34 ELEMENTARY STUDIES IN BOTANY 



may be thought of as a chemical laboratory, which uses cer- 

 tain substances in the manufacture of others. 



25. The energy. Those who have studied physics are 

 aware that energy, the power for work, is as real a thing as 

 the material to work with. It is important, therefore, to 

 discover the source of the energy used in the manufacture 

 of carbohydrates. The chloroplast obtains this energy 

 from sunlight, and it is known that chlorophyll is able to 

 absorb energy from light. It is evident that this absorbed 

 energy, in some form, is used in the chloroplast. It follows 

 that carbohydrates can be produced by green plants only 

 when exposed to the light, and that at night the process is 

 suspended. In fact, many green plants may live through 

 the winter, in the form of bulbs, tubers, etc., without any 

 opportunity to manufacture food. It must be evident, 

 therefore, that a process which is suspended for a consider- 

 able period during every twenty-four hours, and that may 

 be suspended for months, is not a process of living, which 

 involves the use of food, for living must go on continuously. 

 It is simply a manufacture, which has nothing to do with 

 the process of living except that it provides the material 

 that is used in the process of living. It holds the same 

 relation to the process of living that the baker holds to us 

 in manufacturing bread. 



It is important to observe that light is essential not only 

 to the manufacture of carbohydrates, but also to the manu- 

 facture of chlorophyll itself. If light is withdrawn from a 

 green plant for a considerable period, the plant loses its green 

 color, as when a board lies for some time upon the grass, 

 or when earth is heaped about celery to blanch it. When 

 potatoes " sprout " in a dark cellar, the young shoots are 

 pallid, but if exposed to light they become green. 



26. The process. The manufacture of carbohydrates 

 by green plants has received a name suggestive of the process. 

 It is called photosynthesis, which means putting together in the 



