4.11 Plant Stem Structure

Question 1

What are the three parts of plant stems you need to know?

Answer 1

• xylem vessels
• sclerenchyma fibres
• phloem tissue

Question 2

What is the function of the xylem?

Answer 2

• to transport water and mineral ions up the plant - transpiration
• to provide support for the plant

Question 3

What is the structure of the xylem?

Answer 3

• column of dead cells acting as tubes joined end to end to make an uninterrupted vessel
• pits in the tubes allow movement of substances between vessels
• tubes found in bundles

Question 4

What is the xylem made of?

Answer 4

• dead cells that are longer than they are wide, have hollow lumen, and have no cytoplasm
• the dead cells have no or highly perforated end walls to form an uninterrupted tube, allowing water and mineral ions to pass through easily
• lignin and cellulose microfibrils in the cell walls strengthen the tubes and hence support the plant

Question 5

What is the function and position of lignin in the xylem?

Answer 5

• waterproofs the cell walls by restricting the entry of water and solutes
• stiffens the cell walls which provides the plant with greater tensile strength
• it is laid down in spirals/rings in the cell walls

Question 6

How is the xylem formed?

Answer 6

• the tonoplast is broken down
• the cell organelles, cytoplasm, and cell surface membrane are also broken down by the action of enzymes - this is known as autolysis
• the leaves behind dead empty cells that form a tube

Question 7

What is the function of the sclerenchyma?

Answer 7

to provide support for the plant

Question 8

What is the structure of the sclerenchyma?

Answer 8

• made of bundles of dead cells that run vertically up the stem like the xylem
• cells are longer than they are wide and have a hollow lumen
• cells have end walls and no pits in the cell wall, unlike the xylem
• cell walls are lignified for support and have more cellulose than other plant cells (more cellulose microfibrils for support)

Question 9

What is the function of the phloem?

Answer 9

to transport organic solutes (such as sugars and amino acids) around the plant - aka translocation
(have no role in supporting the plant)

Question 10

What is the structure of the phloem?

Answer 10

• living cells arranged in tubes called phloem sieve tubes
• two types of cells: sieve tube elements and companion cells

Question 11

What is the structure and function of sieve tube elements in the phloem?

Answer 11

• are living cells joined end to end to form sieve tubes
• contain perforated end walls called sieve plates that allows solutes to pass through
• have no nucleus, a very thin layer of cytoplasm, and few organelles (unusual for living cells)
• the cytoplasm of the cells are connected by the perforated sieve plates

Question 12

What is the structure and function of companion cells in the phloem?

Answer 12

• the sieve tube elements cannot survive on their own (due to lack of nucleus and other organelles) so require a companion cell
• companion cells have a nucleus, mitochondria, ribosomes, and RER unliked the sieve tube elements
• they carry out metabolic functions for both the sieve tube elements and themselves (like providing energy for the active transport of solutes)

Question 13

How are the xylem, sclerenchyma, and phloem positioned in the stem?

Answer 13

• the xylem and phloem are found in vascular bundles
• the sclerenchyma fibres are just outside the vascular bundle
• order from outside to inside stem: sclerenchyma -> phloem -> xylem