6 THE CHEMISTRY OF THE FARM 



sphere'. Such accumulation would be injurious to the 

 health of animals, but is prevented by the growth^ of 

 plants. It has been calculated that an acre of forest, 

 producing annually 5,755 lbs. of dry matter, will con- 

 sume the carbonic acid produced by the respiration of 

 15"4 men. 



Besides carbonic acid, plants are apparently capable 

 of absorbing a small quantity of ammonia through 

 their leaves. The uncombined nitrogen of the atmo- 

 sphere is not, apparently, appropriated by the leaves 

 of green plants. When rain occurs after severe 

 drought, water may be taken up to some extent 

 through the lea^fes. 



Plants which have no chlorophyll cells, and possess 

 consequently no green colour, do not decompose car- 

 bonic acid. We have familiar examples of such plants 

 in the broomrape and dodder of our clover fields, and 

 in the common fungi. The broomrape and dodder are 

 fed by the juices of the plants on which they live as 

 parasites. The fungi derive their carbon from the 

 decayed vegetable matter in the soil. 



2. Formation of Organic Matter. — The oxygen gas 

 given off by a green plant exposed to light is so nearly 

 equal in volume to the carbonic acid decomposed, that 

 apparently the whole of the oxygen contained in the 

 carbonic acid is returned to the atmosphere; the re- 

 action is, however, really more complicated, as water 

 is probably decomposed at the same time as the car- 

 bonic acid. 



The exact nature of the reaction which takes place 

 when carbonic acid is decomposed in the chlorophyll 

 cell is still unknown. It is probable that formalde- 

 hyde is first produced, according to the following 

 equation :— ^ 



