THE PLANT. 17 



decomposition takes place in the green parts of plants 

 and oily under the influence of daylight. It is not 

 necessary that the sun shine directly on the leaf or 

 green sl.oot, but this causes a more rapid decomposi- 

 tion of carbonic acid, and consequently we find that 

 plants winch 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 different 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 nearly or quite the whole 

 year, the duration of daylight is sufficient 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, and plants from continual growth develop them- 

 selves in that short time. 



It will be recollected that carbonic acid constitutes 

 but about 5 5 * of 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 by 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 icith the presence of atmospheric air^ the 

 oxygen of that air unites with the carbon, and forms 



