Phloem Flashcards
1
Q
Phloem
A
Transports assimilates (sucrose & amino acids) up and down – bidirectional flow. The sucrose is dissolved in water to form sap. The phloem consists of sieve tubes (of sieve tube elements), parenchyma (packing) & companion cells.
- Sieve tube elements – elongated sieve tube elements lined up end to end to form sieve tubes.
- They contain no nucleus and little cytoplasm, leaving space for mass flow of sap to occurs.
- Sieve plates – perforated cross-walls at ends of sieve tubes.
- Sieve plates support the tube, keeping the lumen open.
- They also serve a mechanism to block the sieve tube after injury or infection, the sieve plates
become blocked by callose deposition. - The perforations allow movement of sap from one element to the next.
- Companion cells – carry out processes so that sieve tube elements can survive.
- Large nuclei and a dense cytoplasm with many mitochondria to produce ATP needed for
active loading of sucrose. Linked to sieve tubes by plasmodesmata. - Parenchyma cells which act as packaging tissue to separate and support the vessels.
