50 PHYSIOLOGICAL CHEMISTRY. 



haled, during the vegetative process, in the same, or nearly the same, 

 proportion as that in which it originally existed in the carbonic acid ; 

 and the new substance produced contains hydrogen and oxygen in the 

 proportions to form water. The production of starch in growing veg- 

 etables is therefore represented by the following formula : 



CARBONIC ACID. WATEE. STARCH. 



600 2 + 5H 2 - 12 = C 6 H 10 5 . 



The production of starch in this way by vegetation is a phenomenon 

 of the first importance in the economy of living beings. It is the only 

 natural process known to take place on the earth by which oxygen is 

 set free from its actual combinations. It is a reduction of two com- 

 pounds in which the oxygen affinity of carbon and hydrogen was fully 

 satisfied, resulting in the formation of an organic matter capable of 

 reoxidation. The new substance so produced has therefore a power 

 of combination, which may be afterward brought into activity under 

 requisite conditions, and which is the theoretical basis of all force mani- 

 fested by the living organism. 



There are two conditions requisite for the formation of organic mat- 

 ter by vegetable tissues: First, the access of solar light, either by 

 direct sunshine or by diffused daylight, and, secondly, the existence in 

 the living plant of the green coloring matter known as " chlorophylle." 

 Green vegetables, which absorb carbonic acid and exhale oxygen in the 

 sunshine, cease to do so when daylight disappears, and remain inactive 

 in this respect during the night. On the other hand, colorless vege- 

 tables, and the uncolored portions of green plants, have no reducing 

 action, even in the daytime. 



The materials for this reduction process in vegetables, namely, car- 

 bonic acid and water, are supplied from the atmosphere and the soil. 

 As the atmosphere contains about .05 per cent, of its volume of carbonic 

 acid, and as the column of air above each square metre of surface, at the 

 ordinary barometric pressure, weighs a little over 10,000 kilogrammes, 

 this would give, by weight, 1.5 kilogrammes of carbonic acid to the 

 square metre, equivalent to 30,390 kilogrammes, or rather more than 

 thirty tons, over each acre of land. From this abundant reserve, the 

 carbonic acid is supplied for vegetation. It is absorbed directly by the 

 foliage in contact with the atmosphere, and, brought down in solution 

 by the rain, it is taken up by the roots and transferred to the leaves 

 by the vegetable juices. The activity of the reducing process has been 

 measured by Boussingault.* He found that a single fresh oleander leaf 

 in sunshine decomposed in two successive days nearly 49 cubic centi- 

 metres of carbonic acid ; and, as a mean of six similar experiments, 

 each square centimetre of leaf surface decomposed 1.33 cubic centi- 

 metres of the gas, exhaling an equal volume of free oxygen. It is 

 estimated by Hoppe-Seyler, that, considering the amount of oxygen 

 consumed by animal organisms and the time during which these organ- 



* Coraptes rendus de 1' Academic des Sciences, Paris. Tome LXL, pp. 498, 502. 



