TRANSLOCATION OF WATER 



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pressure becomes less, and, as a consequence, suction tension 

 develops (Art. 42). The upper cell now begins to draw water 

 from the one beneath it, which up to that moment was saturated 

 with water and, therefore, showed no suction tension. The loss of 

 water creates a suction tension in the second cell and it begins to 

 draw water from the third, and so on, until the lower cell is 

 reached. Then this cell begins to absorb water from the con- 

 tainer, in which its lower part is immersed. 



It must be remembered that the moving force that drives the 

 water current from cell to cell is the difference in the suction 

 tension and not in the absolute magnitude of the osmotic pressure. 

 Let the osmotic pressure of the first cell 

 be 10 atmospheres, for instance; and of 

 the second, 20 atmospheres. As long as 

 they are saturated with water they are in 

 equilibrium with one another, as the sur- 

 plus of pressure in the second cell is 

 balanced by the greater tension of its walls. 

 But, as soon as suction tension arises in 

 the upper cell, it begins to draw water 

 from the lower one, in spite of the fact that 

 osmotic pressure is greater in the second 

 cell. In order to make the water current, 

 induced by evaporation in the upper cell, 

 move through a series of cells, suction 



tension must be distributed in these cells in regularly decreasing 

 order. In the upper cell, suction tension must be the greatest; 

 in the following cell it must be less; in the next, still less, etc. 

 The least suction tension must be in the lowest cell which absorbs 

 water from the surrounding medium. 



The difference in suction tension of two neighboring cells is 

 determined by the resistance met by the water current in passing 

 from one cell into the other. This resistance, in its turn, depends 

 on the rate of the water current. It increases as the rate of the 

 water current augments. Ursprung was able to trace the increase 

 of suction tension in the cells of the palisade parenchyma of the ivy 

 leaf, in proportion as these cells were farther from the large fibro- 

 vascular bundles supplying them with water. In the third cell 

 from the bundle, he found a suction tension equaling 12.1 atmos- 

 pheres; in the two hundred and tenth cell, 32.6 atmospheres. In 



Fig. 81. — Diagram illus- 

 trating translocation of 

 water in leaf parenchyma. 



