Transcript Plant structure & growth

Topic 9: Plant Science
9.1 Plant Structure & Growth
Plan diagram: STEM of dicotyledon
Distribution of tissues in a stem
Structure
Function
Epidermis
Thin layer of cells
for protection
Xylem
For water
transport
Phloem
For transporting
organic molecules
Pith
Storage and
support
Cambium
Dividing tissue
Cortex
Storage and
support
Differences: monocotyledonous vs.
dicotyledonous plants
Monocotyledons
Dicotyledons
Parallel veins in narrow
leaves
Branching, net-like veins in
broad leaves
Embryo has ONE cotyledon
Embryo has TWO cotyledons
Flower parts arranged in
threes
Fibrous adventitious roots
Flower parts arranged in
fours or fives
Tap root with lateral
branches
Eg. bean, daisy, sunflower
Eg. maize, wheat, oat
Distribution of tissues in a leaf
Relationship between tissue
distribution in a leaf and function
Palisade cells contain lots of chloroplasts
for absorbing light.
 Palisade cells arranged “end-on” to ensure
each cell receives maximum strength light.
 No chloroplasts in upper epidermis to
ensure light reaches palisade layer.
 Chloroplasts present in spongy mesophyll
cells to ‘mop up’ any unused light.

Relationship between tissue
distribution in a leaf and function
Waxy cuticle to prevent water loss from
epidermal cells by evaporation.
 Air spaces in spongy mesophyll layer to ensure
adequate supply of CO2 to photosynthesizing
cells and ease of removal of O2.
 Stomata located on leaf lower surface to allow
gases in and out of leaf.
 Presence of vascular bundle to supply water
from roots and method for removal of products
of photosynthesis.

Modifications of roots, stems,
leaves for different functions
Bulbs – these are
underground storage
structures that contain
reserves of nutrients to
ensure survival.
 Fleshy leaf bases closely
packed on a short stem.
 Eg. onion, garlic

Modifications contd.
Stem tubers –
swollen tip of rhizome
or underground stem
 Storage of
carbohydrate for
growth of new plants
 Eg. potato

Modifications contd.
Storage roots –
these contain stores
of carbohydrate for
the plant to use later.
 They are usually
swollen tap roots
(primary roots)
 Eg. carrot, sweet
potato

Modifications contd.
Tendrils – slender
stem-like structure to
wrap around or hook
a support.
 Sensitive to touch, so
faster growth on the
opposite side occurs.
 Eg. vine, ivy

Apical v Lateral Meristems
Dicotyledonous plants have apical (primary) and
lateral (cambium) meristems.
 Meristems generate new cells for growth of the
plant.
 Apical meristems found at the tips of stems and
roots increase the length of these sections.
 Stems and roots may also grow in thickness or
in diameter through cell divisions in lateral, or
secondary, meristems, found just under the
surface along the length of the stem or root.

Phototropism
Phototropism is generally described as a
plant's response to light.
 There are two kinds of phototropism:
positive phototropism, or the growth of a
plant stem towards light, and negative
phototropism, or the growth of a plant's
roots away from light.
 Phototropic responses are caused by
auxins, which modify cell walls.

More on phototropism
Auxins concentrate on the side of a plant
stem away from light, in order to stretch
rigid plant cell walls into growth toward
light.
 Auxin is produced in the tips of shoots and
roots.

Yet more on phototropism


Auxin acts as a growth
promoter, possibly by
causing H+ ions to be
secreted into cell walls.
This causes a loosening
of the connections
between cellulose
fibres, allowing cell
expansion.
Phototropism diagram
http://www.biologyonline.org/3/7_meristems.htm
 http://www.biologyonline.org/3/5_plant_hormones.htm
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