TOPIC 3 Practicals

Question 1

What is the diameter of a tree trunk and rate of water flow like in the middle of the day?

Answer 1

• low diameter of tree trunk
• high rate of water flow (greatest rate of transpiration)

Question 2

Why does diameter of tree trunk decrease in the day?

Answer 2

• transpiration is at its greatest
• more tension (more negative pressure) in the xylem
• pulls walls of xylem vessels inwards
• causes trunk to shrink in diameter

Question 3

What happens when xylem vessel is broken and air enters it?

Answer 3

tree can no longer draw up water - continuous column of water is broken, so water molecules can no longer stick together

Question 4

Does water leak out when xylem vessel is broken?

Answer 4

no - air is drawn in as it is under tension

Question 5

How can you measure the rate of water loss in a plant?

Answer 5

using a potometer

Question 6

Give 3 key points of setting up a potometer

Answer 6

• cut leafy shoot under water
• seal all joints with waterproof jelly
• set up potometer underwater and allow to fill completely with water

Question 7

How do you reset a potometer?

Answer 7

• open tap on reservoir
• push syringe down until bubble is pushed back to start of scale on capillary tube

Question 8

Give 3 examples of conditions that could be changed when measuring rate of water loss from a plant

Answer 8

1. temperature
2. humidity
3. light intensity

Question 9

How do you set up a woody stem for a ringing experiment?

Answer 9

section of outer layers (protective layer and phloem) is removed from the complete circumference of the stem

Question 10

What is found in the region of the stem above the missing ring of tissue in a ringing experiment?

Answer 10

• stem swells
• samples of liquids in swollen region are rich in sugars and dissolved organic substances
• tissues below the ring die

Question 11

What do the observations of the ringing experiment show?

Answer 11

• sugars of the phloem are accumulating above the ring (causing the swelling)
• interruption to flow of sugars to region below ring has caused death of tissues

Question 12

What does the ringing experiment prove?

Answer 12

phloem (not xylem) is responsible for translocating sugars in plants

Question 13

How is radioactive 14C incorporated into sugars during photosynthesis?

Answer 13

• 14C used to make radioactively labelled carbon dioxide
• if plant grown in atmosphere of 14CO ₂ the 14C isotope will be incorporated into sugars produced during photosynthesis

Question 14

How does a tracer experiment work?

Answer 14

• trace radioactive 14C in sugars in plant
• take thin cross sections of plant stem
• place on x-ray film
• film = blackened where it has been exposed to radiation produced by 14C in sugars
• blackened tissue corresponds to where phloem tissue is in stem
  = phloem alone is responsible for translocation of sugars