Xylem Flashcards
What is Xylem
The main tissue transporting water around a plant
What is protaoxylem
First xylem, which can grow and stretch as walls haven’t been fully lignified
What is metaxylem
Consists of mature xylem vessels made of lignified tissue
Describe general structure of xylem
Continuous tube with no living contents
Lignin deposited in walls
Bordered pits to allow water to move out
Evidence for movement of water through Xylem
If cut end of shoot is in eosin dye, dye can be seen through transport stream
Autoradiography
Ringing bark kills phloem but not xylem
Describe autoradiography
Water containing deuterium
Taken up in normal way
Identified and tracked with photographic film
Describe symplast pathway
Water moves by diffusion from root hair cells to xylem through symplastic pathway
It moves through plasmodesmata
Describe apoplastic pathway
Water moves through cellulose cell walls from roots
Due to loose open structure of cellulose
Reaches endodermis
Casparian strip is present, which is waterproof
Therefore water has to enter symplastic pathway from here
Cohesion-Tension theory
Theory of transpiration
Water molecules are dipolar
Stream of water molecules due to cohesive forces
When water molecule is lost by evaporation tension is created in column
Another molecule of water pulled up to replace it
Adhesion
Water molecules stick to xylem
Adhesion is attraction between unlike molecules
Transpiration stream
Root hair cells
Xylem
Leaf
Spongy mesophyll
Stomata
Root Pressure
When transpiration rate is low
Active transport pumps in ions
This creates gradient which draws water in
