Perspiration suction

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Under transpiration is understood that force that the water at the upper end of a closed water column apparently by evaporation up suck can. In fact, the water is attracted by the adhesion to the wall of narrow tubes and this so-called capillarity can also overcome gravity . When water evaporates at the upper end of a capillary that is in a water basin, further water is pushed in from below by the atmospheric pressure . Since the atmospheric pressure is limited, water rises in this way no more than 10 meters. When water in a vertical capillary automatically rises to a height of 10 meters, the atmospheric pressure at the base of the capillary is balanced. Further suction would lead to the formation of a vacuum at the foot point , which prevents further water from rising.

According to the controversially discussed cohesion theory , however, the release of water from the leaves of a plant allows water to be transported to greater heights than the atmospheric pressure actually allows. Water evaporates through the stomata of the leaves and creates a suction that ensures that the water is drawn up from the root through the xylem vessels . Greater heights than 10 meters can be overcome against gravity because the cohesive forces of the water (in narrow capillaries) also allow the formation of a negative pressure - threads of water are practically pulled upwards.

Osmosis , i.e. the gradient of the water potential , is sufficient for water transport over large differences in altitude. Osmotic pressure is already generated in the roots (see root pressure ).

Measurements show that the transpiration thesis for water transport in plants is not consistent over large differences in height.

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Individual evidence

  1. Spektrum-Verlag : Compact Lexicon of Biology: Cohesion Theory of Water Piping , accessed on 27 Sep. 2019
  2. U. Zimmermann, A. Haase, D. Langbein, F. Meinzer: Mechanisms of Long-Distance Water Transport in Plants: A Re-Examination of Some Paradigms in the Light of New Evidence , in Philosophical Transactions: Biological Sciences vol. 341, no. 1295, The Transpiration Stream (Jul 29, 1993), pp. 19-31, accessed Sep 27. 2019