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TEXTBOOK OF PLANT PHYSIOLOGY 



sent greater difficulties than do the many meters traversed in the 

 conductive tissues. The ease with which water may be translo- 

 cated through the vessels and its slow movement from cell to cell 

 may be seen from the following simple experiment. When a suc- 

 culent, leafy stem with numerous parenchyma cells, for instance of 

 the balsam plant, is cut and placed into water, it will remain fresh 

 for a long time, as the water readily enters through the cut vessels 

 and ascends to the leaves. When the cavities of the vessels are 

 sealed up, however, by keeping for a time the cut end in liquid 

 gelatine, then cleaning the surface and replacing the stem into 

 water, the leaves and the tip of the stem will begin to wilt. This 



Fig. 80. — Diagram showing the course taken by water current in the plant. The 

 black arrows show movement of water in liquid form; the light arrows, that of 



water vapor (after Smith, et al.). 



shows the slow permeability to water of the parenchyma tissues of 

 the cortex and pith. 



The rapidity with which water will move in the vessels and in 

 the parenchyma is dependent upon the method of transport. It 

 flows through the vessels as through hollow tubes, obeying the 

 general laws of hydrodynamics. In the parenchyma cells water is 

 translocated osmotically and its movement meets with consider- 

 able resistance. 



60. The Osmotic Translocation of Water. — Imagine a vertical 

 series of cells of which the lowest dips in water, while the other cells 

 are above its surface (Fig. 81). 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 this fluid. As soon as evaporation begins, the upper 

 cell loses part of its water, the volume of the cell decreases, turgor 



