-102 GROWTH AND WORK OF PLANTS 



pay}.].; fqu/tfo , sunlight shining into the cell; fifth, the chemical 

 changes. The chlorophyll absorbs some of the rays of light and 

 this furnishes the power, we might say, to break up the carbonic 

 acid compound in such a way as to separate its elements. But 

 according to chemical laws these elements cannot long remain 

 separated under these conditions. They quickly unite into other 

 compounds or into a different compound. The different steps 

 in the process are very complicated and some of them are not well 

 known. But the first compound which we can definitely recog- 

 nize in the plant as a result of this chemical change is the substance 

 sugar* The symbol for grape sugar is C 6 H 12 O 6 , because one 

 molecule of sugar contains 6 parts of carbon, 12 parts of hydrogen 

 and 10 parts of oxygen. In the making of one molecule of sugar, 

 therefore, 6 molecules of the carbonic acid are necessary as will be 

 seen by the following simple mathematical formula, 6(CH 2 O 3 ) = 

 C 8 H 12 O 18 . Now subtract the sugar symbol from this as follows, 

 C 6 H 12 O 18 C H 12 O 6 = i2 ; i.e., there are 12 molecules of pure 

 oxygen set free in the plant cell which the plant cannot use in this 

 process. Much of this pure oxygen escapes by way of the sto- 

 mates into the open air again. It is an interesting fact that during 

 this process by which sugar is made in plants, pure oxygen is 

 added to the air and thus in general the proper balance or pro- 

 portion of this gas is maintained; otherwise, the air w r ould in time 

 become so depleted in oxygen that it could not sustain animal 

 life, since animals use oxygen of the air in respiration. It is also 

 interesting to note that, in this process, the green plants use 

 carbon dioxide from the air, which animals give off during res- 

 piration. Green plants, therefore, in this respect, as well as in 

 so many others, perform a very important work in nature. 



170. How starch is formed from sugar. Sugar is changed 

 to starch in the plant by the loss of one molecule of water. The 



* It is interesting to note that the proportion of hydrogen and oxygen in 

 the sugar compound is the same as that of a molecule of water, i.e., H 2 O. 

 The same is true of most sugars, and of starch. Those compounds of car- 

 bon, hydrogen and oxygen which contain the H and O in the proportion 

 , are known as carbohydrates. 



