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Structure, development
and functioning of conifers
Conifers as principal tree species in dry regions
The Pinyon pine-Juniper community is an extreme, but
widely distributed type in SW USA
Zion Canyon
Pinyon pine and Juniper
Pinus monophyllum
Pinyon pine with its crooked trunk and
reddish bark, is found in dry, rocky places at
elevations of 4,000-8,000 ft where yearly
precipitation is only 10-20 inches. Tap roots
stretch down 40 or more feet into the soil
while lateral roots stretch as far. Very slow
growth rates: a 6-10 foot diameter tree, 10
feet tall will be 80-100 years old.
Juniperus osteosperma
Utah juniper grows in dry, rocky or sandy
locations in the high plateau country from
5000 to 9500 feet above sea level. It is the
most common juniper of the Pinyon-juniper
woodlands of the arid western intermountain
basins. It is commonly 10 to 20 feet high with
a maximum trunk diameter of 1 to 2 feet.
Many conifers are xerophytes
Xerophyte: a plant that can live where water
supply is scanty or there is physiological
drought
Xerophytes have adaptations of leaves, stems
and/or roots.
Mesophyte: a plant that lives in places where the water
supply is neither scanty nor abundant
Two features of conifers:
First, some basic nomenclature about patterns of
plant growth and development
Morphology and anatomy of needles
Anatomy of wood
Three basic tissue systems that running through the plant
Three basic tissue
systems that running
through the plant
This is based on
angiosperms – but
conifers do have the same
basic organization!
Fig. 31.6A
Tissues
A
tissue
is a cooperative unit of many similar cells
that perform a specific function within a
multicellular organism
Tissues usually have cells that are specialized for
particular functions
For example the vascular tissue system conducts
water and nutrients from roots to leaves through
specialized cells for water conduction and conducts the
products of photosynthesis, sugars, from leaves in
different but equally specialized cells.
Unfortunately…
… biologists are rather lax in their
use of the word “tissue”.
Xylem
Water
from roots tissue
Trachieds
Parenchyma
Conducting
tissue
Sugars
from leaves
Phloem
tissue
The Secondary
Phloem in Pinus
has Sieve and
Albuminous Cells
and Parenchyma
with dark contents.
But the principle holds true that a tissue is a group of specialized
cells, frequently of different types, performing a specific function.
… photosynthesis?
Conifer needles (or fronds)
The site of photosynthesis
Exchange between the needle and the
atmosphere of CO2 (into the needle) and
water vapour (out of the needle).
Why is water loss inevitable?
Gaseous exchange takes place through a water film
on the cells inside of the needle
Stomata
Stomata have central openings surrounded by two photosynthetic
guard cells. Usually stomata are open during the day and closed at
night but can close if the leaf dehydrates. Guard cells change the
shape of the opening by changing their own shape.
Epidermal cell
Guard cell
Thick inner wall
Stoma
Chloroplast
In Taxus caespitosa and other conifers stomata are arranged in rows
Cross-section through
a pine needle
The foliage of many but not all conifers also
contains resin ducts. The lining cells secrete
resin into the duct in response to leaf injury.
Resin duct
Cross-section through a pine needle
Transfusion tissue
Endodermis
Phloem
Xylem
Guard cell
Stoma
Mesophyll
Cuticle
Epidermis
Hypodermis
The xylem and
phloem are
surrounded by
undifferentiated cells
called transfusion
tissue. This is
surrounded by an
endodermis, which
typically controls
passage of materials
between conducting
tissue and ground
tissue
Conifer needles are generally thicker and tougher than many angiosperm
leaves in part because they have a layer of thick- walled cells, the hypodermis,
below the epidermis. They also tend to have a thick cuticle.
Leaf cross section of Taxus (yew)
The needle is broader than that of the pine,
but still has only one vascular bundle
The mesophyll is differentiated into palisade and spongy layers
The endodermis is not so clearly developed as in the pine
Leaf cross section of Podocarpus (a conifer)
This needle is still broader, yet contains only one vascular bundle
Center of needle
Mesophyll is differentiated into palisade and spongy layers
Endodermis
How does wood form?
What is it that we are seeing
when we look at tree rings?
Production of xylem and phloem
tissues by the vascular cambium
Secondary xylem is a complex tissue
In conifers it consists of:
tracheids that conduct water upwards,
are long in the vertical direction, and
have bordered pits
parenchyma, thin walled cells
ray cells running horizontally
through the xylem and are composed
mainly of parenchyma and some
tracheids
Cross secction of a young pine stem
Direction of growth
Cambium
Ray initials
Late wood
Early wood
Cambium and secondary xylem of a conifer
Rays
Tracheids with
bordered pits
Parenchyma
Cambium and secondary
xylem of a conifer
Esau 1965
Production of secondary xylem and
phloem by the vascular cambium
Each time a cambial cell C divides one daughter cell retains its
status as an initial and the other, the derivative D or D
differentiates into a xylem X or phloem cell P
Tracheids and rays, pine
Radial longitudinal section
http://www.uri.edu/artsci/bio/plant_anatomy/43.html
Pits
Tracheids with bordered pits, pine
Tranverse
longitudinal
section
Circular bordered pits of pine tracheids as seen
in face view (left) and in side view (right).
Bordered pits
The torus at the center of the bordered pit
moves and seals the pit when a tracheid aspirates
http://www.uri.edu/artsci/bio/plant_anatomy/images.html
Xylary resin duct in Pinus
Ray
Epithelial cell
Tracheid
Transverse section
Sections you need to have read
7.2
31.5
31.6
Courses that deal with this topic
ESC 221 Dendrology and Autecology