. Botany of the living plant. Botany. THE W^ATER-RELATION ^9 been confirmed by determinations ot the osmotic pressures in tiie saps oi various leaves. These pressures have always been found adequate to resist the ; On the other hand there is also a propellent factor which may act frotn below. If a rooted stem be cut a few inches above the soil on a warm day, and the cut surface be immediately examined with a lens, it will appear dry. A drop of water placed upon it will at first be absorbed. But in a short time it will be found that water exudes from the lower cut sur
. Botany of the living plant. Botany. THE W^ATER-RELATION ^9 been confirmed by determinations ot the osmotic pressures in tiie saps oi various leaves. These pressures have always been found adequate to resist the ; On the other hand there is also a propellent factor which may act frotn below. If a rooted stem be cut a few inches above the soil on a warm day, and the cut surface be immediately examined with a lens, it will appear dry. A drop of water placed upon it will at first be absorbed. But in a short time it will be found that water exudes from the lower cut surface, coming out from the cavities of the vessels. If a tube be fitted upon it with a tight joint, the fluid exuded will collect and rise in it. In the case of a potted Fuchsia plant about three feet high it has been seen to rise in a few days to a height of as nmch as 20 feet. Or a tube may be fitted as in Fig. 66^ by which the pressure from below may be measured in inches of a column of mercury forced up by it. This demonstrates Root-Pressure, a second factor which may influence the movement of the Transpiration-Stream. These two factors can act simultaneously, though both vary independently. In the daytime normally the suctional influence is the greater, and this explains the fact that ,^,,aâge,,cnf""iofâ dciuonstratiug when first cut the surface of the stump root-pressure. The tube g is fitted on ^ the cut stem s. Water is absorbed appears dry, and water exudes only after by the roots and forced into the tube, cr J i J ^^^ (.jjg pressure can be measured ui some time has elapsed. The continuous terms of the height of a mercury . column {Q), which it is able to raise. movement of the Transpiration-Stream is s. chiefly due to these two factors, though it is possible that others participate with them in the case of tall trees. The water of the Transpiration-Stream is not pure water. Ihis may readily be showm by evaporating some of the water extruded upon a clean gla
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
