254 THE MAINTENANCE OF THE INDIVIDUAL 



Place the plant in full sunlight for a few hours, then remove the 

 vaselined and untouched leaves, and treat both in the manner de- 

 scribed in the last experiment. The leaves to which no air penetrated 

 will be shown to have no starch. 



The need of carbon dioxide in the process of starch making may also 

 be demonstrated by a relatively simple experiment. If plants are 

 grown under similar conditions in two bell jars, but in one case the 

 carbon dioxide in the atmosphere is removed by means of soda lime, 

 while the other plant is left in the bell jar containing normal air, the 

 latter continues to grow while the one lacking carbon dioxide does 

 not increase in size. 



By burning a plant in a hot flame, it can be ultimately reduced to 

 mineral ash equaling about 4 to 5 per cent of the entire weight. Ac- 

 cording to Raber, from 1 to 55 per cent of the plant is consumed, 

 while from 40 to 95 per cent, roughly speaking, consists of water. 

 Since a green plant is immobile and since it has no way of obtaining 

 material except from the air, water, and the soil that surrounds it, 

 it may be safely assumed that if food is found in the plant body, 

 it must be made there. That foods are found in plants is common 

 knowledge. We eat roots, stems, fruits, and leaves of plants. Grains 

 form our staples of food. Roots and various types of fruits form 

 part of our dietary, while herbivorous animals live upon grasses and 

 fodder crops. This brings us then to the sources of the raw 

 materials out of which these elaborated foods must be formed. 



Carbon Dioxide as Raw Material 



Carbon dioxide is not only a product of respiration of animals but of 

 plants as well. A man gives off about nine hundred grams of carbon 

 dioxide daily into the air. Carbon dioxide also gets into the air from 

 the combustion of inflammable materials. Volcanic eruptions and 

 other sources of combustion increase the amount, while decaying 

 organisms and the oxidation of rocks and soils add a very appreciable 

 amount daily to the store. While it is estimated that there are only 

 two grams of carbon in each ten liters of air, nevertheless the fact 

 that carbon dioxide is universally available in the air and oceans close 

 to the surface of the earth shows that it may readily be made use of 

 by growing plants. Its need in food manufacture is well illustrated 

 by the statement that the world crop of wheat requires annually one 

 hundred and fourteen million tons of carbon dioxide in order to pro- 

 duce the seventy million tons of carbohydrates which form this crop. 



