CALBON OBTAINED FROM THE SOiL. 33 



the sun goes down, a change takes place : an exami- 

 nation will now show that it is carbonic acid which 

 passes off from the leaves, and oxygen that is being 

 absorbed. It is just the reverse of what goes on during 

 the day. 



a. This curious fact shows why it is that plants 

 grow so rapidly in the long days of summer. The 

 nights are then comparatively a small portion of the 

 day, so that for by far the greater part of the twenty- 

 four hours the plant continues to absorb carbonic acid, 

 and to build itself up w T ith the carbon thus obtained 



b. In Greenland and Kamschatka the summer is not 

 more than two or three months, but during that time 

 it is always daylight, the sun scarcely going below 

 the horizon at all. Certain plants are thus enabled 

 to grow so fast as to mature and ripen their seed, even 

 in that short summer. We see how this beautiful 

 provision of nature tends to equalize different climates. 

 If the nights of the short Greenland summers were 

 even so long as our shortest, their crops would never 

 ripen; but as they have nearly perpetual day, they can 

 get enough food from their fields to sustain life during 

 a large part of their long winter. 



SECTION IV. CARBON ALSO OBTAINED BY PLANTS FROM 



THE son,. 



We see that plants are able to obtain much carbon 

 from the air, but it is found that a considerable quan- 

 tity comes from the soil also. This is all, in one form 

 or another, drawn in through the roots. The rain 

 water which falls upon the surface, and all of the 

 spring water found there already, contains some car- 

 bonic acid dissolved. This water entering the roots, 

 carries with it a variety of substances in solution, 

 which the plant seems to use or not as it may require: 

 among these is carbonic acid. This is probably the 



