How Plants Obtain Water

Most gardeners know that plants acquire water and minerals through
their root systems, and leave it at that. But the process is not
quite that simple. The actively growing, tender root tips and almost
microscopic root hairs close to the tip absorb most of the plant's
moisture as they occupy new territory. As the root continues to
extend, parts behind the tip cease to be effective because, as soil
particles in direct contact with these tips and hairs dry out, the
older roots thicken and develop a bark, while most of the absorbent
hairs slough off. This rotation from being actively foraging tissue
to becoming more passive conductive and supportive tissue is
probably a survival adaptation, because the slow capillary movement
of soil moisture fails to replace what the plant used as fast as the
plant might like. The plant is far better off to aggressively seek
new water in unoccupied soil than to wait for the soil its roots
already occupy to be recharged.
A simple bit of old research magnificently illustrated the
significance of this. A scientist named Dittmer observed in 1937
that a single potted ryegrass plant allocated only 1 cubic foot of
soil to grow in made about 3 miles of new roots and root hairs every
day. (Ryegrasses are known to make more roots than most plants.) I
calculate that a cubic foot of silty soil offers about 30,000 square
feet of surface area to plant roots. If 3 miles of microscopic root
tips and hairs (roughly 16,000 lineal feet) draws water only from a
few millimeters of surrounding soil, then that single rye plant
should be able to continue ramifying into a cubic foot of silty soil
and find enough water for quite a few days before wilting. These
arithmetical estimates agree with my observations in the garden, and
with my experiences raising transplants in pots.