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
What is Softwood?
• Gymnos: wood is just tracheids – soft – tracheids for both transport & strength
What is Hardwood?
• Dicots: wood is vessels & fibres – hard
– vessels for conduction, fibres for strength
• many exceptions, e.g. balsa wood is from adicot (Ochromalagopus)&whitecypresspine
wood (often used for flooring) is denser than many hardwoods)
What is Secondary Phloem?
Secondary phloem
• only functional for about 1 year • is crushed and dies
• is in a vulnerable position
– ringbarking, stock eating & abrasion
What is ringbarking?
• Ringbarking: cutting through the bark to the level of the vascular cambium.
• Why to trees die rather slowly after ringbarking?
– The foliage is still supplied with water and minerals
through the sapwood.
– Thus photosynthesis can continue and keep the
foliage functioning.
– HOWEVER, photosynthates (sugars) can not reach
the root system.
– The roots will have some stored starch that can
support (via respiration) the roots for a limited time
– After this the root tips will gradually die and so will the tree
What is Bark?
• two uses ‘bark’ • ‘loose’use: – everything to the outside of the VC – eg. ringbark • ‘strict’use:cellstooutsideofcorkcambium – aka phellem or cork • the CC and all its products = periderm • all cells to outside of VC under stress – cells crushed – bark shed/furrowed
What are Lenticels?
lenticels
• a small group of loosely packed cells in cork
• gas exchange for bark tissues
• also occur on roots and some fruits
What distinguishes Monocot Stems?
Monocot stems • noVC,noCC • X&PinVBs • but VBs scattered, not in ring • more, smaller VBs near outside • still .... X to inside, P to outside • usually a couple of large vessels, with some smaller vessels betn • + air space • AS: first formed X destroyed by stretching • NoVCbetn X&P • VB surrounded by sclenchyma sheath – (not a bundle cap) • Scattered VBs + scl. sheath as no need to link VCs in VBs • No clear distinction betn cortex/pith
What are Specialised Stems?
Specialised stems
• No matter how specialised:
– always: nodes, internodes, axil. buds – leaves often reduced/absent
• Rhizomes: horiz. stems below gnd
• Runners: hor. stems, above gnd, long
internodes, eg strawberries
• Stolons: lots of different definitions!
– usually ‘above gnd rhizomes’
– but ….. tubers borne at end of stolons – Kikuyu: spreads by rhizomes & stolons
• Tubers: swollen end of a stolon
– eg. spuds, ‘eyes’ are axil. bud + scale leaf
scar
• Bulbs: mainly overlapping fleshy leaves,
short stem
• Corms: solid stem tissue, with a few papery scale-like leaves
• Cladodes/cladophylls: stem resembling & functioning as a leaf
– eg pads of pricklypear
What other types of stems are there?
- Thorns – modified axillary bud
- Spines – modified leaf - cacti
- Prickles – epidermal outgrowth - rose
- Tendrils – can be modified leaf, leaflet, branch or inflorescence for climbing
