The plants that have really
taken over the land environment are the ones that have been
able to literally rise above the others, the
vascular
plants. Because of their internal
distribution system of tubes, these plants can get water to the tops
of tall trees. Water moves
into the plants from the soil by diffusion,
but the force generated by diffusion will only move the water until
gravity stops it. The force
of water diffusing into the bottom of a plant generates
a force called root pressure, but that's only
good for maybe a meter's rise, probably
less. Adhesion,
the attraction of water molecules to the molecules
of the plant, will help water climb a plant's
tubes, but also not very far. So how's it get up to where
it's needed in the taller vascular plants?
Water needs to get
to the the
leaves so it can be used in photosynthesis, but the leaves
also need carbon dioxide from the atmosphere for the
process. When pores are
open to allow CO2 in, water also evaporates out,
a process called
transpiration. But
as water is used and lost, more water is drawn up the
tube system through
xylem, tough thick-walled
tubes from the roots. At this point, cohesion
becomes critically important: water, trapped in a tube, holds
together molecule-to-molecule (they are literally like tiny magnets
locked together) and exerts a force that will hold a narrow
column together for significant heights.
The other
part of the tube system, the
phloem,
carries sugary water from the leaves down to the parts of
the plant that will either store the fuel, often as simple
starches, or use the fuel because they don't themselves
photosynthesize (like roots and brown stems).
|