In
many plants this stream of water does not simply pass from cell to
cell, but moves through tubes that apparently have been formed for
the specific purpose of aiding the movement of water through the
plant. The rapidity of this current is often considerable.
Ordinarily, it varies from one foot to six feet per hour, though
observations are on record showing that the movement often reaches
the rate of eighteen feet per hour. It is evident, then, that in an
actively growing plant it does not take long for the water which is
in the soil to find its way to the uppermost parts of the plant.
The work of leaves
Whether water passes upward from cell to cell or through especially
provided tubes, it reaches at last the leaves, where evaporation
takes place. It is necessary to consider in greater detail what
takes place in leaves in order that we may more clearly understand
the loss due to transpiration. One half or more of every plant is
made up of the element carbon. The remainder of the plant consists
of the mineral substances taken from the soil (not more than two to
10 per cent of the dry plant) and water which has been combined with
the carbon and these mineral substances to form the characteristic
products of plant life.
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