

 water in plants which permits this movement. There are 

 two kinds of movements in the plant body: the mass 

 movement of water and solutes together which occurs in 

 the dead wood cells, and the osmotic movements which 

 occur in living cells. So we may picture to ourselves the 

 solutes constantly moving through the tissues in fulfill- 

 ment of osmotic laws, but always we must have in mind 

 that the protoplasts change, and as they change they 

 affect the osmotic movements. A kind of molecule which 

 may be able to enter a protoplast at one time may not 

 be able to enter it at another. 



In the preceding section reference was made to osmotic 

 pressure. It was said that when molecules of a solute 

 become equally abundant on both sides of a cell wall, a 

 sort of osmotic equilibrium is established. That is, the 

 osmotic pressure of that particular solute is equalized ; the 

 pressure is no longer " high " on one side of the wall and 



