Plant stems

what are the different parts of the stem?

vascular bundles containing the xylem and the phloem

sclerenchyma fibres

structure of the xylem vessel?

• very long tubes

• made from dead cells joined end to end

• hollow lumen - no cytoplasm

• no end walls - uninterrupted tube allows water and mineral ions to pass

• walls thickened with lignin to support the plant

• pits in the cell walls where there is no lignin - water and mineral ions can transport across

function of the xylem vessel

• transports water and mineral ions up the plant

• provides support

structure of sclerenchyma fibres

• made of bundles of dead cells

• hollow lumen

• have end walls

• thickened cell walls with lignin

• no pits

• more cellulose than other cells

function of sclerenchyma fibres

• to provide support

• nothing to do with transport

structure of phloem tissue

• contains sieve tube elements and companion cells

• sieve tube elements are living cells joined end to end and forms sieve tubes

• sieves have lots of holes to allow solute to pass

• sieve tube elements have no nucleus, very thin layer of cytoplasm joined to adjacent cells through the holes in the sieve cells, few organelles

• they need companion cells as they have no nucleus and can’t survive on their own

• companion cells carry out living functions for both themselves and the sieve cells (energy for active transport)

function of phloem tissue

• transport organic solutes from where they are produced to where they are needed (translocation)

• nothing to do with support/structure

• active transport

the position of the plant fibres in the stem

• xylem and phloem cell in vascular bundles with phloem on the outside

• sclerenchyma cells associated with vascular bundles on the outside of them

transverse vs longitudinal cross section

• transverse = sections cut through each structure at a right angle to its length

• longitudinal = section taken along the length of a structure

how to dissect plant stems

• use scalpel and cut thin cross section (transverse or longitudinal)

• use tweezers to place section in water to prevent them from drying out

• transfer each section to a dish of toluidine blue O stain for 1 minute

• this stains the lignin blue green and then rest of the tissue pink purple

• rinse off excess stain with water

• place sections on slide

• place under microscope and adjust till you get a clear image

• make a labelled drawing