[Chap. XIII SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 113 



often due to other causes, among which may be mentioned the coagula- 

 tion of protoplasm. 



Sugar manufacture by bacteria. One outstanding exception to the mak- 

 ing of sugar b}' photosynthesis may be mentioned here, but it will be 

 discussed in more detail in a later chapter. A few kinds of bacteria are 

 known to make sugar from carbon dioxide and water in the absence of 

 light. The energy necessary for this process is obtained by the oxidation 

 of iron, sulfur, or nitrogen that has preyiously been reduced. A few other 

 kinds of bacteria ( purple bacteria ) are unique in containing a pigment 

 which, like chlorophyll, is effectiye in photosynthesis. The accompan\'ing 

 purple pigment (C4sH(!.i0.i) is similar to the carotenoids described in 

 Chapter IV. 



The amount of sugar made. The amount of sugar made by plants ^'aries 

 so greatly in different plants and under dissimilar conditions that it is 

 difficult to make a general estimate of it. The results of many experiments 

 show that under fa\'orable conditions a square yard of leaf surface makes 

 on the ayerage about 0.5 gram of sugar per hour. At this rate about four 

 months would be required for a square \'ard of leaf surface to make 

 sugar equiyalent to the food a man consumes in one day. In Chapter 

 XVm an acre of corn is compared with an acre of young apple trees 

 with respect to seyeral plant processes. This acre of corn has about two 

 acres of leaf surface. During the growing season it makes about 10 tons 

 of sugar, an ayerage of about 2 pounds per plant. The acre of young 

 apple trees (10,000 leaves per tree) makes about 8.7 tons of sugar, an 

 average of about 44 pounds per tree. On the basis of data from many 

 sources it is estimated that the plants of the United States make nearh- 

 three-fifths of a cubic mile of sugar each year. For the plants of the 

 world, 9 cubic miles of sugar annualh' seems to be a conseryative 

 estimate. 



Now let us recall the fact that for every 180 grams (almost 0.4 lb.) of 

 sugar made, 674 Calories of light energy are transformed to available 

 potential energy in this sugar. The potential energy in the sugar made 

 annually in the United States alone is estimated to be about 14 X 10'' 

 Calories. Yet large as this figure is, it probably is below the one that 

 would represent the energy liberated annually on the earth today by 

 plants, man, and all other living organisms. Through the burning of 

 coal, oil, and gas alone we liberate about 5 X lO^*''* Calories of energy 

 each year. The energy was transformed from radiant energ\' to potential 

 energy by the plants of the distant past. The facts upon which these 



