SUNLIGHT NECESSARY FOR PLANT GROWTH 11 



Plants must manufacture carbohydrates not only for their 

 own use, but also for the use of animals. This process 

 cannot go on unless the leaves are exposed to the light. 

 The name photosynthesis, which is given to this process, 

 means that the work is done in the presence of light. In 

 the manufacture of carbohydrates the leaves use as raw 

 materials two substances which are very common in nature 

 and easily obtained by the plant. One of these substances 

 is water, which the plant roots absorb from the soil while 

 the other is carbon dioxide, sl gas which is present in small 

 quantities in the air but which is constantly being renewed, 

 so that there is always an abundance. Water is made up 

 of one part of oxygen and two parts of hydrogen, while 

 carbon dioxide consists of one part of carbon and two 

 parts of oxygen. These are the elements that enter into 

 the composition of a carbohydrate. In this photosyn- 

 thetic process the elements of the water and the carbon 

 dioxide are separated and in the presence of light are re- 

 combined to form carbohydrates. During this process 

 oxygen is given off from the leaf as a waste product. 

 Therefore, in the sunlight the leaf is constantly taking in 

 carbon dioxide and giving off oxygen. If an actively growing 

 water-plant is submerged in a glass of water and exposed 

 to bright light, bubbles of oxygen may be seen coming 

 from the plant and rising to the surface. Shading the 

 glass diminishes the rate at which the bubbles appear. 



