16 QUARTERLY JOURNAL. 



1. Water. This substance is also a necessary ingredient in the 

 food of animals, but in them it serves the purpose of a diluent or 

 solvent of the alimentary principles, and does itself contribute di- 

 rectly to nutrition. It serves this same purpose of a solvent also 

 in plants, for the carbonic acid, the ammonia and the inorganic 

 constituents are introduced in a state of solution in water. But 

 water is also directly nutritive in plants ; its elements combine 

 with the carbon of the carbonic acid and form the non-nitrogen- 

 ized proximate principles. 



The part played by water in vegetation is then doubly impor- 

 tant, for it not only serves as an indispensable article of food 

 which is converted into the substance of the plant, but by its sol- 

 vent properties it serves to introduce the other articles of food 

 from the soil and from the atmosphere. 



The necessity of water for vegetation and the sources whence 

 it is derived, are so generally understood that they require no fur- 

 ther illustration. 



2. Carhonic acid. Carbon is the preponderating element of 

 plants, constituting more than fifty per cent of their weight. It 

 is introduced in the form of carbonic acid, which is derived from 

 the soil and from the atmosphere. 



The carbonic acid derived from the soil is absorbed by the roots, 

 passes into the trunk and from thence into the leaves and ends by 

 being exhaled, without change, if no new force intervenes. 



" Such is the case with plants vegetating in the shade and du- 

 ring the night season : the carbonic acid of the soil permeates 

 their tissues and is diffused in the air. Plants are commonly said 

 to produce carbonic acid during the night ; this is incorrect : plants 

 then only transmit unchanged the carbonic acid which their roots 

 have pumped up from the soil. 



" But suppose this carbonic acid, whether derived from the soil 

 or from the atmosphere, to be in contact with the leaves and green 

 parts, and the light of the sun to fall on them, immediately the 

 whole scene is changed : the carbonic acid disappears ; minute 

 bubbles of oxygen are evolved from every point of the leaves and 

 the carbon is fixed in the tissues of the plant. 



" And it is a point most worthy of remark and fitted to arouse 

 attention, that these green parts of vegetables, the very ones that 

 have been found capable of exhibiting this wonderful phenome- 



