Lesson 3

Question 1

What are xylem vessels?

Answer 1

Continuous column of dead cells arranged from end to end, with completely dissolved cross-walls to form long hollow tubs that transport water from root to leaves

Question 2

Describe differences between tracheids and vessels

Answer 2

Tracheids are more elongated with tapering ends

Question 3

Why do vessels and tracheids need to be thickened with lignin?

Answer 3

• Impermeable to water do solutes and water molecules can’t leak out
• Water can travel in one unbroken stream
• Also prevents vessels from collapsing under pressure

Question 4

What are pits?

Answer 4

Holes in vessels and tracheids which allow movement between adjacent vessels

Question 5

Describe structure and purpose of sieve tubes

Answer 5

• Transports photosynthates (sucrose and amino acids) up and down plant cell
• Formed from cells called sieve elements placed end to end
• Thin cellulose wall at end of these cells perforated to form sieve plates acting like plasmodesmata

Question 6

Why are companion cells important?

Answer 6

Because companions cells have nucleus and can provide ATP for sieve cells