20 CHEMICAL MANURES. 



in this train of ideas ; I would not retrench my words, 'for I believe 

 to act intelligently we must have a clear idea of the principles upon 

 which we act. But I hasten to return to the practical in the assimi- 

 lation of carbon. 



The assimilation of carbon is included, as we have said, in two 

 facts : Plants absorb carbonic acid from the air, and decompose it. 



To prove that leaves absorb carbonic acid, introduce the leafy 

 branch of a vine into a globe of glass, making a current of air to 

 pass through it. 



Before entering the globe, the air will contain from three to four 

 ten-thousandths of its volume in carbonic acid ; when it comes out, it 

 will contain but two ten-thousandths, more or less. The leaves have 

 acted like a crucible. All plants and trees effect by their foliage 

 what you see this branch of a vine produce under your own eyes. 



But for this three conditions are necessary : 



1st. The plants must receive the direct action of the sun. 



2d. The temperature of the atmosphere does not descend below ten 

 to twelve degrees above zero. 



3d. The plants must be provided with leaves. 



The suppression of one of these three conditions is enough to stop 

 the phenomena, and measurably to strike the plant with inertia. 



Leaves lose the power of absorbing carbonic acid in the dark. As 

 soon as light fails, the leaves, in direct opposition to what they had 

 done before, absorb oxygen and give off carbonic acid. 



In vour climate the assimilation of carbon ceases almost entirely 

 below ten to twelve degrees. It would be imprudent to make this an 

 absolute rule, as all plants are not affected in the same degree by the 

 lowering of temperature. 



The leaves are essentially the seat of the assimilation of carbon ; 

 neither roots, trunk nor branches share in this important office. 



We will now proceed to a more practical order of ideas, and one 

 pertaining especially to agriculture. 



The quantity of carbon that plants fix during the season the acre 

 reaches as high as 8888 pounds. 



Here arises a new question. All plants do not attain this standard. 

 Whence comes the difference ? The leaves do not present the same 

 amount of surface. 



If, from this point of view, we compare some of the plants in 

 which we are most interested, such as the sweet potato, the beet, the 

 Irish potato and wheat, we find that the sweet potato, which fixes 

 7111 pounds of carbon the acre, gives a leaf-surface fifteen times 

 that of the soil cultivated ; that the beet, which fixes 1776 pounds 

 of carbon, gives a leaf-surface five times that of the soil. The same 

 remarks are applicable to the Irish potato and wheat, which absorb 

 but 1511 and 1244 pounds of carbon, and give a much reduced leaf- 

 surface. 



Lastly, to complete the study of the assimilation of carbon, I 

 must add, that if the atmosphere is the principal source from which 

 plants derive it, they, however, draw a certain quantity from the 



