EFFECT OF LIGHT ON PLANTS 



155 



out of the air, is another of the raw materials. This gas enters the 

 leaf through the stomata and thus comes in contact with the living 

 cells of the leaf which are the manufacturers. 



Demonstration 9. To show the effect of light on green leaves. 



Place oxalis or nasturtium plants near a window. After several days, 

 notice the position of the blades of the leaves. Notice also the leaf 

 stalks. Account for the position of the leaves and stems. 



Effect of light on plants. Evidently sunlight has something to 

 do with the life of a green plant ; for 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, as those of 

 a sprouting potato kept in 

 darkness. 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 growing on a window 

 sill, or where the light comes 

 in from one side, you have 

 doubtless noticed that the 

 stem grows towards the 

 source of light and the leaves 

 tend to arrange themselves 

 so as to receive as much 

 light as possible on their up- 

 per surfaces. The illustra- 

 tion here shows very plainly 

 the effect of light on a grow- 

 ing plant. A hole was cut in one end of a box and barriers were 

 erected in the interior of the box so that the seeds planted in the 



Wright Pierce 

 Explain why this plant has grown toward the right 

 of the box instead of the left. 



