324 



PLANT PHYSIOLOGY 



end in liquefied gelatin gel, cleaning the cut surface, and replacing 

 the stem in water, the leaves and the tip of the stem will begin 

 to wilt. This plugs the vessels with gelatin. This demonstrates 

 the slow penetration of water through the parenchyma tissues of 

 the cortex and pith, which are not filled with gelatin. 



The difference in the rate at which water will move in the 

 vessels and in the parenchyma is dependent upon the method of 

 transport. It flows through vessels as through hollow tubes, 

 obeying the general laws of hydraulics. In the parenchyma 

 cells, water is translocated osmotically, and its movement meets 



with considerable resistance. 



Imagine a vertical series of cells of which 

 the lowest dips into water, while the other 

 cells are above its surface (Fig. 103). In 

 order to simplify the scheme, assume that 

 only the upper cell evaporates water, while 

 the other cells are protected against water 

 loss by impermeable lateral walls. When 

 all of the cells are saturated with water, 

 _ _ _ _ _ _ there will be no movement of water. As 



"^ 'ri^'"' "" soon as evaporation begins, the upper cell 



Fig. 103. — Diagram il- ^ . 



lustrating translocation loses part of its Water, the volume of the 

 of water in leaf paren- ^.^jj decreases, turgor pressure becomes less, 



chyma. 7 o ± ^ 7 



and, as a consequence, suction tension 

 develops (Art. 6). 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 

 container, 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 their suction 

 tensions and not in the absolute magnitude of their osmotic 

 pressures. Let the osmotic pressure of the first cell be 10 atmos- 

 pheres, 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 surplus of pressure in the second cell 

 is balanced by the greater tension on its walls. But as soon as 

 suction tension rises in the upper cell, it begins to draw water 



