In fact, they are active
only when they are in actual process of growth. It follows,
therefore, that water absorption occurs near the tips of the growing
roots, and whenever a plant ceases to grow the water absorption
ceases also. The root-hairs are filled with a dilute solution of
various substances, as yet poorly understood, which plays an
important tent part in the ab sorption of water and plant-food from
the soil.
Owing to their minuteness, the root-hairs are in most cases immersed
in the water film that surrounds the soil particles, and the
soil-water is taken directly into the roots from the soil-water film
by the process known as osmosis. The explanation of this inward
movement is complicated and need not be discussed here. It is
sufficient to say that the concentration or strength of the solution
within the root-hair is of different degree from the soil-water
solution. The water tends, therefore, to move from the soil into the
root, in order to make the solutions inside and outside of the root
of the same concentration. If it should ever occur that the
soil-water and the water within the root-hair became the same
concentration, that is to say, contained the same substances in the
same proportional amounts, there would be no further inward movement
of water.
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