. The theory of horticulture. hey do perspire is susceptible ofdirect proof, and is by no means a mere matter ofinference. 70. We do not indeed see vapour flying off fromthe surface of plants; neither do we from that of ani-mals, except when the air is so cold as to condensethe vapour; yet we know that in both cases perspira-tion is perpetually going on, and it would appear thatin plants it takes place more abundantly than in ani-mals. If a plant covered with leaves is placed undera glass vessel, and exposed to the sun, the sides of thevessel are speedily covered with dew, produced bythe conde
. The theory of horticulture. hey do perspire is susceptible ofdirect proof, and is by no means a mere matter ofinference. 70. We do not indeed see vapour flying off fromthe surface of plants; neither do we from that of ani-mals, except when the air is so cold as to condensethe vapour; yet we know that in both cases perspira-tion is perpetually going on, and it would appear thatin plants it takes place more abundantly than in ani-mals. If a plant covered with leaves is placed undera glass vessel, and exposed to the sun, the sides of thevessel are speedily covered with dew, produced bythe condensation of the insensible perspiration of theplant. If the branch of a plant is placed in a bottleof water, and the neck of the bottle is luted to thebranch, so that no evaporation can take place, never- ACTION OF LEAVES. 45 theless the water will disappear; and this can onlyhappen from its having been abstracted by the branchwhich lost it again by insensible perspiration. Hales,an excellent observer, devised many experiments. 46 VITAL ACTIONS. connected with, this subject ;* among others the fol-lowing, which he relates thus:— August 13. In thevery dry year 1723, I dug down 2h feet deep to theroot of a thriving baking pear tree, and laying barea root half an inch in diameter (fig. 7) I cut off theend of the root at i, and put the remaining stump(i n) into the glass tube d r, which was an inch indiameter, and eight inches long, cementing it fast atr; the lower part of the tube dz was eighteen incheslong and a quarter of an inch diameter in bore. . .Then I turned the lower end of the tube (z) upper-most, and filled it full of water, and then immediatelyimmersed the small end z into the cistern of mercuryat the bottom, taking away my finger which stopped up the end of the tube z The root imbibed the water with so much vigour, that in six minutestime the mercury was raised up the tube d z as high as 2, namely, eight inches The next morning at eight oclock the mercury was fallen
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