<62 THE CULTURE OF FARM CROPS. 



Thus starch consists of 12 parts of carbon; 20 parts of hy- 

 drogen and 10 parts of oxygen; while it has been seen that 

 water, consisting of 2 parts of hydrogen and 1 part of oxy- 

 gen, the 20 parts of hydrogen and 10 of oxygen in the 

 starch are equivalent to precisely 10 parts of water. But 

 it is not certain that starch is made up of carbon and wat- 

 er; it is more probable that the three element: exist in 

 starch in other forms of combination. It is certain however 

 that carbonic acid is the source from which the carbon of 

 the vegetable substance is procured : because carbon is in- 

 soluble in water and is a solid cubstance, and plants cannot 

 take any solid matter into their circulation and their food 

 must always be in solution in water. This part of our 

 subject however will be more fully treated in a future chap- 

 ter and under its appropriate head. 



The air contains one part of carbonic acid in 2,500 and 

 this proportion seems to be the most suitable for the health- 

 ful growth of plants. The sun light has a great influence 

 upon this nutritive function of this acid. When plants are 

 exposed to the sunshine, it has been found that they grew 

 more vigorously in an artificial atmosphere containing one- 

 twelfth of its bulk of carbonic acid ; but when this propor- 

 tion was increased the plants were injured. When the 

 carbonic acid amounted to one-half the atmosphere, the 

 plants perished in 7 days; and when the proportion was 

 two-thirds, the plants stopped growth immediately. In the 

 shade, any increase of the carbonic acid above the normal 

 amount in the atmosphere viz one twenty-five hundredth 

 (.0004) proved to be injurious. This fact is of im- 

 portance; for the reason that although an increase 

 in the quantity of carbonic acid in the air, might stimu- 

 late vegetable growth, yet it would seriously and even fatally 

 disturb the balance of nature, because the air would then 

 be unfit for the respiration of animals; and moreover al- 

 though plants would grow more luxuriantly in such an at- 

 mosphere, in perpetual sunshine, yet they would suffer in 

 the shade; and w r ould also certainly require a proportion- 

 ate increase in the supply of other food, to complete their 



