shoots and leaves & tree growth

stomata

specialized cells on the epidermal surface that facilitate gas exchange

1 degree and 2 degree veins

major (largest) veins

bulk flow of materials and structure of the leaf

3+ degree

minor (smaller) veins

conduction within photosynthetic tissues and preventing leaf tearing

common vascular pattern in eudicot leaves

reticulate/netted

common vascular pattern in monocot leaves

parallel

development of leaf vein patterns

leaf venation begins to form after leaf primordium extension and blade extending

veins begin to develop first at the distal end and continue towards proximal end

where does axillary meristem develop

axil of each leaf

axillary meristem produce branches

SAM of axillary meristem grow and divide- forms a branch

looks and behaves just like SAM of main shoot

consists of repeating phytomers

each leaf on the branch will have its own axillary meristem with the potential to grow and create another branch

growth and extent of branch under hormonal control of main shoot SAM

loss of the shoot apex

releases buds from apical dominance= loss of auxin produced by SAM

cause it to branch and become bushy

if main shoot apex is damaged, can be replaced by shoot from axillary bud under hormonal control

negative gravitropism

shoots sense, and grow away from a gravitational pull

how do shoots sense gravity

statocysts associated with vascular tissues are involved

phototropism

herbaceous shoots can sense and grow towards light

lamina

leaf blade- captures light and conducts photosynthesis

petiole

leaf stalk- holds lamina away from stem and allows for movement

leaf tissues found in leaf and function

epidermis- protects leaf and helps control water loss

mesophyll- middle of leaf

• palisade- does most of photosynthesis

• spongy- helps gas exchange

veins- vascular tissue

• xylem- brings water from roots to leaf

• phloem- carries sugar from leaf to plant

stomata- let gas in and out

what is leaf morphology affected by

leaf lifespan- reflects energetic investment

adaptation- long-term selection to environment

age or immediate environment

characteristics of evergreen leaves

long leaf lifespan

thick cuticle (epidermis)

dense tissue (mesophyll)

hard tissue (sclerenchyma)

found in scrubland, boreal forest

characteristics of deciduous leaves

short leaf lifespan

thinner cuticle (epidermis)

thinner tissue (mesophyll)

little or no hard tissue (sclerenchyma)

found in poor growth environments, winter and drought-deciduous

hydrophyte

water-adapted

less cuticle (epidermis)

stomata on aerial portions (epidermis)

large pockets of air tissue (mesophyll)

xerophyte

dry, arid-adapted

thick cuticle (epidermis)

dense trichomes (epidermis)

extra layers of epidermis

water storage tissue (mesophyll)

have stomata inside a cavity filled with trichomes

mesophyte

non-drying and non-aquatic environments

denser veins and close loops more efficient for water flow

leaf morphology change due to age

become more complex as they grow older

leaf morphology change due to season

spring: thinner and larger

summer: thicker, develop waxy coating

leaf morphology change due to environment

shade bottom canopy- shallower lobes, larger and thinner

sun top canopy- deeper lobes, smaller and thicker

explain how and why trees may increase stem growth to avoid shade / neighbour competition

they need sunlight to survive

grow stems faster- more energy in vertical growth

sense shade- detect changes in light quality

hormones help- auxin

adjust branching

excurrent canopy form

leading shoot grows more than branches

strong apical dominance, single straight stem, axillary meristem sensitive to auxin

crown form= pyramidal or conical

decurrent canopy form

leading shoot does not grow more than lateral branches

weaker apical dominance, single stem may not always be as evident

crown form= rounded, domed, vase shaped

fastigate canopy form

leading shoot grows more than branches

lateral branches at acute angles to leading shoot

moderate apical dominance, single stem often evident

crown form= columnar

effects on crown form

spacing, forking, injury, natural adventitious shoots

spacing

wide spacing of trees allows light into lower branches- lower branches likely to be retained

close spacing of trees causes live crown to recede rapidly- lower branches die off

forking

loss of leading shoot- 2+ branches grow as leading stems

injury

exposure to constant wind, salt spray or blowing snow causes growth reduction and can modify tree canopy

humans- via pruning and other mechanical stress

natural adventitious shoots

some trees naturally create adventitious shoots (cloning)

red

high quality light

far red

low quality light

activated to Pfr

red light levels ok!

deactivated to Pr

far red light levels too high or red light levels too low= competition

apical dominance

auxin produced by SAM can suppress axillary meristem- low concentration of auxin towards base of stem= axillary meristem grow and form branches

SAM controls bud growth and occurs via auxin