mits only a single ion such as potassium
(along with analogs from the Periodic Table)
in controlled quantities. There are ion chan-
nels for potassium, calcium, magnesium, etc.
(See reference)
In the usual situation, a root is located
in soil, which has at least some water in it.
Roots absorb water through root hairs and as-
sociated fungi called mycorrhiza. (Mycorrhiza
will be ignored in the sequel because, for our
purposes, they act as simple extensions of
the root hairs.)
Absorption by root hairs is done via
the process of osmosis. In osmosis, a semi-
permeable membrane separates two com-
partments with (possibly) different solutions
containing various dissolved substances
(solutes). In osmosis, water moves from the
compartment with a lower solute concentra-
tion to the compartment with a higher solute
concentration.
Here the effect of a solute (sugar in this
case) is to “pull” water from the pure water
compartment into the compartment contain-
ing sugared water.
In the case of plant roots, each root is
provided with a large number of cellular pro-
trusions called root hairs, which extend from
the root into the soil as shown here:
The liquid portion of soil, called the soil
solution, contains both water and dissolved
materials of various kinds. Much of this water
is absorbed into or adsorbed onto the surface
of soil particles. The rate of water absorption by the
plant is proportional to the difference in solute con-
40
centration between the root cytoplasm and the soil
solution. Soil solution contains, in addition to water,
various dissolved minerals plus exudates from plants,
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May 2018