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Plant Science
Why exactly are plants so important?
The main groups of plants
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Bryophytes (mosses and liverworts)
filicinophytes (ferns)
coniferophytes (conifers)
angiospermophytes (flowering plants)
Mosses and Liverworts
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No roots
No vascular system
Do not produce flowers
They are not woody (due to lignin)
Ferns
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Do have roots and a vascular system (xylem and phloem)
They are not woody (due to lignin)
Conifers
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Advanced vascular system
Have roots stem and leaves
They have woody tissue
Flowering plants
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Advanced vascular system
Have roots stem and leaves
Do have woody tissue
Form flowers
Dicotyledonous and Monocotyledonous
Plants
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Flowering plants are divided into dicotyledons (dicots)
and monocotyledons (monocots). The sunflower is a
typical dicotyledon (Helianthus). Maize (Zea) is a
typical monocotyledon. All grasses are monocotyledons.
Three differences between the structures of
dicotyledonous and monocotyledonous plants are:
Dicotyledon
The seed has two cotyledons
Leaves have veins that are net-like
Vascular bundles in the stem are in a ring
Monocotyledon
The seed has one cotyledon
Leaves have veins that are parallel
Vascular bundles in the stem are scattered
Grouping plants
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Hydrophytes are plants that are adapted to live in aquatic conditions.
Xerophytes are plants of permanently dry and arid conditions.
Mesophytes are plants adapted to conditions of average water supply.
Plant Structure and Growth
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The Stem
Think about vascular bundles and where they are in the leaf and the stem. A vascular bundle has phloem,
xylem and cambium. In the stems of dicotyledons the vascular bundles are arranged in a regular ring. In
the stems of monocotyledons the vascular bundles are spread out within the stem.
Roots
The Leaf
The waxy cuticle prevents water loss
from the upper surface of the leaf.
The upper epidermis has no
chloroplasts and allows the maximum
amount of light to pass through. The
palisade mesophyll is packed with
chloroplasts in order to absorb the
maximum light for photosynthesis.
The spongy mesophyll is made of
loosely packed cells with air gaps.
The air gaps allow the exchange of the
gases oxygen and carbon dioxide. The
lower epidermis contains stomata
which allow gas exchange. The
vascular bundles contain xylem and
phloem. The xylem transports water
to the leaf. The phloem transports
sugar away from the leaf. The
vascular bundle also provides support
to the leaf.
Xylem
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Xylem vessels are cells that have been lignified. The contents die and they become long hollow tubes
attached end to end. The cells are xylem vessel elements and they form a xylem tube.
Xylem: support and transport.
Phloem
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Phloem is used to move sucrose. Phloem is alive and is made from two types of cell: sieve elements and
companion cells. The sieve elements lie end to end forming a continuous tube called a sieve tube. Phloem
is able to transport sucrose (and amino acids) by mass flow. Source cells actively transport sucrose into the
phloem sieve elements.
Source: photosynthetic tissue
The sink:
storage organs
Different plant structures
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Bulbs are a plant organ modified for food storage made from layers of fleshy leaves: for example onion
bulbs. They are packed with food reserves in the form of starch. Potatoes are underground stem tubers. A
carrot is a modified tap root. Tendrils are modified stems (or leaves: the sweet pea has leaf tendrils) that
allow a plant to secure its position.
Meristems
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New Growth
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The cells of a plant that are capable of dividing repeatedly are called meristems. Meristems generate new
cells for growth of the plant. Dicotyledonous plants have apical and lateral meristems. In addition cambium
is meristematic tissue. Apical meristems can be called primary meristems and occur at the tips of the stem
and root, they are responsible for primary growth. Lateral meristems form from the cambium. When the
lateral meristem grows it causes the secondary growth of the plant, this results in an increase in the
circumference of the stem.
Auxin
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Auxin is a plant growth substance that has a function in phototropism. Such substances control plant
growth. Auxin is made by the cells in shoot tips. It then diffuses down both sides of the shoot. The auxin
collects on the dark side of the shoot. Auxin stimulates shoots to grow towards light, by stimulating cell
elongation on the dark side of the shoot. Such growth is called positively phototropic.
Roots
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The root system provides a large surface area for mineral ion and water uptake by
means of branching roots and root hairs. Water is transferred to the xylem by two
pathways, the apoplast pathway (mainly) and the symplast pathway. The water
moves by the process of osmosis. This happens because the solute concentration
inside the root cells is greater that that in the surrounding water. Mineral ion uptake
by the roots is by active transport. Active transport requires the use of globular
proteins within the plasma membranes called “pumps”. Mineral ions move by
active transport up a concentration gradient.
Many species of plants live in a mutualistic relationship with fungi that live in soil.
In this relationship the fungal hyphae receive a supply of sugar from the plant root
cells. In return, the fungal hyphae are able to provide the plant with a greater
supply of mineral ions.
Apoplast pathway
In this pathway the water does not enter the cell but moves along the cell walls.
The cells of the endodermis have a Casparian strip which is impermeable to water.
To pass the endodermis, the water must follow the symplast pathway.
Symplast pathway
In this pathway the water passes through the cell wall and through the plasma
membrane into the cytoplasm. The water passes via cytoplasm.
The stem and support
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Terrestrial plants support themselves by means of thickened cellulose, cell turgor
and xylem.
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Imagine a balloon inflated in a shoebox made of thin card. The balloon is pushed
up against the cardboard and this gives the structure strength. This strength is what
the plasma membrane of the plant cell does when it pushes up against the cellulose
cell wall. When a plant cell is full of liquid and the plasma membrane is pushed up
against the cell wall the cell is known as turgid. If the plasma membrane is not
pushing against the cell wall the cell is described as flaccid.
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Some plant cells have cell wall corners that have been specially thickened. Tissues
made of these cells are called collenchyma.
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Xylem vessels (where are they?), in addition act to strengthen the leaves, roots and
stem of a plant. Xylem vessels are strengthened with spirals or circles of lignin.
Transpiration
• Transpiration is the loss of water vapour from the leaves and stems of
plants.
• The transpiration stream is the stream of water that is drawn up the stem
through the xylem by the evaporation of water from the leaves. This is
called transpiration pull. Water is a polar molecule and because of this
molecules of water are loosely attached to each other (this is known as the
cohesive property of water). Water molecules stick together by cohesive
forces because of hydrogen bonding between water molecules. Adhesion is
the force with which molecules cling to the surrounding surfaces. A
continuous stream of water runs from the roots all the way up the stem and
to the leaf cells. Xylem vessels are cells that have been lignified. The
contents die and they become long hollow tubes attached end to end.
• Xylem: support and transport
Conditions that effect the rate of
transpiration
• The abiotic factors light, temperature, wind and humidity, affect the rate of
transpiration in a typical terrestrial plant.
• Light: Stomata tend to be open in the light. This will increase the rate of
transpiration.
• Temperature: Causes increased evaporation. This will increase the rate of
transpiration.
• Wind: Maintains a concentration gradient by removing water vapour;
increasing transpiration.
• Humid air: high humidity slows transpiration.
Stomata
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Guard cells can regulate transpiration by opening and closing stomata. Stomata are the pores on the lower
epidermis of leaves; they open into spaces in the spongy mesophyll. The plant hormone abscisic acid
causes the closing of stomata. When guard cells are full of water, they are called turgid, and the stomatal
pore becomes larger.
Xerophytes
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Xerophytes are plants of permanently dry and arid conditions.
Adaptations of xerophytes that help to
reduce transpiration
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Rolled leaves:occurs in marram grass, which means that the grass can grow on sand
dunes. Rolled leaves enclose the stomata, which reduces transpiration.
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Reduced stomata:this reduces the outlets from which moist air can diffuse.
Deep roots:this allows the plant to absorb water from deep in the ground.
Stomata in pits:this traps moist air outside the stomata, lowering the rate of
evaporation.
CAM metabolism:Crassulacean acid metabolism. This occurs in succulent
xerophytic plants such as Sedum. In these plants the stomata are open at night only.
The carbon dioxide is fixed (stored in malate) and is released during the day, when
the stomata are closed.
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Hydrophytes
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Hydrophytes are plants that are adapted to live in aquatic conditions.
Water Hyacinth
Hornwort
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The bases of the leaf stalks are swollen. This is because they are full of air spaces, forming a
spongy tissue. This allows the plants to float.
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The Hornwort has many finely divided and submerged leaves. This allows water to flow
through the leaves while preventing leaf damage.
Reproduction in flowering plants
The seed
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For example the dicotyledenous seed, french bean (Phaseolus vulgaris):You should be able to draw and label the inside and outside of this seed.
You should label: testa, micropyle, embryo root, embryo shoot and cotyledons.