REN R 121 Lec 11/12

Secondary Growth and Wood Structure

cork cambium

a lateral meristem (secondary growth) that produces water-repellent cork cells with lots of suberin (form the outer bark to protect the tree against injury and desiccation); also produces a layer of parenchyma called phelloderm on the interior side

phloem

transports sap from sources (leaves during the summer) to sinks (areas with less sugar) through the sieve elements (helped by parenchyma companion cells) due to a pressure gradient (caused by sugar concentration); a common target for pests as it is very nutritious

sieve plates

separate sieve elements to help protect against damage/insect pests and ensure the direction of flow 

vascular cambium

a lateral meristem (cells that continuously divide) that produces secondary growth of phloem (on the outside) and xylem (on the inside) 

turgor

an internal cell pressure caused by water needed for expansion (growth); trees cannot grow when they are dehydrated (and they need carbohydrates - source of energy - and nutrients as well)

spring and night

most trees in temperate areas grow during the season of —— (ended late June- early July) before growing slower; specifically, they grow during the —— portion of the day (2am-6am) due to favorable water potentials (they store energy so they wont lose much water)

parenchyma

where non-structural carbohydrates (starch) are stored in living —— cells in otherwise dead wood (keeps other living cells alive); also for vascular rays to help transport nutrients/water between the xylem/cambium and the cortex

dehydration

plants lose water during photosynthesis as CO2 diffuses in through the stomata (they are open so water can transpire); they close during —— but then the plant cannot photosynthesize - the cells will lose tugor and collapse

cytorrhysis

the complete and irreversible collapse of cell walls under extreme desiccation (dehydration)

carbon starvation

an inability to carry out metabolic, defense, or hydraulic functions due to a lack of stored carbohydrates

cohesion-tension theory

water molecules cohere (stick together) due to hydrogen bonds - thus causing unbroken columns of water molecules to form - and transpiration in the leaves creates negative pressure (tension) that pulls the entire water column upward from roots; how water moves through the xylem

xylem

contains tracheids (water conducting cells which are connected by bordered pits - allows movement of water) and vessel members (water conducting cells connected by both bordered pits and perforation plates); both of these are dead at maturity while there are living fibers (sclerenchyma - gives structure) and parenchyma (maintain metabolism for living cells)

gymnosperms

lack vessel members and fibers (only have tracheids)

vessel elements

many vessels linked together by perforations plates that are longer and wider and tracheids

pits 

(and in angiosperm vessels, perforation plates) allow water to pass from one element to another while minimizing the spread of air bubbles

Hagen-Posieuille equation

Q = πPr4/8nl; used to describe laminar flow through a long cylindrical pipe but doesn’t account for the resistance through the pits/perforation plates; proportional to the fourth power of radius

large vessel elements

conduct disproportionately more water; however, are at the risk of cavitation or embolism; 75-90% loss of conductivity would be likely to kill the tree (eg. 80% loss causes 50% chance of death)

cavitation

(embolism) the breakage of the water column which lets in air thus renders it unable to conduct water (high pressure allows air to move easier); high levels means the leaves will not receive enough water - thus death

air-seeding

due to drought (dehydration) where cavitation allows air bubbles to form and, due to negative pressure, the air moves easily to other vessels through the pits and cannot conduct water

freeze-thaw embolism

occurs when dissolved gases in xylem sap form bubbles when frozen and then expand during thawing under moderate negative tension; wider vessels are at greater risk of this because they can produce larger bubbles (more solute)

solutions to cavitation

can have smaller pits/perforation plates (for drought-induced embolism) and narrower vessels (for freeze-thaw embolism); however, these would reduce conductivity and the plant would need more wood to supply water 

sapwood

water-conducting xylem tissue with living parenchyma; towards the outside of the rings and a lighter colour; inner cells die first due to cavitation, aging (programmed cell death) or a pathogen (resulting in tyloses - the outgrowth of neighboring parenchyma cells to stop water flow by killing xylem) to then form heartwood

heartwood

fully dead, non-conducting xylem tissue that mostly serves a mechanical role and accumulates natural ‘extractives’ like resin and tannins to deter insects and fungi; towards the inside and is a darker color; forms due to the blockage of sapwood cells and accumulation of extractives; fast-growing ‘diffuse porous’ species have less

tree rings

found in only seasonal environments (temperate and boreal forests); produced each year due to growth and dormant seasons; may not always be visible; richer sites and better years tend to yield wider ——

ring-porous

found only in angiosperms (hardwood); clear separation between early wood with long, wide vessels and late wood with much smaller vessels; eg. oak, elm, ash trees - may also be semi if the separation is continuous rather than distinct; leaf out later (focus on growth) and have higher stem conductivity/ density

diffuse-porous

only in angiosperms (hardwoods); no clear separation between early wood and late wood; apparently random or oddly patterned distribution of vessel widths and no clear annual growth rings; eg. aspen, maple; leaf out earlier because xylem is functional for several years; less stem conductivity and less heartwood 

gymnosperm rings

only have tracheids (softwood - no fibers - with more sapwood because tracheids undergo less embolism; however, there is lower hydraulic conductivity), so more uniform size although some species produce larger tracheids early and smaller tracheids later (that are darker and denser/mechanical in function even though it is less dense overall)

resin ducts

conduct resin for defense against pests; are mostly found in xylem but can be found in other tissues

lacticifers

secrete latex for defense against pests and wound-healing, mostly found in phloem and cortex