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42' 



ever air and water are available. The 

 land plants, with which we are chietly 

 concerned in the production of food for 

 the human race, obtain water from the 

 soil, and carbon dioxide directly from 

 the air in contact with the leaves. An 

 interesting fact in rctV-rence to these 

 raw materials is that they are also "ul- 

 timate wastes" when food is beino; used. 

 This means that when living bodies are 

 using foods, carbon dioxide and water 

 are excreted because they cannot be 

 broken up in the body as a preliminary 

 step to the formation of new combina- 

 tions. From food to waste is the work 

 going on in all living bodies ; from waste 

 to food is the added work going on in 

 all green plants. 



The active agent in the manufacture 

 of carbohydrates is the "chloroplast," 

 which needs definition. Chloroplasts 

 are minute green bodies within the cells 

 that give green color to foliage. As the 

 name suggests, they comprise two con- 

 spicuous substances: the plastid is the 

 living substance (protoplasm), while 

 the chlorophyll is a green pigment. 

 The living plastid does the work, while 

 the chlorophyll supplies the conditions 

 for work; in fact, the chloroplast may 

 be thought of as a chemical laboratory 

 which uses raw materials in the manu- 

 facture of carbohydrates. 



In order to work, the chloroplast 

 must have a supply of energy, and this 

 is obtained from sunlight. It is known 

 that chlorophyll is able to absorb en- 

 ergy from light, for when light passes 

 through it, certain rays are retained, 

 and it is these retained rays that sup- 

 ply the energy with which the chloro- 

 plast works. It is an interesting fact 

 that the rays of light not absorbed give 

 a green color; that is, leaves are green 

 because the green-producing rays are 

 not being used. If the energy for 

 photosynthesis is obtained from sun- 

 light, it is evident that at night the 

 process is suspended; in fact, many 

 plants live through the winter without 

 any opportunity to manufacture cai'bo- 



iiydrates. It must be evident that a 

 process which is suspended for a con- 

 siderable period during every twenty- 

 four hours, and which may be sus- 

 ])cnded for months, is not a process of 

 living, for living must go on continu- 

 ously. It is simply a manufacture that 

 ])rovides material used in the process of 

 living. 



The ])rocess of carbohydrate manu- 

 facture has been called "photosynthe- 

 sis" because the word means "putting 

 together in the presence of light." The 

 first step in the process is the breaking 

 up of water and carbon dioxide into 

 their constituent elements. Water con- 

 sists of hydrogen and oxygen, and car- 

 bon dioxide of carbon and oxygen. To 

 break up these two substances in our 

 university chemical laboratory requires 

 a great display of energy in the form of 

 heat or electricity, but it is accom- 

 plished by the chloroplast in the labo- 

 ratory of the leaf without any unusual 

 display of energy. Following this 

 breaking up of the raw materials, the 

 freed elements are put together in new 

 combinations, this being the "synthe- 

 sis" referred to in the name. It must 

 not be supposed that a carbohydrate is 

 the result of the first synthesis, for it is 

 reached only after a series of chemical 

 changes. 



The final product of photosynthesis 

 is reached when a carbohydrate is 

 formed. In the production of a carbo- 

 hydrate, not all of the elements of the 

 raw materials are used. As much oxy- 

 gen is left over as entered with the 

 carbon dioxide, and this oxygen is a by- 

 product which is being given off when 

 green plants are engaged in photosyn- 

 thesis. (The name carbohydrate, mean- 

 ing carbon and water, is given because 

 it contains carbon and also hydrogen 

 and oxygen in the same proportion as 

 in water.) The total result seems to be 

 to get the carbon out of the carbon di- 

 oxide and combine it with water, and 

 therefore the process is often called the 

 '•fixation" of cnrlion : tliat is, getting 



