Xylem

Xylem is a plant transport tissue that carries water from the roots to the rest of the plant

• sieve tube elements
• companion cells
• Living parenchyma cells

- Are long cells with thick walls that are impregnated with lignin

- As the cell develops, the lignin waterproofs the walls of cells

•  As a result, cells die and their end walls and contents decay
• = dead cells with no contents

- Strengthens vessel walls

- Prevents vessel from collapsing

• This keeps vessels open even at times when water may be in short supply

- Ligning thickening forms patterns in the cell wall

- Patterns may be:

• spiral
• annular (rings)
• reticulate (network of broken rings)

- This prevents the vessel from being too rigid and allows flexibility of the stem or branch

- In some places, ligninfication incomplete

• leaves pores in wall of vessel, called pits or bordered pits
• These allow water to leave one vessel and pass into another adjacent vessel, or pass into the living parts of plant

Xylem tissue can carry water and minerals from roots to the very top of plant because:

• made from dead cells- forming continuous column
• narrow tubes- water column does not break easily and capillary action effective
• pits in lignified pools- allow water to move sideways from one vessel to another
• patterns with deposited lignin- xylem can stretch as grows and stem can bend

Flow of water not impended because:

• no end walls
• no cell contents
• no nucleus or cytoplasm
• lignin thickening prevents walls from collapsing