9.1 Transport in the Xylem of Plants

Structures of a Leaf

• Upper Epidermis → Protection, prevents water loss

• Palisade Mesophyll → Photosynthesis

• Spongy Mesophyll → Gases, photosynthesis

• Stoma → Pores for gas exchange

• Xylem → Channel for water, made of hollow, dead cells reinforced with lignin rings

• Phloem → Living cells for glucose and organic molecule transport

• Guard Cells → Control opening of the stoma

Abiotic Transpiration Factors

• Temperature → as temperature increases rate of transpiration increases as evaporation and the point of saturation of the air increase

• Wind Speed → as wind speed increases rate of transpiration increases as wind moves humid air away from the leaf, maintaining a humidity gradient

• Humidity → as humidity increase rate of transpiration decreases as the gradient is decreased

Mineral Ions

taken into the root via active transport, raising the solute concentration in the root and increasing osmosis

Xerophytes

• Plants adapted for arid conditions

• Saguaro Cactus → thicker waxy cuticle, stomata in pits, reduced leaves

• Marram Grass → low to the ground, rolled leaves with stomata on the inside

Halophytes

• Plants adapted for saline conditions

• Glasswort → altered flowering schedule, sequestration of ions in vacuoles, restricted ion entry at the root, excretion of salt through glands

Capillary Tubing

• Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)

• This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface

• The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm) 

Filter Paper

• Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties

• When placed perpendicular to a water source, the water will hence rise up along the length of the paper

• This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)

Porous Pots

• Porous pots are semi-permeable containers that allow for the free passage of certain small materials through pores

• The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves of plants

• If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative pressure that draws more liquid

Potometer

• Device used to estimate transpiration rates

• Distance moved by an air bubble is recorded every minute to indicate rate of water uptake