Ho does water get transported through a stem of a
plant or flower.
A simplified answer:
Vascular plants (plants with tubes) have two sets of tubes that
transports materials throughtout the plant's tissues. "Xylem" tubes
carry water primarily and are designed to facilitate water upward
from the roots. "Phloem" tubes carry sugar and other materials made
with photosynthesis dissolved in water downward to feed the plant's tissues.
The mechanisms that allow water to move against gravity are
interesting. Xylem tubes act a little like paper towels absorbing
water against gravity. The roots concentrate absorbed water and
create a pressure that helps force water upward. This is why cutting
a rooted plant often shows signs of "bleeding". The main mechanism
is the evaporation of water from the leaves of most plants. Water is
polar and sticks together to a certain point and when water molecules
evaporate, they pull on the water molecules behind them forming a
chaining effect all the way to the roots.
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Update: June 2012