Text-book of structural and physiological botany . distribute themselves to themost dehcate parts of the leaves, where, in a manner compar-able to the perspiration of animals, the water again disappearsin the form of vapour, by transpiration. It has been ascer-tained that in actively grow-ing plants the quantity ofwater thus absorbed oftenamounts in a few days tomany times the weight andvolume of the whole woody parts of thevascular bundles serve asconduits for this powerfulstream, as may easily beproved in plants fromv/hich the bark has beenpeeled off, and whose pithhas decayed, by
Text-book of structural and physiological botany . distribute themselves to themost dehcate parts of the leaves, where, in a manner compar-able to the perspiration of animals, the water again disappearsin the form of vapour, by transpiration. It has been ascer-tained that in actively grow-ing plants the quantity ofwater thus absorbed oftenamounts in a few days tomany times the weight andvolume of the whole woody parts of thevascular bundles serve asconduits for this powerfulstream, as may easily beproved in plants fromv/hich the bark has beenpeeled off, and whose pithhas decayed, by causing theroots to suck up a colouredinfusion. The chief causesof these great movements ofwater are : capillarity, anddiffusion \osmose~\; but anaction of suction may alsobe distinguished from oneof propulsion. In everypart of a plant where growthor evaporation is takingplace, the part which re-quires water will obtain itfrom the surrounding parts, and as this suction gradually ex-tends to larger and larger circles, the most distant parts, Fig. 343.—Apparatus for measuring the root-pressure. A manometer, a, b, c, is fixedinto the upper part of the plant p growingin a pot, the top having been cut oif. Thedifference in the height of the mercury inthe two arms b and c indicates the intensityof the pressure by which the water suckedup by the roots is forced out at the cutsection. The Life of tlie Plant. 161 in general the roots, are at length compelled to absorb in conjunction with this force is the roots absorb a greater quantity of water than the plantrequires ; and this, therefore, exercises a pressure whichdrives the water that has been already absorbed higher andhigher up the stem ; and often, as in grasses, Aroideae, &:c,,even causes it to exude in drops at the margins and tips ofthe leaves. Experiments on the intensity of the root-pres-sure, performed by cutting off the upper part of the stem,and attaching a manometer to the section, s
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Keywords: ., bookcentury1800, bookdecade1870, bookpublishernewyorkjwileysons
