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 with 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 by 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 which 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 may 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 
