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Structure and Function of
Forest Trees
Elements of Forestry
Kenneth Williams
Fisheries Extension Specialist
Langston University Aquaculture
Extension Program
Trees

Longest lived organisms on earth.
 Heights of 100 meters diameters of 10
meters.
 Due to longevity, trees must withstand
extremes of climate.
 Trees allocate much of their energy to
diameter growth. Not so in other plants.
 Physiology of trees a young science.
3 TISSUE SYSTEMS OCCUR IN
PLANTS

Dermal Tissue System
– Function: Protection from the environment and
water loss.

Vascular Tissue System
– Function: Conduction of water, nutrients, sugars
and hormones throughout the plant.

Ground or Fundamental Tissue System
– Function: Storage, support, filler tissue and site of
photosynthesis.
Dermal Tissue System

Function: Protection from the
environment and water loss.
 Tissues:
 a) epidermis - single layer of cells on
primary (herbaceous ) plant parts.
 b) periderm or bark - a corky tissue that
replaces epidermis on secondary (woody)
plant parts.
Vascular Tissue System

Function: Conduction of water, nutrients,
sugars and hormones throughout the plant.
 Tissues:
 a) xylem - conducts water and nutrients up
roots, stems and leaves.
 b) phloem - conducts water, sugar, hormones,
etc. primarily down roots, stems and
leaves, but also can move up at times.
Ground or Fundamental Tissue
System

Function: Storage, support, filler tissue
and site of photosynthesis.
 Tissues:
 a) cortex - outer region of stems and
roots.
 b) pith - center of stems.
 c) mesophyll - middle of leaves and
flower petals
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PLANT ORGANS, TISSUE SYSTEMS, TISSUES AND CELL TYPES
3 TISSUE SYSTEMS OCCUR IN PLANTS
Dermal Tissue System
Function: Protection from the environment and water loss.
Tissues:
a) epidermis - single layer of cells on primary (herbaceous ) plant parts.
b) periderm or bark - a corky tissue that replaces epidermis on secondary
(woody) plant parts.
2) Vascular Tissue System
Function: Conduction of water, nutrients, sugars and hormones throughout the
plant.
Tissues:
a) xylem - conducts water and nutrients up roots, stems and leaves.
b) phloem - conducts water, sugar, hormones, etc. primarily down roots, stems
and leaves, but also can move up at times.
3) Ground or Fundamental Tissue System
Function: Storage, support, filler tissue and site of photosynthesis.
Tissues:
a) cortex - outer region of stems and roots.
b) pith - center of stems.
c) mesophyll - middle of leaves and flower petals

MERISTEMS AND GROWTH

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Primary Growth - growth in length that gives rise to
primary (herbaceous) tissues called the primary plant
body.
 Apical meristem or apex - the growing points
located at the tips of stems and roots

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Secondary Growth - growth in width or diameter
that gives rise to secondary (woody or corky) tissues
called the secondary plant body.
Apical meristem
Shoot diagram
Shoot growth
Fixed shoot growth – 1 period of spring
growth, often over quickly. Ex. Dogwood.
 Free shoot growth – continual growth
throughout season (most in spring) ex.
Cottonwood
 Recurrent flushing – several periods of
growth throughout season. Ex. White oak

Shoot growth
Free growth – fast growing hardwoods –
poplar, willow, cottonwood and sweetgum.
 Fixed growth – slower growing hardwoods
and conifers – northern oak, sugar maple,
white pine.

Free growth
Ex. willow
Fixed growth
Ex. Red oak
Recurrent flushing – several periods of growth
throughout season. Ex. White oak and fast
growing conifers ex. Red pine.
 Each growth spurt ends in new bud formation.
 Then the new buds grow in the next cycle 3-7
times per year.
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Root growth

Similar to shoots
 Taproot is the 1st root to grow. Branch roots
form off of it. Taproot may not persist.
 Root growth depends on soil and moisture
conditions/ Ex. Red maples on upland sites
have large taproots but have a shallow rot
system in poorly drained soils.
Root growth

Most growth in length occurs in the long
roots.
 Most surface area is in the short of fine
roots. The fine roots are also called feeder
roots. They take up nearly all the water and
nutrients.
 Larger roots provide a conduction path and
mechanical support.
Fine roots, the mycorhizae

Most of the fine roots are actually a fungus
called mycorhizae plus tissues from the
tree root.
 Mycorhizae increase root surface area,
increase water and nutrient uptake and
increase resistance to pathogens and
nematodes.
Fine roots, the mycorhizae

The mycorhizae receive most of its food
from the host tee.
 This is a very important fungal/tree
partnership and crucial to tree survival.
 All trees are mycorhizal
 Without this fungus trees will stunt and may
die.
Root growth

Mychorrizal
fungi
Flowering and reproduction

Flowers develop as vegetative shoot apical
meristem transforms into a reproductive
meristem.
 Flower shoots have a determinate growth
pattern, they grow, flower and meristem
disappears.
Conifer reproduction
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Male and female cones are borne
separately on the same tree.
Male cones are usually lower than
female cones to reduce self
pollination.
Male cones are small (1-2 cm)
release pollen in spring 1 year after
forming.
Female cones large, seeds released 2
years after pollination.
Conifers are wind pollinated
Angiosperm reproduction

Pollination may be by wind, insects, birds
or bats.
 Flower and seed development may be in a
few weeks ex. Poplars or several years, ex.
Some oaks.
 Male and female reproductive parts may be
in the same flower or male and female
flowers on separate trees.
Angiosperm reproduction

Seeds dispersed in spring can usually
germinate immediately. Ex .silver maple
 Many trees disperse seeds in fall and winter.
These seeds remain dormant until spring.
Ex. Some oaks
 Some seeds can remain in the soil for years
until conditions are right for germination.
Secondary growth

Diameter growth occurs In the vascular
cambium, the large sheet of cells below the
bark.
 It ensheaths the entire plant body in a single
layer of cells..
 During growth cambium divides to produce
xylem to the inside and phloem to the
outside of this tissue.
Secondary growth

Cambium activity is greater in spring than
summer making the cells at this time larger.
 This results in annual rings. Usually 1 ring
per year.
 Rings may be missing or incomplete or
sometimes more than 1 per year.
Cross section
Cross section
Cross section
Xylem

3 functions:
 Conduction of water by cells called
vessels. These cells are dead.
 Support by thick walled vessel cells.
 Storage – parenchyma cells transport and
store starches. These cells may be in rays.
 Hardwoods more efficient than conifers in
moving water due to structural differences.
Sapwood (living
parenchyma cells)
can be 300 yr. old
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Heartwood
Healthy
heartwood (dead
parenchyma cells)
Fungus rotted heartwood
Phloem
Cells form sieve tubes – vertically arranged
files of living cells with end walls that are
porous.
 Phloem transports sugars and organic
substances formed in leaves to other parts of
the tree and roots.
 Phloem is only functional a few years
before the cells are crushed in tree growth
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Xylem and
phloem
Tracheids
Similar function as xylem in
hardwoods, support and conduction
Cork cambium and bark
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Periderm or outer bark formed from the cork
cambium which originates in the epidermis.
Cells to the outside of the cork cambium have a
thick layer of a waxy substance called suberin.
Suberin is water repellant and resistant to bacteria
and fungus.
New periderms are continuously formed.
Cork cells are dead at maturity.
Cambium
Cork
cambium
of cork
oak
Leaf structure
Gas movement through leaves
Respiration

Respiration provides energy for growth,
maintenance and the construction of new
cells.
 Respiration is the opposite of
photosynthesis. The oxidation of sugars o
carbon dioxide and water, releasing usable
chemical energy.
Respiration

The chemical equation for respiration is:
 (CH2O)6 + 6 O2
6 CO2 + 6 H2O
 Photosynthesis occurs mostly in leaves.
 Respiration occurs in all living cells.
 Carbohydrates not used in respiration are
stored as starches. An important tree
reserve.
 Many trees have enough stored starch to refoliate the tree 3 times. This allows the tree
to recover from insect, weather and disease
damage.
Growth
substances or
plant
hormones
Effects very
complex and
not always
understood.
Secondary compounds
1000’s of chemicals produced by trees,
many with no known function.
 Some may be defensive.
 Many have commercial value, ex. The
terpenoids.
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Water

Single most important factor in tree growth
and survival.
 Water makes up 80% of living cells.
 Most water in trees lost through
transpiration.
 There is a continuous column of water from
soil to roots to leaves in trees. This is called
the soil-plant-atmosphere continuum.
Transpiration
Macronutrients
Micronutrients
Light
Shade tolerance – ability of a tree to grow
in the shade of other trees.
 Photoperiod – duration of daylight.
Provides signals for bud set and leaf fall and
other physiological functions.

Temperature
Trees grow between 0C – 40C
 They become dormant when temperatures
begin to drop.
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Tree forms
Excurrent growth form – single main stem
ex. Pine tree.
 Decurrent growth form – multiply stems,
ex. Elm tree.

Tree forms
Excurrent growth
Decurrent growth
Tree longevity
Oldest bristlecone pine – 4844 years old
 Found in Nevada 1965.

Tree longevity

Growth rate slows as trees age.
 They become less resistant to pests and
disease.
 May be killed from storms , fungal rot etc.
 Physiology of tree longevity poorly
understood.
Largest trees

Giant sequoia over 300 ft. high
THE END