CIRCULATION" OF SAP. 95 



we may say, so largely diluted that our chemists tell us 

 that it averages only about one twenty-five hundredth 

 part of the bulk of the atmosphere, while it abounds in 

 many soils, springs, brooks, wells, caverns in the humus 

 of forests, besides occurring combined with bases, form- 

 ing carbonates of lime, magnesia, soda, strontia, baryta, 

 as well as in the various oxides of metals, some of which 

 are always found in fertile soils. The position which 

 Dr. Masters takes in regard to the source, as well as the 

 way in which carbonic acid is utilized by plants, does not 

 differ essentially from that held by several other Euro- 

 pean botanists, but he gives the theory in a better form 

 than I have seen it elsewhere. 



Dr. Balfour, however, a very eminent English author- 

 ity on vegetable physiology, says that "carbonic acid is 

 readily taken up, either in its gaseous state by the leaves, 

 or in combination with water hy the rp.ots," We cer- 

 tainly can scarcely conceive of any inherent power of 

 choice so potent in the roots as to enable them to reject 

 this gas which is so necessary to their health and growth, 

 and it is well known to be far more abundant in the soil 

 about the roots than in the atmosphere. In a recent 

 (1885) edition of the "Manual of Agriculture," by Geo. 

 B. Emerson, I find the following in relation to this sub- 

 ject : " Carbonic^ a^cidjs the most indispensable and 

 abundant article of the toodjDf all plants.. It enters the 

 plant dissolved in water, and either remains in that state, 

 or the vital action of the plant, in the light of the sun, 

 decomposes the acid and throws back most of the oxygen 

 into the atmosphere, but retains, a portion which per- 

 forms important offices, and also retains the carbon. 

 This forms the solid parts of every plant." 



Among men who combine science with practice the 

 foregoing is the most generally accepted theory of the 

 manner in which plants obtain the great bulk of their 

 carbon. It is open to some objection, but it will mislead 



