428 



THE AMERICAN MUSEUM JOURNAL 



carbon out of a gas and "fixing'' it in 

 a solid. Since hydrogen and oxygen are 

 both gases, carljon is the only solid that 

 enters into the fabric of the plant, and 

 this solid is obtained from a gas that 

 exists in the air. 



The carbohydrates thus formed in the 

 plants are usually starches or sugars, 

 and they are freely transformed into 

 one or the other. Starch is spoken of 

 as the storage form, but when the car- 

 bohydrate is being used and is moving 

 through the plant, it is in the form of 

 sugar, for a substance must Ije in solu- 

 tion to be carried about, and therefore 

 sugar is spoken of as the transfer form 

 of a carbohydrate. 



When it was first discovered that 

 green plants take in carbon dioxide and 

 give out oxygen, it was natural to sup- 

 pose that this gas exchange represented 

 the respiration of plants. Since the 

 gas exchange in the respiration of ani- 

 mals is just the reverse, the opinion be- 

 came current that plants and animals 

 differ in their "breathing." Since this 

 impression is still current, its correc- 

 tion should be emphasized. It is clear 

 that photosynthesis has nothing to do 

 with respiration, for respiration is asso- 

 ciated with what may be called the act 

 of living, and therefore is carried on by 

 every living thing all of the time. If 

 respiration stops, the plant or animal 

 is dead ; in fact, we use respiration as a 

 sign of life. Therefore plants and ani- 

 mals "breathe" alike, both taking in 

 oxygen and giving out carbon dioxide; 

 but green plants carry on the process 

 of photosynthesis also, in connection 

 with which carbon dioxide is taken in 

 and oxygen is given out. The confusion 

 arose from the fact that during the 

 day, when photosynthesis is going on, 

 the amount of gas exchange involved in 



the manufacture of carljohydrates is so 

 much greater than the amount involved 

 in respiration, that the latter was not 

 noticed. If the observation had been 

 extended into the night, however, it 

 would have been discovered that only 

 the gas exchange of respiration was 

 being carried on. 



Carbohydrates are by no means the 

 only foods that plants make, and there- 

 fore photosynthesis is not their only 

 process of food manufacture. Another 

 conspicuous group of foods is the group 

 of proteins, which may l)e regarded as 

 foods in the most advanced stage as liv- 

 ing protoplasm is largely composed of 

 proteins. Carbohydrates, therefore, may 

 be thought of as the first stage of food, 

 and protein as the last stage. It is 

 known that neither light nor chloro- 

 phyll is required for the manufacture 

 of protein, for the process goes on in 

 living cells removed from light, and in 

 plants containing no chlorophyll. It is 

 known, however; that carbohydrates are 

 used, and that to the carbon, hydrogen, 

 and oxygen supplied by them, the ele- 

 ments nitrogen, sulphur, and often 

 jihosphorus are added, and these ele- 

 ments are obtained from their combi- 

 nations in the salts of the soil. 



The role of green plants in the world, 

 therefore, is evident. It is only by 

 them that food can be made from that 

 which is not food. For this reason they 

 are the only independent organisms, 

 that is, independent of the work of 

 other organisms. When we see the 

 phrase "nothing but leaves," with its 

 implication of failure, we must realize 

 that leaves stand for the most funda- 

 mental of all the work of the earth, 

 without which tliere would be no world 

 of livinff beings. 



