plant biology lecture 9- vascular tissues revisited

about xylem tissue

• conducts water + minerals

• dead at maturity

• lignified (secondary cell walls)

about phloem tissue

• conducts mixture of sugars, proteins, amino acids, hormones, etc.

• alive at maturity

• non-lignified

what are tracheary elements

• primary xylem conducting cells

• lack a protoplast

what are the two types of tracheary elements

vessel elements and tracheids

what are vessel elements

• xylem conductive cells in angiosperms

• Have perforations on end walls

• Joined end to end to form vessels

• Typically aligned with pits

what are tracheids

• xylem conductive cell in gymnosperms

• no perforations

• aligned with tapering walls

• lined with pits

• longer and narrower

what are pits

• an area where two adjoining cells have a closer connection

• connects neighbouring vessel elements and allows movement of materials between vessels

what are the 2 types of pits

simple pit: secondary wall comes straight in

bordered pit: secondary overhangs the pit field and created cavity

what are annular cell walls

secondary cell walls laid in rings to allow for expansion

what are helical cell walls

secondary cell walls laid in criss cross pattern to allow for cell wall expansion

where can helical and annular cell walls be found

further down the stem in the zone of cell elongation and maturation

how does a tracheary element develop

1. xylem cell begins with a nucleus, large central vacuole, cytoplasm and primary cell wall

2. cell deposits secondary cell wall and end walls expand

3. programmed cell death occurs— cell walls, tonoplast and nucleus disintegrate

4. leaves primary cell wall with thickenings and perforations

what are sieve elements

• primary conducting cells of phloem

• only has primary cell walls

• alive at maturation by highly modified (loses nuclei, tonoplast, ribosomes, golgi apparatus, and cytoskeleton)

what are the 2 types of sieve elements

sieve tube elements and sieve cells

what are sieve tube elements

• phloem conductive cells in angiosperms

• wider

• has sieve plates

• larger pores

• stacked end to end

• has companion cells

what are sieve cells

• phloem conductive ells in gymnosperms

• narrower and longer

• no sieve plates, but has sieve areas

• smaller pores

• has albuminous cells

what’s a sieve area

sites with a concentration of pores

• connects cytoplasm between adjoining cells

• sieve cells

what’s a sieve plate

area on the end of a cell with larger pores compared to sieve cells

• allows for efficient flow of material

• sieve tube elements

how do sieve tube elements develop

1. young sieve tube element nucleus splits into 2

2. the young cell divides into 2 (sieve tube element and companion cell)

3. sieve tube element nucleus, tonoplast, ribosomes, G.A and cytoskeleton degenerates

4. the remains of the sieve tube elements (cytoplasm and cell membrane) press against cell wall and leave open space for movement of materials

what are companion cells

specialised parenchyma cells that act as life support for sieve tube cell and delivers substances to sieve tube element

• connected to sieve tube element through plasmodesmata

• sieve tube cell

what are albuminous cells

specialised parenchyma cells that traffick substances in/out of sieve cells