RELATION OF WATER TO THE PROTOPLASM 67 
present in any particular cell will depend upon the 
behaviour of the protoplasm from time to time. Such 
substances are usually being continually produced in all 
growing Cells, and in most others in which chemical 
changes are proceeding. Hence such cells are continually 
absorbing water, and are consequently so full that a con- 
siderable stretching force is exerted on the cell-wall which 
bounds them. Cells in such a condition are called turgid, 
and the condition itself is known as turgor or turgescence. 
The equilibrium which is attained by such a cell is reached 
when the distension caused by the entering osmotic stream 
is balanced by the elastic recoil of the extensible cellulose 
wall. In some cases the tension set up in a tissue by the 
turgescence of the cells is sufficient to force the water, by a 
process of filtration, through the walls of the outermost 
ones, so that it escapes in drops or in a slow stream. This 
may often be seen on the edges or apices of blades of grass 
in the early morning. It is of great use also in forcing 
water into the axial woody cylinder of roots, as will appear 
later. Occasionally the turgescence becomes so great as to 
lead to rupture of the cell-walls, as is the case in some 
pollen grains, and sometimes in certain fleshy fruits. 
That the condition of turgescence in cells is attended by 
a stretching of the cell-walls can be seen by taking a piece 
of a plant which is turgid, such as the stalk of a rhubarb 
leaf, and, after carefully measuring its dimensions, steeping 
it for some time in a ten per cent. solution of common salt. 
On removing it, it will be found to have become flaccid, 
and a remeasurement will show that both its length and 
thickness have diminished. Turgescence is not, however, 
due simply to physical causes ; the protoplasm which lines 
the cell has a regulating influence over the passage of the 
water into and out of them. When a turgid pulvinus of 
such a plant as Robinia or Mimosa is stimulated by rough 
handling of the leaf, the latter falls backward from its 
expanded position, and the fall is found to be due to the 
escape of water from the cells of the lower side of the pulvinus. 
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