COMPOSITION OF SOIL 137 



definite way in the stem ; that through these, in the inner bark of a 

 woody stem, food passes down from the leaves, while in the woody 

 stem inside, soil water passes up to the leaves. 



Again you might ask the question, how do solid foods pass 

 through such tiny tubes? Not in a solid state, but in the form 

 of dissolved food substances. We shall find that these foods are 

 actually digested or made soluble by means of certain peculiar 

 substances called enzymes which are found in the cells in different 

 parts of the plant. Some of these enzymes seem to have the 

 ability to change solid foods into soluble form, while others change 

 the soluble foods back into insoluble substances. In this way we 

 have starches, proteins, and oils stored in different parts of the 

 plant. It will be the purpose of this unit to explain to us these 

 various processes so that we may really understand how the plant 

 makes food, how it transports it, and how it stores its surplus 

 which is used for the benefit of the animal world, including man 

 himself. 



PROBLEM I. WHAT DO PLANTS TAKE FROM THE SOIL? 



Composition of soil. As any one knows, the soil is composed 

 of different substances in different localities. Contrast the black 

 soil of Minnesota or Illinois with the sandy soil of Maine or Cali- 

 fornia, or the red clay of Virginia. If we examine a small mass 

 of garden soil carefully, we find that it is composed of numerous 

 particles of varying size and weight. Between these particles, 

 if the soil is not caked and hard packed, we can find tiny spaces, 

 which are formed and enlarged when the soil is tilled. They 

 allow the air and water to penetrate into the ground. If we exam- 

 ine some soil under the microscope, we find considerable water 

 clinging to the particles, thus forming a delicate film around each 

 one. 



Under the microscope, also, most soils are seen to contain par- 

 ticles of different kinds. Some are tiny pieces of rock, like those 

 still being formed where solid rock is exposed to the weather. 

 Rain, cold, and ice, working alternately with heat, chip off pieces 

 of rock. These pieces in time may be worn smaller by the action 

 of winds, running water, and in some places by glaciers. These 



