Download Plant Structure

Plant Structure
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Plant cells differ in many respects from animal cells
Thick cell walls, chloroplasts, large water vacuoles in center
Cell membrane permeated with lignin for extra support and rigidity
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Many basic types of plant cells, fill three basic functions
> Growth - meristem cells
> Support - parenchyma
> Transport - xylem, phloem
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Meristem cells are undifferentiated, can divide quickly to build up new tissues
Parenchyma - most common cell type, most of the plant body (pith)
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Phloem cells conduct food and hormones, (ph ~ f)
Conduction can be either up or down
Sieve cells are the conducting elements
Companion cells maintain the sieve cells
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Xylem cells conduct water and dissolved mineral nutrients (K, P, Ca etc..)
Conducts either up or down (not both)
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Xylem dies after differentiating (dead tubes)
Two types of xylem
> Thinner, more primitive tracheids (all tracheophytes)
> Wider, more advanced vessels (mostly angios)
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Break in water column (cavitation) poses serious problems for trees
Narrower tracheids are much better at recovering from cavitation
Conifers do better in cold, dry, sandy habitats where the water supply is often interrupted
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Three different types of tissues found in plants
> Epidermal tissue - outer layer, skin or bark, root hairs, guard cells (stomata)
> Ground tissue - “background” tissue, mostly parenchyma
> Vascular tissue - xylem, phloem - gathered into vascular bundles
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All plants develop in the same way
Root - shoot system, send roots down into the soil, send shoots up into the air (shoot =
stems, leaves, flowers)
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Growing tips are called apical meristems
Apical meristems at tip of stem, tip of root
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Primary growth (up and down) is controlled by apical meristems
Many herbaceous annuals (weeds, flowers) have only primary growth, never get “thick”
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Secondary growth = lateral thickening
Secondary growth from lateral meristem
Lateral meristem is a thin cylinder of tissue that rises through the body of the plant
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Apical meristem controls growth of stems
Tender meristem tissue is protected by a terminal bud
Buds are really shortened stems, protective bud scales are modified leaves
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As the stem grows, bud scales fall off, leaving scars that look like little rings around the
stem
We can use these bud scale scars to determine the age of a stem!
Count backward from the tip….
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Lateral branches come from smaller axillary buds, which emerge from the stem just above
where the leaf meets the stem (axil)
Leaves and axillary buds emerge at special growing points on the stem called nodes
The spaces between the nodes are called the internodes
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When leaves fall off, they leave a crescent-shaped leaf scar
Look closely and you’ll also see teeny little bundle scars, holes where the xylem and
phloem entered the leaf
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Within the stem, the cylinder of lateral meristem tissue forms vascular cambium, which
makes new xylem and phloem cells
So xylem and phloem also form a cylinder that rises through the plant
Outside the cylinder of the vascular cambium = cortex, inside = pith
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Vascular cambium always forms xylem towards the center of the stem
Vascular cambium always forms phloem towards the outer edge of the stem
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Vascular bundles in dicots are nicely organized as a ring of little ovals
Vascular bundles in monocots are scattered throughout the pith (“monkey faces”)
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Phloem dies each year, replaced each spring - new phloem lies just under the epidermis
Can kill the plant by “girdling” it - making a shallow cut all the way around the stem slices
through all the active phloem cells
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Xylem gradually accumulates in the center as the stem thickens (= wood)
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Each year’s xylem forms an annual growth ring - can use tree rings to age tree, study
climate patterns - How ??
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Spring wood light & thick, summer wood darker and thinner
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Primary root - main body of the root
Secondary root - lateral extensions of root
Root hairs - finger-like extensions of epidermal cells, absorb water etc…
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Unlike the shoot, there is no waxy cuticle surrounding the root
Outer layer is thick cortex, used for food storage
Inner circle is the stele, surrounds the vascular tissue
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Vascular tissue in roots forms a big X - larger cells are xylem (red), smaller cells are
phloem (green)
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Outer cells of the stele control the flow of water into the root
Outer layer of cells in the stele are called the endodermis
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Endodermis cells are wrapped by a thin waterproof strip - Casparian strip
Casparian strip seals every edge of the cell except the outside facing edge
Only way water can penetrate the stele to the xylem is to pass through the cells of the
endodermis, can’t go over, around, between
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Root tip is an apical meristem
Root tip is covered by the root cap, a very tough layer of cells - Why??
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Delicate meristem would be ground away by friction with the soil, root cap cells give their
lives to protect the meristem
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Just behind the apical meristem is a zone of elongation, where cells are stretching out,
growing
Above this zone is the zone of differentiation, where new cells are starting to differentiate
(mature) into xylem, phloem, etc…
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Thin blade of tissue, primarily responsible for photosynthesis (stems can help)
Upper and lower epidermis covered with a waxy cuticle
Upper and lower surface dotted with numerous stomata (sing. stoma), guard cells - mostly
upper epidermis
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Below the upper epidermis are elongated rectangular cells called palisade parenchyma (lots
of chloroplasts)
Middle of leaf is spongy parenchyma, lots of water, air spaces for gas exchange
Vascular bundles pass through the spongy parenchyma
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Leaves consist of a flat blade on a stalk called a petiole
Swollen and curved base of the stalk is called a stipule (crescent shaped cross section - leaf
scars)
Large midrib passes through the center of the blade, carries vascular bundles
Great diversity of leaf shapes, arrangement on stem, leaf margins, different types of hairs
on leaf…Patterns of leaves help botanists sort out their relationships, identify plants