HOW ROOTS ABSORB FOOD 5 



and to secure plant food from the soil. The stem is to raise the 

 leaves up to the light and air. There are many forms of stems. 

 See the cross-section of a woody stem in figure 2. Many vines 

 bring their leaves up to the light by climbing on supports of vari- 

 ous kinds. Running vines extend the plant over larger areas to 

 seek more air and light. Trees and other plants with rigid stems 

 grow tall in competition with other plants near them. If they 

 grow in more open places with less competition, they may become 

 more branched and their arms often 

 reach horizontally a long distance 

 into the air. 



The purposes of leaves are (1) 

 to expose the green chlorophyll of 

 the plant to the sunlight, (2) to 

 "breathe" carbon dioxide and oxy- 

 gen, and (3) to manufacture starch 

 and sugar. The work of leaves has 

 already been described. 



Important Work of Roots and 

 Root-hairs. — The chief purpose of 

 plant roots is to take in food from „, heMt^d7"^ap n £ool^ «£i 

 the soil. The root-hairs are far the £ r oXr°ba?k° win6 layer; d ' inner bark: 

 most effective in this work. The 



plant food taken in by roots must be in solution in the films of 

 soil water which cling around the grains of soil. 



The root-hairs are very numerous on the smaller rootlets, 

 particularly on the young growth. They form a velvety covering 

 of the roots and greatly increase the absorbing surface. (Fig. 3.) 

 It is through the root-hairs that all, or nearly all of the food from 

 the soil is taken into the plant. 



How Roots Absorb Food. — Liquids may pass through a mem- 

 brane by a process called osmosis. When two liquids are separated 

 by a membrane they tend to trade places and mingle with each 

 other. The thinner liquid passes through the membrane faster 

 than the denser liquid. 



The soil water, containing some plant food in solution will thus 

 enter the plant through the root membrane. A little waste mate- 

 rial from the cell sap will escape at the same time. This trade of 

 the two liquids through the surface of the root is necessary for the 

 growth of the plant. If there be enough moisture in the soil the 

 flow inward far exceeds the out-go, and growth will be rapid. When 



