THE PLANT, IT 



decomposition takes place in the gi'een-poxts of plants 

 and only under the influence of daylight. It is not 

 necessary tliat the sun shine directly on the leaf or 

 green shoot, but this causes a viore rapid decomposi- 

 tion of carbonic acid, and consequently we find that 

 plants which are well exposed to the sun's rays make 

 the most rapid growth. 



The fact that light is essential to vegetation ex- 

 plains the conditions of difl'erent latitudes, which, so 

 far as the assimilation of carbon is concerned, are 

 much the same. At the Equator the days are but 

 about twelve hours long. Still, as the growth of 

 plants is extended over nearjy or quite the whole 

 year, the duration of daylight is suflicient for the re- 

 quirements of a luxuriant vegetation. At the Poles, 

 on the contrary, the summer is but two or three 

 months long ; here, however, it is daylight all sum- 

 mer, aud plants from continual growth develop them- 

 selves in that short time. 



It will be recollected that carbonic acid constitutes 

 but about -g-gVs- ^f the air, yet, although about one- 

 half of all the vegetable matter in the world is de- 

 rived from this source, as well as all of the carbon 

 required l)y the growth of plants, its proportion in 

 the atmosphere is constantly about the same. In 

 order that we may understand this, it becomes 

 necessary for us to consider the means by which it is 

 formed. In the act of burning, carbon unites with 

 oxygen, and always when bodies containing carbon 

 are burnt ivit/i the presence of atmosj^heric ah\ the 

 oxygen of that air unites with the carbon, and forms 



