PROBLEM 2. The Part Stems and Roots Play in Making Food 



soil water enter the cell? First let us con- 

 sider the water itself; later we can con- 

 sider the dissolved salts (mineral matter). 

 In diffusion the movement of each sub- 

 stance is independent of the movement 

 of every other. The root hair is com- 

 pletely surrounded by a membrane. The 

 vacuole within the root hair contains 

 cell sap which is a solution of various 

 substances in water. Because of the many 

 compounds dissolved in the cell sap the 

 concentration of water is relatively low. 

 The soil water on the other side of the 

 membrane is normally a weak solution 

 of minerals; it has few minerals and a 

 relatively large percentage of water. In 

 other words, the weak solution has a 

 crreater concentration of water molecules; 

 it may be 97 per cent water as compared 

 to 80 per cent water making up the cell 

 sap. As a result, water will pass into the 

 root hair. 



If the soil water happens to be a very 

 concentrated solution (have a large per 

 cent of mineral matter and a compar- leaving the water concentration about 



151 



able to pass through the membrane. The 

 protoplasm lining the cell wall is perme- 

 able to them. Each mineral passes inde- 

 pendently of the others. When a mineral 

 in the soil water is present in higher con- 

 centration outside than in the cell sap, 

 it passes from the soil into the root hair. 

 But the movements of minerals are not 

 always simple; electrical charges and 

 other factors play an important part. 

 There is much about their movement 

 that cannot be explained here and also 

 much that is not understood. 



What happens to the compounds once 

 they are in? As more and more water en- 

 ters the root hair, you would expect the 

 cell sap to become a weaker and weaker 

 solution. The water concentration in- 

 side, therefore, would gradually become 

 greater and would soon equal the con- 

 centration of the water outside. But this 

 does not happen because the water that 

 enters does not remain in the root hair 

 cell. It diffuses to a neighboring cell 



ativeh' low per cent of water), water 

 will diffuse out of the cell; the plant 

 would lose water through its roots and 

 dry up in consequence. Diffusion of 

 water from the plant takes place when 

 large amounts of salt are sprinkled on the 

 ground. This is sometimes done to kill 

 unwanted plants but it should not be 

 done if the same soil is to be used for 

 other plants. Under normal conditions, 

 however, water does not diffuse from the 

 root to the soil; it goes from soil to root. 

 We are now ready for the second 

 question. How do the nitrates, sulfates, 

 phosphates, and other mineral com- 

 pounds enter the root hair? They are 

 dissolved in the water of the soil and are 



as it was before diffusion began. Thus 

 water keeps diffusing into the plant. 



The same is true of some mineral sub- 

 stances; they do not become concen- 

 trated in the root hair. They pass right 

 on to the next cell toward the interior 

 of the root. In this way, both water and 

 minerals pass inward from cell to cell. 



What is the structure of a root? By 

 making sections through a carrot, as de- 

 scribed in Exercise 6, you can learn the 

 general structure of a root. In almost 

 all young roots the tissues are arranged 

 in three distinct cylinders. The outer- 

 most cylinder of one layer of cells is the 

 epidermis; inside of this lies the cortex, 

 several to many cells in thickness; in the 



