The Manufacture of Food 37 



physiology. He grew a plant for several years in an accurately 

 weighed body of soil. He then carefully removed the plant, 

 dried it, weighed it, and also reweighed the soil. He found that 

 the increase in dry weight of the plant was more than a thousand 

 times the loss in weight of the soil. This proved that the plant 

 must have obtained most of its materials from some other source 

 than the soil. The plant, of course, used vast quantities of water 

 during its growth, but since water contains no carbon, the only 

 other source of this material must therefore have been the carbon 

 dioxide of the air. 



How the products and wastes are removed. The manufacture 

 of carbohydrates in the leaf goes on only during the hours of sun- 

 light ; the removal of food goes on at all times. The food-con- 

 ducting tissue of the veins furnishes the outlet for the product, 

 which is transferred in the form of sugar. During the day the 

 rate of manufacture is so much greater than the rate of removal 

 of food that starch and sugar accumulate. During the night 

 the movement of food into the stem nearly empties the chloren- 

 chyma. The waste product, oxygen, passes from the cells to the 

 intercellular spaces and out through the stomata to the atmos- 

 phere. 



A leaf, then, is carrying on photosynthesis at its full capacity 

 only when there are sunlight, a favorable temperature, and an 

 abundant supply of water, and when the stomata are open. Even 

 under these conditions the work may be interfered with if more 

 than a certain amount of the products accumulates in the cells. 



The amount of the product. The amount of carbohydrates 

 produced in photosynthesis varies so greatly in different plants 

 and under dissimilar conditions that it is very difficult to make a 

 general estimate of it. The result of many experiments shows that 

 under favorable conditi«dft a square meter of leaf surface makes 

 on an average about i gram of carbohydrates per hour. At this 

 rate a square meter of leaf surface in midsummer would require 

 2 months to produce food equivalent to that consumed by the 



