Structure and growth in plants
We will be focusing on the angiosperms (flowering plants) here.
- What structures do plants have?
- What tissues compose these structures,
and how are they distributed?
- How do plants grow?
- Contrast monocots and dicots.
Refer to chapter 24 in text.
- What structures do (angiosperm) plants have?
- flower for reproduction
- “buds” (meristems) for growth
- leaf to provide optimal situation
for photosynthesis
- stem for transport and support
- root to obtain water and
minerals and anchor plant
Each has special tissues
to enable these specific uses.
Each will be looked at
in greater detail.
(This is a dicotyledonous angiospermophyte)
Root
- The root is for anchoring,
- for storing food products, and especially for
- controlling minerals and water that enter the plant from the soil.
Dicot
(like oaks, carnations, duck weed...)
Monocot
(like reeds, lilies, mostly grasses...)
Root (cont.)
note
- X of xylem,
surrounded by phloem
- endodermis (inside skin)
is final filter for what
crosses into the plant system
- cortex, pith, epidermis
- Extensive branching,
root hairs, and
epidermal cell surface extensions
multiply absorptive surface.
Root (cont.)
note
- root hairs shown here ↓
- Two routes into
vascular tissue:
- Apoplastic: traveling
along cell walls
- Symplastic: traveling
through cytoplasm
(plasmodesmata)
- Casparian strip:
tight junctions between
endodermis cells force
all materials to pass
through cytoplasmic
membranes before
entering xylem.
Special modifications
nodules for nitrogen fixing bacteria
Bacteria of the species Rhizobium
capture N2, and convert it to ammonia
which is absorbed by the plant, and thus
nitrogen enters the food web.
food storage
such as carrots
http://en.wikipedia.org/wiki/Image:CarrotDiversityLg.jpg
Root (cont.)
http://www.uq.edu.au/_School_Science_Lessons/9.209.GIF
geotropism
Auxins move down toward bottom of root,
and inhibit root cell elongation,
making roots bend downward into ground.
http://www.omegagarden.com/images/photos/geotropism.jpg
Root (cont.)
Special modifications (cont.)
Pneumatophores →
Roots with negative geotropism
for obtaining air in waterlogged
Soils. (e.g. cypress knees)
Mangroves ↑ (and other salt-tolerant plants)
often have mechanisms to reduce salt uptake
through the roots.
http://media-2.web.britannica.com/eb-media/50/5650-004-479D6282.jpg
Air roots ↑
True epiphytes have roots for anchoring only, as the
leaves and stems are modified to capture water.
Stem
- The stem is for support (hold aerial parts up),
- and to transport water and nutrients.
- xylem and phloem in vascular bundles, epidermis,
ground tissue (pith, cortex), sclerenchyma
Dicot
(like beans, maples, roses...)
Monocot
(like grasses, daffodils, palms...)
Stem (cont.)
A “plan diagram”
of the dicot stem.
note:
- Xylem is interior
to phloem
- New growth is in
cambium
- This is a young stem,
not a woody trunk...
Stem (cont.)
note
- Vascular cambium
is main site of
lateral (outward)
growth.
aka lateral meristem
-Growth also builds
cork/ bark.
- Successive layers
of 2o xylem are the
“annular rings”.
- New length comes
from apical meristem.
- This lateral, aka secondary,
growth is mostly in
dicots.
(Palms have their own way…)
Lenticels,
splits in
cork,
allow gas
exchange
Stem (cont.)
Lengthening in dicots
note
- terminal bud = apical meristem
(= primary meristem),
end of twig
- axillary bud
base for side twigs
- Mitosis is at the above points.
- Cells elongate in this year’s
internodes.
- Snipping an apical meristem
encourages division in
lateral meristems (pruning).
Stem (cont.)
Special modifications
phototropism
↓
Auxins (plant hormones)
move away from sunny side of stems,
and enhance stem cell elongation,
making stems bend toward sunlight.
Stolons ↓
Horizontal stems,
above or below ground,
for propagation
of new plants.
stem tubers
For asexual reproduction:
Bulbous lateral shoots
with food storage
form near hypocotyl
to generate new plants
(e.g. potatoes and begonias↑)
http://www.zeably.com/Stolon
(Rhizomes are stems for spreading, too.)
- Primary site of photosynthesis
therefore,
- primary site of gas exchange
- Requires vascular tissue
to bring nutrients to leaf,
and carry products away.
http://www.inclinehs.org/smb/Sungirls/images/monocot%20leaf.JPG
Monocotyledonous angiosperms ↑
(like wheat, corn,... tulips)
have parallel veins.
←Dicotyledonous angiosperms
(like sunflowers, ivy.... tulip poplars)
have branched veins.
http://www.sbs.utexas.edu/bio406d/images/pics/poa/Arundo%20donax%20leaf5.jpg
Leaf
Leaf (cont.)
You will not have to
actually draw this,
but you should be able to
identify the following:
cuticle
upper epidermis
palisade parenchyma/
palisade mesophyll
spongy parenchyma/
spongy mesophyll
vein, with
xylem (carries water)
phloem (carries sugars)
lower epidermis
stoma, bounded by
guard cells
http://kvhs.nbed.nb.ca/gallant/biology/leaf_structure.jpg
Leaf (cont.)
- You should be able to draw a “plan diagram”,
showing leaf tissues, but not all those individual cells.
- Uses: light absorption, water retention, gas exchange,
support, water and carbohydrate transport.
Illustration from Allott book, posted online
Leaf (cont.)
Special modifications:
Xerophytes
(adapted to dry environments)
- Leaves reduced or lacking
- Few stomata
- Hairs on leaf to reduce air over stomata
- Modified for water storage
- Waxy cuticle thick to reduce water loss
Hydrophytes
(living in or on the water)
- Buoyant
-Stomata on top surface
-Guard cells may be inactive
-Waxy cuticle thick for less water gain
- If under water, leaves small/divided
Special modifications (cont.)
tendrils →
Thin modified leaves
that coil around. (They may
be modified stems)
When they touch something
they wrap around, serving
as a holdfast for vines.
Thigmotropic:
sensitive to touch.
http://galleries.neaq.org/2011_04_01_archive.html
salt excretion ↑
Salt, sequestered in vacuoles,
excreted from glands in leaf.
Assists in survival in tidal flats.
http://www.ars.usda.gov/is/graphics/photos/sep05/d199-1i.jpg
Leaf (cont.)
↑ red vine
← mangrove
water chestnut↓
air bladders →
Leaf stem swellings that enable aquatic
plants to float.
(In kelp these are called pneumatocysts).
http://dnr.state.il.us/Stewardship/cd/images/768x512/0002120.jpg
Contrast monocots and dicots.
Some of these points are generalizations – there are exceptions.
Stomata on
top and
bottom of
leaf
Stomata
mostly
on bottom
More on flowers and seeds in plant reproduction
Primarily
herbaceous
Herbaceous
or woody
Collenchyma:
support of shoots,
no lignin, uneven, expandable
celery strands →
Sclerenchyma:
tough secondary wall with lignin,
often “dead”
specialized for support
“sclerieds” in nut shells,
“fibers” used for cloth (hemp, or
← flax)
Celery_stem,_light_micrograph-SPL.jpg
http://course1.winona.edu/sberg/IMAGES/potatoCell.jpg
Parenchyma:
thin, flexible,
where walls are not primary function
palisades ~,
← cells w starch plastids in fruit
http://www.sciencephoto.com/image/29014/530wm/B7250301-
PLANT CELL TYPES
For each of the structures (leaf, stem, root)
describe specialized tissues and how they are
able to perform their particular jobs
Trace the path of water
from root hair to stoma.
For any vocabulary that is not self-explanatory
devise mnemonic devices to assist recall.
How can annular rings be used
to date building timbers
in pre-Columbian dwellings?
(Ahem: Can you relate this to gene sequencing?
Or to using Antarctic ice core samples to
estimate ancient atmospheric CO2 levels?)
flower
meristems
leaf
stem
root
endodermis
root hair
apoplastic
symplastic
casparian strip
geotropism
epiphyte
pneumatophore
vascular bundle
ground tissue
pith
cortex
sclerenchyma
cambium
vascular cambium
lateral meristem
terminal bud
apical meristem
axillary bud
internode
phototropism
auxin
stem tuber
hypocotyl
photosynthesis
gas exchange
vascular tissue
monocotyledonous angiosperms
dicotyledonous angiosperm
cuticle
upper epidermis
palisade parenchyma
palisade mesophyll
spongy parenchyma
spongy mesophyll
thigmotrophic
vein
xylem
phloem
lower epidermis
stoma
guard cell
xerophyte
hydrophyte
tendril
air bladder
collenchyma