OSMOSIS 105 



ture, rich in the dissolved substances which the green 

 leaves above it will build into food. How is this task to 

 be performed ? The root-hair has no mouth-like opening, 

 nor can it, like a bit of blotting paper or of sponge, absorb 

 all liquids that it touches. It is a living thing and it is 

 surrounded by a continuous wall made of a substance called 

 cellulose. Cellulose is the substance of which all cell walls 

 are composed when young, and of which many cell walls 

 are composed throughout their existence. 



Even with a microscope we can see no openings in that 

 cellulose wall. Yet evidently if the water is to get in at 

 all, it must get through that wall. Are there invisible 

 spaces between the particles which compose that wall? 

 Although no one has ever seen them, we believe that there 

 are such spaces. We are able to explain the entrance of 

 the water in no other way. We believe that the spaces 

 are large enough to admit molecules of water and other 

 molecules, still larger, which are in solution in the water. 



The root-hair has water in it from the first. As you 

 already know, the young plant cannot grow unless water 

 is present. Seeds will not sprout unless water is present. 

 Root-hairs cannot be formed unless there is water present 

 to fill them. This water inside the plant body has various 

 other substances in it. It is called sap, but sap is always 

 principally composed of water. 



Now the moisture just outside the root-hair is a solu- 

 tion. In it as solutes are those various soil substances 

 which the plant needs. As you know, the molecules of 

 these solutes tend to become equally distributed as far as 

 it is possible for them to travel through the water. Now 

 if the pores of the root-hair are large enough to admit 

 them, and the water inside is continuous with the water 



