1.2.3 - Transplant in Plants (Xylem and Phloem) Part 13 & 14

Question 1

Describe the difference between woody and non-woody plants, in terms of the arrangement of the vascular bundle in the stem.

Answer 1

• In woody plants - The vascular bundles on the outer edges become continuous as it ages, forming a ring.
• In non-woody plants - The vascular bundles are separate and discrete.

Question 1

Describe the structure of the xylem vessels.

Answer 1

• Xylem vessels are long cells with thick walls that have lignin deposited within them.
• Lignin is waterproof, causing the cell and its contents to die. This makes the vessels hollow.
• Lignin also strengthens the walls, preventing it from collapsing.
• The walls also break down so they can line end-to-end, forming a long column.
• Incomplete lignification leaves pores (called pits).
• Wider and shorter than tracheids.
• Narrow for capillary action.

Question 2

What is the function of the phloem?

Answer 2

To transport sugars and other assimulates up and down the plant.

Question 4

What is the function of the xylem?

Answer 4

To transport water and minerals from the roots upwards.

Question 4

There are numerous mitochondria inside a companion cell. Suggest a reason for this.

Answer 4

• Companion cells must also carry out the metabollic processes for the sieve tubes.
• The mitochondria are also needed to produce ATP for active processes such as loading sucrose into the sieve tube elements (translocation).

Question 5

Draw and describe the arrangement of tissues in the cross-section of a root. Include the xylem, phloem, cortex, endodermis and epidermis.

Answer 5

• The vascular bundle is found in the centre of the root.
• Large central xylem = in the shape of an ‘X’.
• Phloem = found in between the arms of the ‘X’.
• Endodermis around the vascular bundle is made of meristem cells (pericycle).
• Cortex surrounding the endodermis contains parenchyma cells.

Question 6

How do xylem vessels differ from tracheids?

Answer 6

• Xylem vessels are shorter and wider than tracheids.
• Tracheids also link end-to-end but with overlapping, sloping ends.

Question 7

Describe the arrangements of the xylem and phloem in a transverse section of a dicotyledonous leaf.

Answer 7

• The vascular bundles form the midrib and veins.
• The leaves have a branching network of veins.
• Each vein has the xylem on top of the phloem.

Question 8

Describe the distribution of tissues in the cross-section of a stem. You must include the xylem, phloem and cambium.

Answer 8

• The vascular bundles are found on the outer edges of the stem.
• Xylem is found towards the inside of each vascular bundle.
• Phloem is found on the outside of each vacscular bundle.
• Inbetween the two, there is the cambium (a layer of meristem cells).

Question 9

Explain how the structure of the sieve tube elements helps it carry out its function.

Answer 9

• They have a wide lumen - so there are no obstructions for the flow of sugars.
• Cells are aligned with pores on the end walls - to form a tube for transportation.
• Little cytoplasm and no nucleus - little obstruction for flow.

Question 11

Describe how the xylem is adapted to its function.

Answer 11

• The xylem vessels are narrow - this helps with capillary action and prevent water columns from breaking.
• Pits allow water to move laterally between vessels.
• The patterns lignin creates in the walls allows flexibility for plant.
• No end walls for water to flow.
• No cell contents allow water-flow.
• Lignin strengthens the walls to stop the vessel from collapsing.

Question 12

Vascular bundles often contain other types of tissue. Suggest a function for these tissues.

Answer 12

These tissues give the bundles some strength and help support the plant.

Question 13

Describe the structure of the sieve tube elements in the phloem.

Answer 13

• Sieve tube elements are hollow - they have no nucleus and little cytoplasm.
• They are aligned end-to-end forming a tube for transportation.
• They also have a wide lumen.
• They are unlignified and therefore living.
• End of the walls are perforated with pores (sieve plates).
• Pores are lined with callose which seals when damaged.
• There are plasmodesmata to allow links between the sieve tube and companion cells.

Question 14

State the role of a companion cell and describe its structure.

Answer 14

• Companion cells - provide the site for processes sieve tubes cannot carry out as they have a lack of structures.
• Companion cells have a full set of organelles.
• They have a large nucleus and a dense cytoplasm.
• They also have numerous mitochondria.
• There are able to pass substances to and from the sieve tubes via plasmodesmata.