Xylem and phloem compared

The two complex tissues run together, look similar in a section, and do opposite jobs. Eight rows separate them completely.

Assignment 1.1

Compare the structural and functional characteristics of xylem and phloem tissues. Present them in a suitable table. Try it on paper first, then check it against the one below.

The comparison

Feature	Xylem	Phloem
Cell types	Vessel cells, tracheids, fibres, parenchyma	Sieve tube cells, companion cells, parenchyma, fibres
Living or dead	Vessel cells, tracheids and fibres dead; parenchyma living	Sieve tube cells, companion cells and parenchyma living; fibres dead
The conducting cell	Xylem vessel — cross walls completely dissolved, an open tube	Sieve tube — cross walls incompletely dissolved, a sieve plate at each join
Nucleus in the conducting cell	Absent — the cell is dead	Absent, although the cell is alive; the companion cell’s nucleus controls it
The cell wall	Lignin deposited on cellulose in the conducting cells	Cellulose, and thin
What it carries	Water with dissolved mineral salts	Food made in the leaves, mainly sucrose
Where it carries it	From the roots, upward, throughout the plant	From the leaves, throughout the plant — translocation
Does it support the plant?	Yes — this is its second function	No — phloem has one function only

Row 4 is the row worth looking at twice

Neither conducting cell has a nucleus, and for completely different reasons. The xylem vessel cell has none because it is dead. The sieve tube cell has none although it is alive, and borrows one from the companion cell beside it. Writing that both lack a nucleus is true and explains nothing.

Water up, food everywhere

Xylem carries water and mineral salts up from the roots and supports the plant; its conducting cells are dead, with open ends. Phloem carries food from the leaves throughout the plant; its conducting cells are alive, with sieve plates, and it does not support.