600 WEED FLORA OF IOWA 



The formula for sugar shows that it is composed of three ele- 

 ments, carbon, hydrogen, and oxygen. These elements are furnished 

 by the carbon dioxide (CO2) which is taken in from the air, and 

 the water (HjO) which is taken from the ground by the roots and 

 conducted to the leaf tissue by the vascular bundles of the plant. 

 Although CO2 and H2O furnish the necessary elements for sugar 

 and starch, these are only the raw materials and some agent or iac- 

 tory is needed to cause these elements to combine and to combine 

 in the right proportions. These factories are the ehloroplasts, which 

 give the green color to the entire leaf. The green pigment (chloro- 

 phyll) is the active agent of the chloroplast in the manufacture 

 of sugar. The process by which these raw materials are combined 

 is not well understood, and it seems that several simpler products 

 are formed before sugar is produced. We know that CO2 plus 

 HjO forms carbonic acid (OH. COOH). The carbonic acid is prob- 

 ably reduced to formaldehyde (H. COH). If six molecules of for- 

 maldehyde were properly combined we would have one of the sim- 

 ple sugars (HeCeOgHg) or better written (CgHiaOg). Two mole- 

 cules of the simpler sugars combined with one molecule of water 

 eliminated will give cane sugar — CoHijOg plus CsHi206=Cane 

 sugar C12H22O11 plus HjO. By a further synthesis starch is pro- 

 duced. 



When formaldehyde is produced as described above, oxygen is 

 eliminated and this forms the escaping bubbles from the submerged 

 leaf. 



In this process of photosynthesis, the ehloroplasts constitute the 

 factory, carbon dioxide and water furnish the raw materials, sugar 

 and starch are the products, and sunlight is the necessary condition 

 without which the machinery will not run. 



Respiration. Plant cells as well as animal cells have much work 

 to do and in order to perform work, energy is needed. Plant cells 

 transform material into cell walls, increase and repair protoplasm, 

 divide and do many other things which require energy. This work 

 never ceases as long as the plant lives. The external indication of 

 it is the absorption of oxygen and the giving out of carbon dioxide. 

 This exchange is spoken of as respiration. It will be noted at once 

 that this is exactly the reverse of what takes place in photosynthe- 

 sis. During the day both carbon dioxide and oxygen are being 

 both absorbed and eliminated. Photosynthesis and respiration are 

 independent processes and must not be confused. 



