PHOTOSYNTHESIS 31 



of carbon dioxide in the plant takes place so rapidly that 

 minute bubbles of oxygen may often be seen rising to the surface 

 of the water. If, for example, we cut off a leafy branch of the 

 common Canadian water weed, known as Elodea canadensis, and 

 fix it under water in such a jar, it is possible to arrange the 

 experiment so that a regular stream of small oxygen bubbles will 

 be given off from the cut end, and it is further possible to adjust 

 the experiment so delicately that the interposition of a dark 

 screen between the jar and the sunlight will cause the immediate 

 cessation of the stream of bubbles, which will start again the 

 instant the screen is removed. This simple and beautiful 

 experiment clearly demonstrates the dependence of the process 

 of decomposition of carbon dioxide upon the presence of 

 sunlight. 



The oxygen liberated in this way is not (with the exception of 

 a relatively small quantity used in respiration) required by the 

 organism, and is accordingly at once discharged into the sur- 

 rounding medium. The carbon, on the other hand, is needed for 

 the manufacture of new protoplasm. It is never actually set free 

 as carbon, but its molecules are probably recombined under the 

 influence of the sunlight with the molecules of water to form the 

 carbohydrate known as glucose or grape sugar. This process 

 may be represented by the equation 



6C0 2 + 6H 2 = 60 2 + C 6 H 12 6 



(Carbon Dioxide) (Water) (Oxygen) (Glucose). 



It is probable that a simpler compound, possibly formaldehyde 

 (CH 2 0), is formed as an intermediate product, while, on the other 

 hand, the glucose appears to be rapidly converted into starch, 

 which is the first visible product of the process of photosynthesis 

 in the plant cell. 



Starch, like glucose and cellulose, is a carbohydrate, and, 

 though differing in many of its chemical and physical properties, 

 has the same general formula as the latter (C 6 Hi 5 ) n . This 

 means simply that the elements carbon, hydrogen and oxygen 

 are present in the same proportions as in cellulose, but they must 

 be linked together differently in the molecule. 



The first step in the actual construction of the proteid molecule 



is then the combination of carbon with the elements hydrogen 



and oxygen to form a carbohydrate. In the higher plants starch 



\first appears in the chlorophyll-containing cells of the leaves in 



