NUTRITION, 141 



and storing the accumulated food for a time. In all green 

 plants oxygen is a by-product. The amount given off about 

 equals in volume the carbon dioxid used in making the 

 foods. 



The condftions under which photosynthesis occurs are 

 three : {a) the presence of chlorophyll, (3) the action of 

 light, and (c) the presence of potassium salts. 



EXERCISE XXXI. 



Demonstration To show that oxygen is a by-product of photosynthesis. 



Collect the gas mixture evolved from a vessel full of aquatic plants by 

 inverting over them a funnel to whose tip is connected a test-tube filled 

 virith water to be displaced by the rising gases. Keep the plants in sun- 

 light. When the tube is filled, test the contents for oxygen by inserting 

 a glowing splinter. 



191. {a) Chlorophyll. — Chlorophyll, as has been shovi^n 

 in Part I, sometimes colors the w^hole protoplasm of the cell, 

 but is more commonly found only in certain special structures, 

 the chloroplasts. The real work of forming the food de- 

 pends, therefore, upon the protoplasm of the chloroplast. 

 The purpose of the chlorophyll is to absorb certain portions 

 of the light which falls upon it. 



192. ip) Light. — The light absorbed by the chlorophyll 

 furnishes the energy necessary to do the work of taking apart 

 the carbonic acid and rearranging the material into a more 

 complex substance. This energy cannot be supplied by the 

 plant itself. An external source of energy is therefore neces- 

 sary. What this source is is unimportant, provided the 

 energy be sufficient. The light of an electric arc serves the 

 purpose as well as sunlight, if its intensity be equal. 



EXERCISE XXXn. 



To show that manufactvre of starch occurs only in cells directly illu,~ 

 minated. 



