the Water mode of plants. Rising of water in trunks of high trees

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Long time the mechanism of rising of water to tops of high trees remained a riddle for phytophysiologists, however now we represent it to ourselves well enough. This problem seemed so difficult as it was known that under the influence of atmospheric pressure the water column can rise not above than on 10 m, and highest of trees reach in height more than 100 m. It was necessary to admit, hence, that still any force exceeding force of atmospheric pressure approximately in 10 times here operates. Now we know that it not that other, as the sucking force of atmosphere generated in the lowest size of its water potential.

We Will remind that the water potential of atmospheric air at relative humidity <100% makes negative size of an order of several hundreds bar. The low water potential of atmosphere causes water loss on transpiratsiju through leaves and continuous moving of molecules of water in a tree trunk from a cage to a cage - on a potential gradient. Loss (evaporation) of molecules of water in the top part of the water column filling ksilemu, forces water a leak on tubes ksilemy upwards for completion of this loss. this caused transpiratsiej water moving, so-called transpiratsionnyj the current, in turn causes water inflow in a plant from soil, also on a gradient of water potential. Owing to tranopiratsii water potential in the top part of a plant more low, than at its basis; besides, quite often speed of evaporation of water is more than speed of its receipt because also cellular walls, and endoderma a root show resistance to water movement. Thereof in the water columns filling ksilemu of high trees, there is a negative pressure, i.e. A tension. To find out it it is easy. If a small cup from forming clay to fill with ink and to press outside to a tree trunk, and then to make on a trunk a cut so that to cut in this place vessels ksilemy ink quickly will be involved in a trunk and will rise on it upwards on some metres. Negative pressure shows also a manometre entered into a tree trunk. It is possible also, measuring diameter of a trunk, to track its daily fluctuations and to be convinced that minima coincide with the periods strengthened transpiratsii.

In due time the opinion was expressed that if in such conditions in water columns there was a tension in them it would be observed kavitatsija - formation of the cavities filled only with water steams and consequently blocking moving of water from below upwards. It is convincingly shown, however that in the pure glass tube filled with water, not containing any dissolved gases, kavitatsii does not occur even at a tension in some hundreds bar; interferes kavitatsii kogezija - coupling of molecules of water under the influence of forces of an attraction. In vessels ksilemy water molecules are connected not only with each other, at the expense of forces kogezii (couplings), but also and with walls of vessels (very much gidrofilnymi) - at the expense of adhesion (sticking); these forces too interfere kavitatsii in vessels ksilemy at usually observed tension.

To Show this mechanism, i.e. prisasyvajushchee action transpiratsii in a combination to cohesive forces of a water column at a water tension, is possible by means of following experience. In the top part of the glass tube filled with water, densely fix cut off oblistvennuju a branch, and the bottom end of a tube immerse in mercury. If glass pure and water does not contain some the dissolved gases mercury owing to transpiratsii leaves can rise in a tube on height more than 760 mm in a tube, i.e. Above that level to which it rises in a barometre at atmospheric pressure in 1 atm. This system purely physical; it does not depend at all on vital properties of vegetative cages. Really, transpiratsija occurs, even if stalk cages are killed by steam if only leaves only have not been killed. In this device it is possible even to replace a branch with any evaporating surface, for example a plaster piece. This convincing experience strengthens our confidence what the combination transpiratsii, kogezii and tension causes rising of water in trunks of high trees (fig. 6.16).