Anatomy and Growth in Woody Plants

1

• Outline for todays lecture
• The plant form (leaves, stems and roots)
• The main tissues within stems and roots (xylem
  and phloem)
• Growth (meristems, tropisms and factors that
  affect resource allocation)

Anatomy and Growth in Woody Plants
World Champ Baobab tree
Natl Champ Bald Cypress
2
Leaves
Function provide a high surface area volume
organ for CO2 acquisition
3
Sun vs. Shade Leaves
Sun leaves
Shade leaves
Area
Thickness
Lobiness
Palisade and conducting tissue
4
Internal leaf structure 30 of leaf is air!
Mesophyll
Cuticle
Upper epidermis
Palisade parenchyma
Spongy parenchyma
Xylem
Phloem
Stomata
Cuticle (grows into leaf at stomatal pores)
Lower epidermis
5
Stomata crucial structure for land plants.
Formed by the epidermis to facilitate gas
exchange, but they also are present on all
above-ground organs
The guard cells have no plasmadesmata to other
cells.
Helical thickenings in guard cell allow pore to
form when water enters
6
stem
Stems
petiole
Function water and nutrient transport, storage,
and structural support of the plant.
node
stem
Internode
Internode
node
petiole
Unlike leaves, stems have indeterminate growth,
and increase in size by adding modules of nodes
and internodes
7
Stems
Esau, 1960
8
Roots
Function in water and nutrient uptake, storage,
and structural anchorage of the plant.
9
Uptake roots
All water and nutrients must go through the
endodermis
Apoplastic pathway along cell walls and dead
cells (xylem conduits)
Symplastic pathway through living cells
10
Lateral roots form from young roots and begin to
grow from within the endodermis. Why?
11
Transport roots
Esau, 1960
12
Stem vs. root
One of the main differences between root and stem
structure in woody plants is the lack of a pith
in roots. The pith consists of undifferentiated
cells in the center of the stem. What might be
one reason this tissue exists in stems and not
roots?
13
Xylem
Within seed plants, there are 3 major wood types
Coniferous
Ring-porous
Diffuse-porous
Angiosperms only
Generally in gymosperms only
Panshin and De Zeeuw 1980
14
In most woody plants, the entire cross-section of
xylem is not functional in water transport
15
Sapwood Width is Variable Examples in Hardwoods
Wood Type RP RP RP DP RP RP DP DP DP DP DP DP DP D
P DP DP
Panshin and De Zeeuw 1980
16
Conifer or softwood xylem
Longitudinal tracheid (earlywood)
Longitudinal tracheid (latewood)
Ray cells
Longitudinal parenchyma cells
17
Main Cells in the Xylem of Softwoods(Gymnosperms)
- 90-95 of volume longitudinal
tracheids. Dead in sapwood and heartwood -
5-10 of volume ray parenchyma. Alive in
sapwood, dead in heartwood - (alsoother
parenchyma, which functions in storage and
defense against herbivory (e.g., resin ducts))
18
Tracheids
They are dead at functional maturity
conifer tracheid length lt 3-7 mm
Function in transporting water and structural
support of the plant
Core, Cote Day 1979
19
Rays

• Theyre alive!
• Ray cells may have inclusions (e.g., oil cysts,
  crystals)
• Function as storage for nutrients, sugars, and
  may play role in conduit refilling

20
Angiosperm or hardwood xylem
Parenchyma
Vessel
Ray
Fiber
21
Main Cells in the Xylem of Hardwoods -Vessels -F
ibers -Rays -Tracheids -Parenchyma
Much more variable proportions than in softwoods
Same function as in softwoods
22
Vessels
Also dead at maturity
Vessels are composed of vessel elements, and
function in water transport
23
Vessel elements combine to make vessels
24
Form of Perforations Varies Among Hardwoods
Simple perforation plate
Vessel element
Scalariform perforation plate
Vessel element
Whole thing is 1 vessel
Hoadley 1990
25
Simple perforation
Scalariform perforation
26
Fibers

• Long, narrow with closed ends
• THICK-WALLED
• Function mainly in mechanical support
• Usually dead

27
Cross section of late-wood tracheid of longleaf
pine (Pinus palustris Mill.) W.A. Côté, Jr.
VESSELS, TRACHEIDS AND FIBERS HAVE SECONDARY
CELL WALLS
Secondary cell layers (in cross section)
S3
S2
S1
Primary wall
Middle Lamella
28
ML
P
Each layer has a unique composition
S3
S2
S1
29
Cell wall layers in tracheids
W
Xylem water
Hydrophilic Hydrophobic
S3
S2
S1
ML
P
Includes primary wall
30
Tracheids are always and vessels are almost
always shorter than the length of the plant, so
water has to travel between at least 2 (and
usually 1000s) of these conduits to travel
from roots to leaves
Betula nigra
Photo Jarmila Pittermann
31
Betula nigra
32
Betula nigra
Photo Jarmila Pittermann
33
Water travels through pits
Betula nigra
Photo Jarmila Pittermann
34
Pit structure and function in angiosperms vs.
conifers
homogenous
torus-margo
35
Pit structure and function in angiosperms vs.
conifers
homogenous
torus-margo
36
Wood generalizations
Survey of 5,600 samples from InsideWood by E.
Wheeler et al. (IAWA, 2007) found -75 of
records have no growth rings, are diffuse-porous
and have simple perf. plates -Ring porosity is a
N. Hemisphere adaptation -Shrubs and small trees
very, very rarely have a few wide conduitsmore
often have many small conduits.
37
Phloem
aka phloem
38
Sieve cell
Angiosperm
Helper cells
Sieve elements
Albuminous cells
Combine to make sieve tubes
Gymnosperm
Companion cells
39
Characteristics of sieve cells and
elements They are ALIVE!! They are highly
modified relative to normal cellsthey have no
nucleus, vacuole, golgi apparatus, ribosomes, and
they have very few mitochondia They do have a
cell membrane and only primary cell walls
40
Sucrose is most common, but some species
transport raffinoseboth are very stable sugars
Sieve element
41
These purple proteins are an artifact of the
sample preparation. They are P-proteins that
surge to clog the phloem when a major change in
pressure is detected.
42
Characteristics of helper cells They are
ALIVE!! They have very dense cytoplasm with
small vacuoles, and many, many mitochondria
43
Woody Plant Growth
44
Primary plant growth occurs at shoot and root
apical meristems
45
Apical meristems lays down the entire primary
plant body
46
Vascular Cambium
A 1-cell thick ring around stem that divides
repeatedly and creates new phloem and xylem,
resulting in secondary growth
47
Vascular Cambium
Many more divisions towards xylem than phloem.
Most of stem cross-sectional area is secondary
xylem, not phloem. Xylemphloem cell number
3-6.
Geometrical issue with this method of growth as
the circumference increases, this stretches the
vascular cambium and then initials must also
divide radially to increase the number of cells
in the cambium.
48
Tropisms plant growth in response to external
stimuli.
2 examples today -Phototropism -Gravitropism
49
Phototropism
Plants tend to grow towards the light
Early work on this was done in monocots, but the
mechanism seems to be the same for woody plants
Light sensing molecules are in apical meristem
50
Indicates there is an inhibitor involved, as well
as an growth promoter
Iono Briggs 1984
51
Gravitropism
How do roots know which way is down?
The organelles containing starch grains
(amyloplasts) are denser than the cytoplasm, so
they settle with gravity
Root cap