496 [As&EMBLY 



earth. Certain tropical plants send out aerial roots j and yet, 

 such facts as these must not lead us, as it was not long ago sup- 

 posed they would, to the conclusion that it is the office of roots of 

 plants merely to fix them to the earth, that they only furnish a 

 pediment to the trunk. The fact that a circulation goes on in the 

 stem, is of itself evidence that the root of a plant has a different 

 function from the top It is their office to take into its circula- 

 tion that carbonic acid that is dissolved in water. As it is the 

 office of gills of fislies to excrete the same gas into water, and it 

 is only Avhen wet that either of them can discharge this duty. 

 Though the absorption of carbonic acid does certainly go on 

 through the leaves, yet it is in solution, and to the roots that this 

 substance is presented in its most concentrated form, and it is 

 only through the root that the processes of agriculture may in 

 any measure increase the supply. There are very few plants, 

 and none of any economical worth, that will grow in a soil of 

 pounded glass, wet with distilled water. When the earth is 

 made two or three feet deep over the roots of shade trees, as it 

 sometimes is in grading our streets, the supply of moisture to their 

 roots is not affected, nor is the supply of salts, and as these trees 

 require little or no nitrogen, it can be only from the deficient 

 supply of carbonic acid, that they die. 



But this gas also exerts an ameliorating influence on the soil. 

 Water charged with it acts on granite, feldspar, gneiss, mica, &,c., 

 disintegrating them and setting free their saline constituents. In 

 fact, scarcely any mineral substance can resist its long- continued 

 action ; so that it is to this substance, as well as to oxygen, that 

 we owe the present alluvion. De la Beche attributes the condi- 

 tion of those stones in the west of England, that have been sup- 

 posed to be of Druidical origin, to corrosive action of this acid. 

 The chalk formation contains an immense amount of this gas in 

 a fixed condition, as it were to counterbalance the reduced carbon 

 of the coal measures. It is eliminated continually in many 

 regions of tlie earth's surface, as in the Limagne d'Auvergne, in 

 France, surcharging the soil and stimulating vegetation enor- 

 mously.* 



• The disintegration of granite is a striking feature of large districts of Auvergne, especially 

 in the neighborhood of Clermont. This decay was called by Dolomieu "La ftJaladie da 



