82 ELEMENTS OF GENERAL SCIENCE 



If the sugar is not needed at once as food for the plant, 

 it may be converted into starch and stored in the leaf to 

 be used during the night, when the plant cannot manufac- 

 ture food. A part of the sugar is converted into cellulose, 

 which makes up the woody fibers which form the supporting 

 skeleton of the plant. Under the name carbohydrates it is 

 convenient to group sugar and such other' substances as 

 starch and cellulose, which are easily produced from sugar 

 by the plant. 



90. The waste material. The oxygen produced when sugar 

 is formed is gaseous oxygen exactly like that in the air, 

 and may pass out through the stomata into the air, where it 

 mingles with the other gases. It may be called the waste 

 material of the process of manufacture which we have just 

 been studying. Bubbles of oxygen may often be seen rising 

 from plants which are growing under water. It is escaping 

 from land plants in a similar way, though it is not visible 

 in the air. Some of this same oxygen, or other oxygen, may 

 be used by the plant in its respiration. 



91. The source of energy. A factory needs energy; and 

 our carbohydrate factory in the leaf must have energy to 

 operate it, Most factories that we are familiar with are 

 operated by energy derived from a waterfall, or from burn- 

 ing coal under the boilers of a steam engine, or by electric 

 power, but the leaf derives its energy from a source that is 

 different from all these and at first appears rather sur- 

 prising the sunlight. Light is as much a source of power 

 or form of energy as is electricity, heat, or falling water, but 

 men have not yet learned so well how to use it. We may 

 cause light to do work on a photographic plate, we know 

 that it will change the colors of the chemicals we use to dye 

 our fabrics, and we may cause it to do other kinds of work 

 for us, but we have not learned how to make it run our 

 factories. The leaf is able to use this energy of light in 

 the manufacture of carbohydrates. If there is no strong light 



