t8€ RETROSPECT. 



CHAPTER XIII. 



Retrospective View of the Preceding Theories. 



The knowledge of the processes of nutrition, in the case of the 

 culture of meadow and of forest land, indicates that the atmo- 

 sphere contains an inexhaustible quantity of carbonic acid. 



On equal surfaces of wood or of meadow land, in which exist 

 the constituents of the soil indispensable to vegetation, we obtain 

 crops without the application of carbonaceous manures ; and 

 these crops contain, in the form of wood and hay, a quantity of 

 carbon equal to, or, in many cases, greater than that produced 

 by cultivated land in the form of straw, corn, and roots. 



It is obvious that the cultivated land must have presented to it 

 as much carbonic acid as is furnished to an equal surface of 

 wood or of meadow land ; that the carbon of this carbonic acid 

 becomes assimilated, or is capable of assimilation, if the con- 

 ditions exist for its reception and conversion into a constituent 

 of plants. 



However great may be the supply of food in a soil, it will be 

 sterile for most plants, if water be deficient. At certain seasons 

 of the year rain fructifies our fields; seeds neither germinate 

 nor grow without a certain quantity of moisture. 



The action of rain is much more striking and wonderful to the 

 superficial observer than that of manure. For weeks and 

 months, the influence which it exerts on the crops is appreciable, 

 and yet very small quantities of carbonic acid and ammonia are 

 introduced to the soil by means of rain. 



Water plays, doubtless, a decided part in the growth of plants, 

 by virtue of its elements ; but, at the same time, it is a mediating 

 member of all organic life. Plants receive from the soil, by the 

 aid of water, the alkalies, alkaline earths and phosphates neces- 

 sary to the formation of their organs. If these substances, 



