7th Grade Science Student ebook

UNIT 5 STRUCTURE AND FUNCTION IN LIVING THINGS

Figure 14.15: Xylem tissues carry

water from roots to leaves through

capillary action and transpiration.

transpiration - the loss of water

through the stomata.

Movement of fluids in plants

The vascular

system of plants

The vascular tissues form a network of tubes that carries water

and nutrients throughout the plant. The vascular system of a

plant is a bit like your circulatory system which carries fluids

throughout your body. But plants don’t have a heart to pump

fluids throughout their bodies. Water enters the plant through the

roots by osmosis. But osmosis cannot push water throughout the

rest of the plant. In vascular plants, xylem cells are joined to form

continuous tubes. Water moves through the xylem by two forces—

capillary action and transpiration (Figure 14.15).

Water moves through the xylem by capillary

action and transpiration.

Capillary action If a thin tube is placed in a cup of water, the water will rise up the

tube in a process called capillary action. Water molecules are

strongly attracted to each other and attracted to surfaces. Those

attractive forces act together to pull water molecules up a thin

tube—like the tubes formed by the xylem of a plant.

Transpiration A stronger force is produced by a process known as transpiration.

Transpiration is the loss of water through the stomata. When the

stomata are open, the plant is able to obtain carbon dioxide for

photosynthesis. Oxygen produced during photosynthesis exits

through the open stomata, along with water vapor. As water exits

the stomata, it draws more water out of the xylem. The strong

attraction of water molecules to each other creates a pull of water

molecules throughout the xylem, similar to a train engine pulling

cars along. When the stomata are closed, transpiration stops.

About 10 percent of the water vapor in Earth’s atmosphere comes

from plants through transpiration.