EFFECT OF LIGHT ON PLANTS 



79 



prevent evaporation, the absence of leaves, as in the cactus, and 

 the turning of leaves edgewise to light are all modifications which 

 help to hold water in the body of the plant. 



Green Plants Food Makers. — We have already seen that green 

 plants are the great food makers for themselves and for animals. 

 We are now ready to learn how green plants make food. 



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

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

 heat and light. Every one 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 shall harness the 

 energy of the sun in everyday work. Experiments have shown 

 that as much as 80 per cent of the radiant energy falling on 

 certain green leaves is absorbed. Part of this energy is used by 

 the leaf ; but part is changed to heat, raises the temperature of 

 the leaf, and is lost to the air if the air is cooler than the leaf. 

 Regulation of this temperature is obtained in much the same way 

 as in our own bodies, by 

 evaporation of water. We 

 sweat; the leaf passes off 

 water vapor, largely through 

 the stomata. 



Effect of Light on Plants. 

 — In young plants which 

 have been grown in total dark- 

 ness, 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. 



Two stages in an experiment to show that 

 green plants grow toward the light. 



We can explain the changed 

 condition of the seedling grown in the dark only by assuming that 

 lack of light has some effect on the protoplasm of the seedling 

 and induces the growth of the stem. If seedlings have been grow- 

 ing on a window sill, or where the light comes in from one side, 

 you have doubtless noticed that the stem grows toward the source 

 of light and the leaves tend to arrange themselves so as to receive 

 as much light as possible on their upper surfaces. The experiment 

 pictured shows the effect of light very plainly. A hole was cut in 



