6 PRINCIPLES OF FARM PRACTICE 



densities, like water and sugar solution, are separated by a 

 membrane through which water may freely pass. The water 

 flows toward the liquid of the greater density, as in the above 

 instance where it enters the bladder containing the sugar 

 solution, or denser liquid, causing it to rise in the tube. The 

 root-hair may be regarded as a small sac filled with a liquid 

 somewhat denser than water. When the root-hair comes in 

 contact with soil water some of the water passes into the 

 root-hair just as water would enter a bladder filled with 

 sugar solution. The water then proceeds from the root- 

 hair into the root, and thence into the various parts of the 

 plant. 



Plants need air. If a plant is deprived of air it soon dies. 

 Not only does the air furnish carbon dioxide for the starch- 

 making, but it also supplies oxygen in much the same way 

 as is done in our own bodies. Oxygen is needed by all parts 

 of the plant all the time. The parts of the plant above 

 ground are surrounded by air and have air currents freely 

 moving among them, so that they are at all times abundantly 

 supplied with oxygen. The roots, on the other hand, being 

 below ground and having only a small supply of air, may fail 

 to get sufficient oxygen or may fail to have the supply re- 

 newed often enough to remove injurious gases as they ac- 

 cumulate in the soil spaces. An example of the effect of 

 insufficient air on the growth of plants may be seen in the 

 low wet spots in a field where some crop such as corn is grow- 

 ing. Plants in these wet spots are generally undeveloped and 

 weak, and often die. This is because the water with which 

 the soil is saturated has cut off the air supply from the roots, 

 thus depriving them of oxygen. 



How to help plants grow. We have seen that plants 

 require sunlight, air, water, and certain substances dissolved 



