Stems Flashcards
What are stems for?
• stems evolved before roots and leaves, thus usually studied first
• stems:
– for the attachment of ‘other bits’ – transport between ‘other bits’
– increase in height and branching – thus increase photosynthesis
– storage
– some photosynthesis
• economics: paper, cork, wood, etc
What are steles?
Steles • epidermis, then cortex, then stele • stele – xylem, phloem and pith • angios & gymnos: X & P is in vascular bundles • no VBs in roots
What are Traheids?
• Tracheids
– more primitive, less efficient
– narrow dia., overlapping slanting end walls – water moves from pit to pit
– a compromise betn conduction & strength
What are Vessel Elements?
• Vessel elements
– advanced, more efficient
– wide dia., end wall breaks down
– an end-to-end series of vessel elements forms a vessel (can be > 1 m length)
– water moves in a ‘straw’ – little flow resistance
What do both vessel elements and tracheas have in common?
• Both vessel elements & tracheids: – dead at maturity – lignified thick cell wall – lignin: • strength • compression resistance • water proofing • why wood is a durable product
Phloem
• Phloem
• transports ↓ organic compounds • made up of:
– sieve tube elements that form sieve tubes with sieve plates between the s.t.e.
– sieve plates have pores
– pores penetrated by cytoplasmic strands
– s.t.e. alive but have a enucleate protoplast –+
– companion cells, parenchyma, fibres
Herbaceous (non-woody) dicot stem
- most tissues are primary
- VBs in an outer ring
- VBs discrete or merge together
- X to inside, P to outside in VB
- sclerenchyma bundle cap to outside of 10ary P
- fascicular VC develops betn X & P
- usually small amount of 20ary X & P just in VBs
Woody Dicot Stem?
Woody dicot stem
• vascular cambium & cork cambium
• secondary growth – inc. in girth
• fascicular cambia linked by interfascicular cambia
• forms a complete ring
• much secondary X (= wood) to the inside
• less secondary P to the outside
What are secondary features of Xylem?
growth rings heartwood softwood
Vascular Cambium
• most VCs are seasonal – due to dry or cold
• (however in some areas …………)
• creates patterns in the wood
• when VC starts ÷ing in spring
– large dia. vessels, few fibres – is spring wood (early wood) – spring wood is light in colour
• by summer VC ÷s slow down
• vessels smaller dia., more tracheids/fibres
• darker in colour
• known as summer wood (late wood)
Annual Growth Rings
• one year’s spring + summer wood
• = annual ring
• annual rings vary in width
• annual rings can be used for:
– aging/dating trees
– ascertaining past growth conditions, e.g.
rainfall, caterpillars, pollutants, fires
• growth rings = annual rings???
• NOT ALWAYS, e.g. a tree in subtropical Brisbane may form several growth rings in one year, depending on rainfall
How many ways can you section wood?
3 ways to section wood
– TS or XS (just seen in prev. slides) – RLS (radial longitudinal section)
– TLS (tangential longitudinal section)
Structure of Rays
• structure of rays – a radiating system – made of parenchyma – betn spp. much variation in width/height • rays – lateral conduction + storage
How do you determine sections of wood?
- How to determine TS/TLS/RLS?
* look at vessels and rays
Describe Sapwood and Heartwood?
- As wood ages:
- some protoplasts of ray cells
- grow thru pits
- ‘balloon’ into cavities of vessels, block them • called tyloses
- prevent conduction (+ fungal spread)
- resins and tannins accumulate
- darker colour – Heartwood
- Sapwood – outer, paler, functional X
