2154. 'The importance of aeration' Dr. Lindley observes, 'cannot be over estimated. It is the one thing which now requires to be secured, in order to render our artificial climates natural. A man's reason, indeed, must tell him that a plant condemned to pass its life in a still atmosphere, is like nothing so much as a criminal set fast in an everlasting pillory. In order to secure motion in the vegetable kingdom, currents of air are made to do the work of the muscles, limbs, and volition of animals. It is not at all improbable that, in addition to the mechanical effect of motion in assisting the propulsion of the sap, it may be important that the stratum of air in contact with the leaves of plants should be incessantly shifted, in order to enable them to procure an adequate supply of food; for we find that water in motion feeds them better than that which is stagnant. Leaves are continually extracting from the air the very minute quantity of carbonic acid which it contains. When the air moves quickly over their surface, fresh supplies of that food are incessantly presented to it, and the operation of abstraction may be facilitated; while, on the contrary, if the air is stagnant, the absorption of carbonic acid may be very much slower. Just as would be the case if a great sponge filled with milk were to be placed in mere contact with a man's mouth: he would be a long time in sucking out its contents if the sponge were immovable; but he would soon possess himself of the milk, if the surface of the sponge were continually shifting.' (Gardeners' Chronicle, for 1846, p. 267.)
