Phloem and Xylem

Phloem
composed of sieve tube elements, companion cells, parenchyma, sclereids and fibres

sieve tube element:
tubular cell
fuse together end to end to form continuous tube--sieve tube
Its maturity:

• nuclear degenerates
• cytoplasm restricted to the edge of the cell
• end wall perforated--sieve pore
• keep alive by companion cell<---connected via plasmodesmata

Xylem
composed of tracheary elements, fibres and parenchyma

Tracheary elements:
Elongated cells that transport water and minerals.
provide high mechanical strength to withstand the high tension of the uprising columns of water give support to the plant

two kinds: -----vessel element -------------- tracheid
different-aaaa------shorter-------------------longer
aaaaaaaaaaaaa---larger lumen---------------smaller

common- Thick and highly lignified
----------When extensively lignified, it would be deprived of water,
----------nutrient supply and die off, cell content degenerates
---------- leaving empty lumen to facilitate the transport of water
-----------and minerals.

vessel element:
align as hollow tubes called xylem vessel
primary wall area covered by secondary wall material--

• first occur as rings or helices called annular thickening and spiral thickening
• next more extensive deposit of lignin reticulate thickening and pitted thickening

Tracheid:
similar to sclerenchyma
tapered end wall overlapping each other
passage of water from cell to cell is facilitate by bordered pits, through pit membranes


Fibres
similar to sclerenchyma but with thicker cell walls
do not involved in conduction of water
provide additional mechanical support