PASSAGE OF WATER 73 



more or until the counter pressure of the stretched walls 

 equals the osmotic pressure (i.e. the power with which, 

 under the given difference in density of the outer and 

 inner solutions, the water from the outside tends to 

 enter the cell). Such a water-distended cell is said to be 

 turgid or in a state of turgor. The pressure within it 

 may equal several atmospheres. Jost gives this pressure 

 for some desert plants as equalling one hundred atmos- 

 pheres, i.e. about 1500 pounds per square inch. 



107. If a cell be in contact with a plentiful water 

 supply, it will become as turgid as the difference in 

 osmotic pressure outside and inside will permit. If a 

 cell adjacent to it is not in contact wdth the external 

 water, there will be a passage of water from one cell to the 

 other, the direction depending upon which cell has the 

 denser solution in its cell sap. Thus, in a plant with one 

 part exposed to evaporation into the air and with 

 the other part in water there will be a constant passage 

 of water into the plant and up through it from cell to 

 cell, by osmosis, and out into the air by evaporation from 

 the wet surface of the cell walls. 



108. In larger land plants, however, this rather slow 

 passage of water from one cell to another b}^ osmosis is 

 too slow to supply the aerial parts with the requisite 

 amount of water. Such plants possess special elongated 

 cells no longer living and often with the separating 

 partitions dissolved out, viz. : the tracheae and tracheids. 

 (See paragraphs 46 to 49.) These serve as tubes 

 through whi(^h the water rises, not as a simple diffusion 

 of molecules but with a mass motion, i.e. as a definite 

 current carrying with it whatever miiy be dissolved. 



109. In these plants then we can trace the water 

 through the following steps of progress. It enters the 

 root hairs by osmosis from the surrounding soil where it 



