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33
Stems and Plant Transport
Lecture Outline
I.
A woody twig demonstrates external structure
A. Stems all have buds, which are embryonic shoots
1. The terminal bud is at the stem tip
a) Buds are covered with bud scales
b) Bud scale scars are left when bud scales fall off
c) The number of bud scale scars indicates the age of the branch
2. Lateral buds are located in the axils of the leaves
a) Leaf scars show where leaves were attached
b) The bundle scar is within the leaf scar and is the remnant of the
vascular bundle
c) Lateral buds may be located above the leaf scars
3. Nodes are the areas where leaves are attached
4. Internodes are spaces between sequential nodes
5. Lenticels are small specks where loosely arranged cells allow diffusion of
gases
II.
Stem growth and structure
A. Apical meristems are located at the end of the stem and produce new primary
tissues and primary growth
1. Plants with only primary growth are herbaceous
B. Lateral meristems produce secondary tissues and secondary growth
III.
Herbaceous dicot and monocot stems differ in internal structure
A. Stems have dermal tissue, ground tissue, and vascular tissue
B. Vascular bundles of herbaceous dicot stems are arranged in a circle in cross
section
1. The epidermis is the outer covering, with a cuticle to retard water loss
2. The cortex contains various types of ground tissue cells
a) The cortex functions in photosynthesis, storage, and support
3. The vascular bundles are arranged in a circle and are continuous with the
vascular bundles of the roots and leaves
a) Xylem is located on the inner side of the bundle
b) Phloem is located on the outer side of the bundle
(1) Fibers are abundant and strengthen the stem
(2) The phloem fiber cap is found in some dicot stems
c) The vascular cambium is located between the xylem and phloem, and
is a lateral meristem
4.
Interior to the circle of vascular bundles is the pith, composed primarily of
large parenchyma cells
a) Between the vascular bundles (between pith and cortex) are pith rays
C. Vascular bundles are scattered throughout monocot stems
1. Monocots also have epidermis, vascular bundles, and ground tissue
2. The vascular bundles are scattered throughout the stem, but still have
xylem towards the inside and phloem towards the outside
3. The cortex and pith are not distinct, but the ground tissue has the same
functions as in dicots
4. Vascular cambium and cork cambium are not found in monocot stems
a) Monocots like palms attain large size by the accumulation of a
modified type of primary growth with much sclerenchyma
IV.
Woody plants have stems with secondary growth
A. Secondary growth is a result of growth in both the vascular and cork cambia
1. Gymnosperms and dicot angiosperms have secondary growth
2. Cells in the vascular cambium produce secondary xylem, which replaces
primary xylem, and secondary phloem, which replaces primary phloem
a) Secondary xylem is wood, and secondary phloem is the inner bark
3. Cells of the cork cambium divide to produce cork cells and cork
parenchyma
a) These cells are referred to as periderm (outer bark), which replaces
epidermis in woody plants
B. Vascular cambium gives rise to secondary xylem and secondary phloem
1. The vascular cambium originally was discontinuous, located between the
primary xylem and primary phloem in each individual vascular bundle
(recall that they are arranged in a circle in the herbaceous dicot stem)
2. As the plant matures, the vascular cambium becomes continuous in a ring
around the stem
3. During division, one of the daughter cells remains meristematic, and the
other divides to form secondary tissue
4. Cells on the inside of the ring of vascular cambium become secondary
xylem (recall that xylem is the inner part of the vascular bundle), and the
cells on the outside become secondary phloem
5. As secondary tissues are laid down, the cells of the primary xylem and
primary phloem become widely separated and become crushed
6. Secondary vascular tissue must now take over the function of the primary
tissues in conduction of fluids
7. The secondary vascular tissues have the same types of cells found in the
primary vascular tissues, but with more fibers
8. As the tree becomes wider, lateral transport is accomplished via rays,
which are chains of parenchyma cells
a)
The rays, which are formed by the vascular cambium, transport
materials from the secondary xylem to the secondary phloem
C. Cork cambium produces periderm
1. Periderm is the functional replacement for the epidermis
2. The cork cambium forms a ring or a series of arcs of meristematic tissue
3. Cells formed to the outside of the cambium are cork cells, dead at maturity
and waterproofed with suberin
4. Cells formed to the inside are cork parenchyma, a storage layer
5. Since the epidermis is lost, stomata are lost
a) Lenticels are openings allowing gas exchanges
D. Common terms associated with wood are based on plant structure
1. The functional secondary xylem is the sapwood and is located outside the
darker, older heartwood
2. The vascular cells of heartwood are filled with pigments, tannins, gums
and resins, and are nonfunctional
3. Hardwood is the wood of flowering plants and is generally harder than
softwoods
4. Softwood is the wood of gymnosperms, lacks fibers and vessel elements,
and is typically softer
5. Annual rings are apparent in trees that grow in areas with marked
seasonality
a) Trees may be aged by counting the rings
b) Each ring has both springwood and late summerwood
(1) Springwood has vascular elements with larger diameters and
thinner walls due to rapid growth
6. When a branch dies and becomes surrounded by new wood, a knot is
formed
a) Knots typically reduce the value of the wood
V.
Transport in plants occurs in xylem and phloem
A. Water and minerals are transported in xylem
1. Water moves horizontally through root tissue until it reaches the xylem
a) Water and dissolved minerals move upward in the plant in the vessel
elements and tracheids from roots to stem to leaves
2. Water movement can be explained by a difference in water potential
a) Water potential is the free energy of water
b) Pure water has 0 megapascals (MPa), about 10 atm
c) In water with dissolved solutes, the free energy is negative
d) Water moves from a region of higher water potential to an area of
lower water potential
e) Water moves by osmosis from the soil into the root because the root
has a higher concentration of dissolved materials than the soil water
3. Tension-cohesion pulls water up a stem
a)
The evaporative loss of water by transpiration draws water up in
xylem cells
b)
This mechanism is based on an unbroken column of water in the
xylem
(1) This column forms because of the cohesiveness of water due to
hydrogen bonding between water molecules
(2) The adhesion of water to the walls of the xylem cells (due to
hydrogen bonding) also maintains the column of water
c) The tension produced by transpiration is enough to pull water up to a
height of 150 m (492 ft), which is taller than any tree
4. Root pressure pushes water from the root up the stem
a) A less important mechanism involves the movement of water from the
soil because of the differences in water potential
b) Guttation is a result of root pressure
B. Sugar in solution is translocated in phloem
1. Sugar produced during photosynthesis is converted to sucrose for
transportation within the plant body
a) Solutions may move in either direction in the phloem, but always from
sources to sinks
2. The pressure-flow hypothesis explains translocation in phloem
a) At the source, sucrose is moved into the companion cells of the
phloem, utilizing ATP as an energy source and the cotransport of
protons
b) Sugars move from the companion cells to the sieve tube element
through the plasmodesmata
c) Water follows by osmosis, increasing the turgor pressure within the
sieve tube element, thereby pushing the solution through the phloem
d) At the sink, sugar is moved out of the sieve tube members, and water
follows osmotically, decreasing the turgor pressure
3. Phloem studies have been aided using aphids, which are insects that feed
from phloem tubes
a) Mouthparts severed from the aphids allow measurements of phloem
flow
b) This technique has verified that sucrose is the primary carbohydrate
transported in the phloem
Research and Discussion Topics

Transpiration has both negative and positive effects on plants. List a number of
advantages and disadvantages. If you were a plant of similar weight, you would
have to drink about 40 L (10 gal.) of water per day!

Imbibition is another flux of water. What is it?