7.9 Investigating transport in Plants

Question 1

Describe how a ringing experiment is carried out

Answer 1

A section of the outer layers (protective layer and phloem) is removed around the complete circumference of a woody stem while it is attached to the rest of the plant

Question 2

What happens after a ringing experiment is begun

Answer 2

The region of the stem immediately above the missing ring of tissue begins to swell

Question 3

What does the region that was swollen contain

Answer 3

Liquid that is rich in sugars and other dissolved substances

Question 4

What happens to the region below the ring

Answer 4

It begins to die and wilts

Question 5

What do the observations of ringing experiments suggest that removing the phloem around the stem has led to

Answer 5

1. The sugars of the phloem accumulating above the ring, leading to swelling of this region
2. The interruption of flow of sugars to the region below the ring causes the death of tissues in this region

Question 6

What is the conclusion of ringing experiments

Answer 6

That it is the phloem, rather than the xylem, that is responsible for translocating sugars in plants.

Question 7

Why is the conclusion of ringing experiments made

Answer 7

Because it is the phloem that is removed, not the xylem. If the xylem was responsible for translocating sugars then you would not have extended the sugars to accumulate above the ring, or tissues die beneath the ring

Question 8

What are radioactive isotopes useful for

Answer 8

Tracing the movement of substances in plants

Question 9

What carbon isotope is used in radioactive tracer experiements

Answer 9

C-14, as CO2

Question 10

How is C-14 used in radioactive tracer experiments

Answer 10

C-14 is made into CO2, which can then be used in the atmosphere of a closed system to produce organic sugars as a result of photosynthesis of plants

Question 11

How are the radioactive sugars traced

Answer 11

Using auto-radiography

Question 12

What does auto-radiography involve

Answer 12

Taking thin cross-sections of the plant stem and placing them on a piece of x-ray film. The film becomes blackened where it has been exposed to the radiation produced by c-14 sugars.

The blackened regions seem to correspond to where the phloem tissue is in the stem. As the other tissues dont blacken the film, it follows that they dont carry sugars and the phloem is responsible for translocation

Question 13

What evidence is there that translocation occurs in the phloem

Answer 13

1. When the phloem is cut, a solution of organic molecules flow out
2. Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in the phloem after a short period
3. Removal of a ring of phloem from around the whole circumference of the stem leads to the accumulation of sugars above the ring and their disappearance beneath the ring