88 



PLANTS MAKE FOOD 



obtain water rapidly enough from the soil to make up for the loss 

 through the leaves. 



a 



Diagrams of a stoma, a, surface view of a closed stoma; b, the same stoma 

 opened. (After Hanson.) c, diagrams of a transverse section through a stoma, 

 dotted Hnes indicate the closed position of the guard cells, the heavy Hnes the 

 open condition. (After Schwendener.) 



Green Plants Food Makers. — We have previously stated 

 that green plants are the great food makers for themselves and 

 for animals. We are now ready to attack the problem of how 

 green plants make food. 



The Sun a Source of Energy. — We all know the sun is a source 

 of most of the energy that is released on this earth in the form of 

 heat or light. Every boy knows the power of a '' burning glass." 

 Solar engines have not come into any great use as yet, because 

 fuel is cheaper, but some day we undoubtedly will directly harness 

 the energy of the sun in everyday work. Actual experiments 

 have shown that vast amounts of energy are given to the earth. 

 When the sun is highest in the sky, energy equivalent to one hun- 

 dred horse power is received by a plot of land twenty-five by one 

 hundred feet, the size of a city lot. Plants receive and use much 

 of this energy by means of their leaves. 



Effect of Light on Plants. — In young plants which have been 

 grown in total darkness, no green color is found in either stems 

 or leaves, the latter often being reduced to mere scales. The 

 stems are long and more or less reclining. We can explain the 

 changed condition of the seedling grown in the dark only by as- 

 suming that light has some effect on the protoplasm of the seedling 

 and induces the growth of the green part of the plant. If seedlings 

 have been growing on a window sill, or where the light comes in 

 from one side, you have doubtless noticed that the stem and leaves 

 of the seedlings incline in the direction from which the li^ht comes. 



