Download 9 - IBBiology Class of 2010

9.1.1 Dicotyledonous stem and leaf structure.
These are low power diagrams that show the
distribution of the different tissue types. Cell structure
not required for this syllabus statement.
is
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 water loss.
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.
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 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 that you know the tissue diagram)
1. Draw a labeled cross sectional
diagram of a dicot stem. Briefly state
the function of each tissue labeled.
Leaf section:
Cuticle is a waxy layer which reduces water
loss through the upper epidermis
Upper epidermis is a flattened layer of cell
that forms the surface of the leaf and makes
the cuticle.
Palisade Layer is the main photosynthetic
region of the leaf.
Vascular bundles contains the transport
system and vascular meristem tissue
Spongy mesophyll contains spaces that allow 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.
2. Draw a cross sectional diagram of a dicot leaf using the diagram in your books.
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
dicotyledonous.
3.Complete the table below to differentiate between monocotyledons and dicotyledons. You may use labeled
diagrams if you wish.
Monocotyledon
Dicotyledon
Examples
Number of
cotyledons
(first leaves)
Roots
Stem tissue
distribution
Leaves
Flowers
For much of the remainder of the unit we focus on dicotyledons (dicots), which are examples of angiosperms.
Be sure to use dicot examples in any research you carry out.
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.
4.What are the functions of the following leaf structures? How does their position/ distribution in the
leaf relate to their function?
Structure
Waxy cuticle
Function
Palisade mesophyll
Spongy mesophyll
Vascular bundle i. xylem
ii. phloem
Guard cells and stomata
Distribution/ function relationship
9.1.4 Modification of root, stem and leaf.
Stem modification:
Bulbs: Onions & Lilies
Short vertical underground 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 maxillary buds.
These should not be confused with the disc-like Corms
found in daffodils and tulips.
stem modification:T he horizontal stems called runners spread out from the main body of the plant.
At point the stem touches the ground new roots form.
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
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




The potato is an underground modification of the root tip.
The 'eaten potato' contains the carbohydrate and protein stores for
the growth.
The 'eyes' are in fact axillary buds.
In effect this diagram shows the branching axillary buds or a stem.
Carrot: Tap Root modification




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.
5. Give named examples of the following modified leaf, root and stem structures:
Example:
Leaf: bulb
How is it modified?
Stem tuber
Root tuber
9.1.5 Apical and lateral meristem in dicotyledonous
Image:
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).
6. Define meristem.
7. Why is one more likely to find cells in mitosis in a meristem than in other plant tissues?
8. Dfferentiate between apical and lateral meristems in terms of location and function in the stem.
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.
10.Compare the functions of apical and lateral growth.
11. What is the function of the axillary bud? What is the trigger to growth of a new shoot or branch?
9.1.7 The role of auxins 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.
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 at least 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.
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) shows 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".
Boysen 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
bending-growth. In combination with
the previous observation this
suggests the growth promoting
substance is chemical - Jensen
experiments of 1913 showed that and
moving down the shaded side.
(g) The coleoptile is decapitated
(h) A gelatin 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.
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.
(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.
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.
12. Define tropism.
13. Compare these types of tropism:
Response to:
Positive or negative?
Phototropism
Geotropism
(radicle)
Geotropism
(plumule)
Hydrotropism
14. What is auxin?
15. Explain, with the aid of a diagram, the role of auxins in phototropism.