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9.1.1 Dicotyledonous stem and leaf structure.
These are low power diagrams that show the distribution of the
different tissue types. Cell structure is not required for this
syllabus statement.
Stem cross section (Helianthus spp)
Tissue types of the plant stem:
Epidermis: surface of the stem made of a number of
layers often with a waxy cuticle to reduce waterloss.
Cortex Tissue: Forming a cylinder of tissue around the outer edge of the stem.
Often contains cells with secondary thickening in the cell walls which provides
additional support.
Vascular bundle: contains xylem, phloem and cambium tissue.
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Xylem: a longitudinal set of tubes that conduct water from the roots upward
through the stem to the leaves.
Phloem (sieve elements) transports sap through the plant tissue in a number of
possible directions.
Vascular cambium is a type of lateral meristem that forms a vertical cylinder in
the stem. The cambium produces the secondary xylem and phloem through cell
division in the vertical plane.
In the centre of the stem can be found the pith tissue composed of thin walled
cells called parenchyma. In some plants this section can degenerate to leave a
hollow stem.
Cell diagram for comparison (the syllabus requires you know the tissue diagram)
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Leaf section:
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Cuticle is a waxy layer which reduces water loss through the upper epidermis.
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Upper epidermis is a flattened layer of cell that forms the surface of the leaf and
makes the cuticle.
Palisade Layer: This is the main photosynthetic region of the leaf.
Vascular bundle: contains the transport system and vascular meristem tissue (xxylem, p-phloem).
Spongy mesophyll: contains spaces that allows the movement of gases and water
through the leaf tissue..
Lower epidermis: bottom surface layer of tissues which contains the guard cells
that form each stoma.
see cell diagram for comparison
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9.1.2 Differences between dicotyledonous and monocotyledonous
structure.
Illustrations of the differences between monocotyledon and dicotyledonous
(a) The fibrous branching roots of the
monocotyledon
(b) The tap root structure with lateral
roots of the dicotyledon .
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9.1.3 Leaf tissue distribution
and function.
Tissues:
(a) Phloem transports the
products of photosynthesis
(sugars, amino acids).
(b) Xylem transports water and
minerals into the leaf tissue from
the stem and roots.
(c) Epidermis produces a waxy
cuticle for the conservation of
water.
(d) Palisade layer which is the
main photosynthetic region.
(e) Spongy layer creates the spaces and
surfaces for the movement of water and
gases.
(f) Lower epidermis contains the
stomatal pores which allow gas
exchange with the leaf.
The xylem and phloem tissues combine
in the vascular tissue to provide support
to the leaf.
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9.1.4
Modification
of root, stem
and leaf.
Stem
modification:
Bulbs: Onions
& Lilies
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Short
vertical
undergro
und
stems.
Many fleshy highly modified leaves for the storage of nutrient.
Can produce new plants by bulb division or the development of one of the many
axillary buds.
These should not be confused with the disc-like Corms found in daffodils and
tulips.
stem modification:
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The horizontal stems called runners spread out from the main body of the plant.
At point the stem touches the ground new roots form.
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If the horizontal runner is broken the small plantlet can establish itself
independently. (asexual reproduction).
The horizontal stems are often an adaptation to finding water.
Stem modification: e.g. Cacti
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Leaves are reduced to spines to
prevent water loss in
transpiration.
The stem is enlarged for the
storage of water.
The stem carries out
photosynthesis.
Root tip modification Stem Tuber/ Potato
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The potato is an underground
modification of the root tip.
The 'eaten potato' contains the
carbohydrate and protein stores
growth.
The 'eyes' are in fact axillary buds.
In effect this diagram show the
axillary buds or a stem.
for the
branching
Carrot: Tap Root modification
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Function: Storage of water.
Carrot plants are often
associated with very sandy soils.
The enlarged root is familiar to
those who have eaten the
vegetable.
The root modification allows the
storage of water in the cortex
and central stele.
The mass of the root stabilizes
the plant in the loose sandy soils.
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9.1.5 Apical and lateral meristem in
dicotyledonous
Plants grow is restricted to 'embryonic' regions called
meristems. Having specific regions for growth and
development (restricted to just the meristematic
tissue), contrasts with animals in which growth takes
place throughout the whole organism.
Apical meristems are found at the tip of the root and
the shoot, adding growth to the plant in these
regions. The apical meristems are described as
indeterminate , this type of growth tends to add length to root and stem in 'module' or
'units' (described below). This tissue remains 'embryonic' for prolonged periods of time
and in some cases over 100's of years. Contrast this with the more determinate growth
of leaves, petals and flowers in which a very precise growth occurs.
Terminology for plant growth and
development.
The diagram below is of the apical or primary
meristem tissue of a plant.
The meristem tissue retains its
capacity to divide and renew.
(a) Shoot apical meristem
(b) Leaf primordial
(c) Axillary bud primordium
(d) leaf
(e) Stem tissue
Root apical meristem:
(a) Root cap.
(b) Root apical meristem.
(c) Ground meristem.
(d) Protoderm.
(e) Epidermal tissue of the root.
(f) Vascular tissue (central stele).
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9.1.6 Growth in the apical and lateral meristems of dicotyledonous plants.
Apical meristem of the apical bud adds new tissue to the stem tip. This addition increases
the length of the stem.
Stem growth:
The tissue added includes the units described below:
1. Adds length to the stem and root
2. Added in modules.
3. Each module is added at the meristem and includes leaf (leaves), internode length of
stem and axillary buds.
Stem differentiation at the apical meristem.
These diagrams
illustrate that
the tissue added
at the apical
meristem differentiates into the various primary plant body structure
(AB).
This tissue diagram is a cross section of the stem of the primary plant
body.
This means that there has been no additional secondary thickening of the
cell walls.
Secondary growth added by the Lateral meristem (cambium) has two types:
1. Vascular cambium that produces secondary xylem and phloem
2. Cork cambium produces some of the bark layer of a stem.
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9.1.7 The role
of auxin in
phototropism
as an example
of the control
of plant growth.
plants, 3rd ed, Wareing and Phillips.
notes and
diagrams based
on Growth and
Differentiation in
A tropism is a bending-growth movement either toward or away from a directional
stimulus.
Phototropism is the bending-growth towards the unilateral source of light.
Auxins are a class of plant growth hormones (growth regulating factor)
Auxins are one of atleast three major classes of plant growth regulators. Unlike animal
hormones plant hormones can provide a range of responses from the tissues.
The most common auxin is IAA ( Indolacetic acid). IAA was discovered in 1932 and is
believed to be the principle auxin in higher plants.
Auxin is associated with the phenomenon of phototropism.
Charles Darwin studies of auxin effects are published a book called, 'The Power of
movement'.
Darwin studied phototropism using the germinating stem of the canary grass (Phalaris
canariensis).
The cylindrical shoot is enclosed in a sheath of cells called the coleoptile.
Darwin set out to determine which
region of the coleoptile is sensitive
to light.
(a) When there is a unilateral light
shinning on one side of a coleoptile
there is a bending growth
movement towards the light.
(b) Decapitation of the tip results in
no bending growth suggesting that
this region is possibly sensitive to
the light stimulus.
(c) The opaque cap prevents light
from reaching the tip without
damaging the tip as in (b). There is
no bending-growth response.
(d) The buried coleoptile (except
tip) show that it is not the lower
stem section that is responding to
light but rather the tip.
Darwin's experiments suggest that
the tip is the region sensitive to
light. Darwin concluded, " when seedlings are freely exposed to a lateral light some
influence is transmitted from the upper to the lower part, causing the latter to bend".
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Boysen- Jensen experiments of 1913 showed that the substance traveling down the
coleoptile stem was of a chemical nature.
(e) The mica plate is an un-reactive substance that is inserted on one side of the stem.
With the mica in place and the unilateral distribution of light there is no bending-growth.
This suggests that the growth promoting substance is prevented from moving down the
shaded side of the stem.
(f) Note that when the mica placed on the exposed side it does not prevent bendinggrowth. In combination with the previous observation this suggests the growth promoting
substance is chemical and moving down the shaded side.
(g) The coleoptile is decapitated
(h) A gelatine block permeable to
chemical diffusion is placed
between the stem and the root tip.
(i) The reconstructed coleoptile still
shows the bending-growth
response with the unilateral
distribution of light.
Again these Boysen-Jensen
experiments add confirmation that the growth promoting substance is chemical in nature.
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The elegant experiments of Paal (1919) confirm the work of Boysen-Jensen.
(j) Decapitation of the coleoptile tip.
(k) Replacement of the coleoptile tip but asymmetrically over one side of the coleoptile
stem.
(l) With NO LIGHT, there is a bending-growth, with the overlapped side experiencing the
growth.
Paal also suggested that in the dark or light from all sides the growth promoting
substance was sent uniformly down the stem.
Auxin, the growth promoting substance, was first isolated by F. W. Went in 1926. The
actual structure shown above for auxin was not determined until 1932.
(m) Went's experiments extended the
work of Paal, Boysen and Jensen by
isolating the auxin onto agar gel.
(n) The gel was cut up into block as a way
of quantifying the dose of auxin used.
(o) The agar block (containing auxin) are
placed asymmetrically on the stem.
(p) The angle of bending-growth was
measured.
Bio-assay: Went then developed a technique known as a bio-assay which the effect of a
chemical (auxin in this case) is measured by its effects on living tissue.
This graph shows the results of changing the number of coleoptile tips placed on a single
agar block. In effect this means the more tips on the block the greater the concentration
of auxin.
This graph suggests:
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As the number of coleoptiles (conc auxin) are increased the degree of bendinggrowth increases.
That around 6-8 coleoptiles on the agar further increases result in a reduced
increase in bending-growth.
Since Auxin (IAA3) was synthetically
produced more rigorous quantitative bioassay can be performed.
This graph measures the bending-growth against the concentration of IAA3.
Note that the graph suggests:
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Increasing IAA3 increases the bending-growth angle.
Optimal angle of bending-growth is achieved between 0.2- 0.25 mg
Higher levels seem to have reduced-bending growth.
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Transport of Auxin:
Auxin is transported through the cell membrane of the adjacent plant cells by protein
carriers in the plasma membrane.
These carriers transport the anion of auxin in polar direction, from the top of the cell to
the bottom of the cell.
However the stimulus of light would seem to result in the introduction of these carriers
into the side of the cell membranes so that the IAA3 can now be laterally transported.
Whilst not part of the examination syllabus for IB Biology look at the many connections
that can be made to the various parts of the syllabus including, cell structure; plasma
membrane; cell transport.
The role of auxin:
Since IAA3 is a 'hormone' there must be some link between the signal molecule and the
sub cellular responses and the cellular responses. It appears that it is the receiving cell
that determines the exact cellular response rather than IAA3 having very specific
responses across all cells.
As we have noted one of the major functions of auxins is the promotion of growth.
Research has shown that in some tissues IAA3 promotes mitosis whilst in other tissue, it
promotes cell enlargement.
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Click4Biology: Topic 9.1 Plant structure and growth
OCC | LabBanks | StudentBlog | TeacherBlog | Audio | Reading | Brights | Edge| EOL
9.1 Plant structure and growth / 9.2 Transport in angiospermophytes / 9.3 Reproduction in
angiospermophytes /
Plant structure and growth
9.1.1 Dicotyledonous stem and leaf structure.
9.1.2 Differences between dicotyledonous and monocotyledonous structure.
9.1.3 Leaf tissue distribution and function.
9.1.4 Modification of root, stem and leaf.
9.1.5 Apical and lateral meristem in dicotyledonous.
9.1.6 Growth in the apical and lateral meristems of dicotyledonous plants.
9.1.7 The role of auxin in phototropism as an example of the control of plant growth.
notes and diagrams based on Growth and Differentiation in plants, 3rd ed, Wareing and
Phillips.
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