WORK OF LEA VES: PHOTOSYNTHESIS 109 



absorbed by water, the two together form carbonic acid (CO 2 + 

 H 2 O = CH 2 O 3 = the symbol for carbonic acid).* 



173. There are five principal requirements in the process 

 of photosynthesis^ during which the formation of starch and 

 sugar in green plants takes place. First, the living plant cell; 

 second, the presence of carbonic acid; third, the presence of 

 chlorophyll ; fourth, sunlight shining into the cell ; fifth, the chem- 

 ical changes. The chlorophyll absorbs some of the rays of light 

 and this furnishes the power, we might say, to break up the car- 

 bonic acid compound in such a way as to separate its elements. 

 But according to chemical laws these elements cannot long re- 

 main separated under these conditions. They quickly unite into 

 other compounds or into a different compound. The different 

 steps in the process are very complicated and some of them are 

 not well known. But the first compound which we can definitely 

 recognize in the plant as a result of this chemical change is 

 glucose or grape sugar. { The symbol for grape sugar is C 6 H 12 O 6 , 

 because one molecule of sugar contains 6 parts of carbon, 12 

 parts of hydrogen and 6 parts of oxygen. In the making of 



* In the case of aquatic plants the water in which they grow absorbs 

 carbon dioxide from the air, thus forming the carbonic acid which they 

 absorb from the water. 



f For reference. It is now evident what photosynthesis means. The first 

 part of the word comes from the Greek word $wrc/s = light, and the last part 

 of the word has reference to the synthetic process, or union, or putting to- 

 gether. The process was formerly considered a process of assimilation and 

 was called carbon dioxide assimilation. True assimilation, however, is 

 brought about entirely by the living substance of the plant (see paragraph 

 180). In "carbon dioxide assimilation " sunlight supplies the energy for one 

 step in the process, and it seems better to use the term photosynthesis, though 

 if the nature of the process is kept clearly in mind there should be no very 

 great objection to saying carbon dioxide assimilation. Some use the term 

 fixation of carbon. 



{ Glucose is also called grape sugar, or starch sugar. It is interesting to 

 note that the proportion oT hydrogen and oxygen in the sugar compound is the 

 same as that of a molecule of water, i.e., H 2 O. The same is true of most 

 sugars, and of starch. Those compounds of carbon, hydrogen and oxygen 

 which contain the H and O in the proportion H 2 O, are known as carbo- 

 hydrates. 



