Transcript Document

Plant structure and function
(parts of chapters 35, 36 and 37)
Unlike animals, plants remain in one place and
produce food through photosynthesis.
In the process of photosynthesis plants (and
other photosynthetic organisms such as algae,
other protists, and cyanobacteria) trap the
energy in sunlight and store it in chemical
The energy stored in chemical bonds can then
be used to fuel metabolic processes.
Figure 10.2
Plants and photosynthesis
 This
 In
process is called photosynthesis.
this class we will not discuss the
process of photosynthesis in detail. It is
covered in depth in Bio 101.
 In
photosynthesis carbon dioxide (CO2)
and water (H20) and the energy provided
by light are used to make glucose.
CO2 + 12 H20 + energy  C6H12O6 +
6O2 + 6 H20
 The
organelle plants use to carry out
photosynthesis is the chloroplast.
 In
plants chloroplasts are concentrated in
the leaves, which generally are thin and
flat to allow maximum exposure to light.
Fig 10.3
Basic structure of plants
 To
carry out photosynthesis plants must
obtain water and minerals from the soil,
CO2 from the air, and light from the sun.
 The
structure of plants reflects their need
to carry out these tasks.
Basic structure of plants
 Plants
have three basic organs:
 These
organs are organized into two
systems: the largely below-ground root
system and the above-ground shoot
system (stems and leaves).
 Roots
perform several tasks. They
Anchor the plant in place
Absorb minerals and water
Store organic nutrients such as sugars (e.g.
carrot, sugar beet, turnip).
 Roots
systems may have a central
taproot with lateral roots branching off
from it (e.g. dandelion).
 Alternatively,
a root system may have no
obvious main root, but instead be a
fibrous system with many small roots
growing from the stem, each of which has
its own lateral roots (e.g. grasses).
 The
entire root system anchors a plant in
place, but absorption of water and
minerals occurs mainly at the root tips.
 At
the root tips huge numbers of root
hairs increase the surface area
Root hairs
 Root
hairs are extensions of individual
epidermal root cells and are not
multicellular structures
(as lateral roots are).
 Root
hairs are permeable to water and
adhere closely to soil particles allowing
efficient absorption of water and nutrients.
 Most
plants forms mutually beneficial
relationships with fungi, which facilitate
absorption of water and minerals.
The plants and fungi form mycorrhizae:
symbiotic associations of plant roots united with
fungal hyphae (hyphae are tiny filaments that
form the bulk of a fungus).
Most plants form these symbiotic mycorrhizal
relationships and they greatly enhance the
plants growth. [a symbiotic relationship is a
close, mutually beneficial relationship]
Mycorrhizae (white) growing on a root
 The
fungal hyphae grow over the root and
penetrate into it and may in some cases
form a mantle or layer over the root.
 The
fungus benefits from a steady supply
of sugar donated by the host plant.
Plant receives numerous benefits:
Fungus greatly increases surface area for absorption
(can be as much as 3 meters of hyphae per cm of
plant root length).
Fungus selectively absorbs phosphate and other
nutrients and supplies them to plant.
Fungus may secrete growth factors that promote root
Fungus may produce antibiotics that protect the plant
from pathogenic bacteria and fungi in the soil.
 Plant-fungus
symbiosis may have been
one of the early adaptations that allowed
plants to colonize the land, which probably
initially was quite nutrient poor.
 Fossils
of some of the earliest plants show
Shoot Systems
 Shoot
systems consist of stems and
 Stems
are elongated structures comprised
of nodes and internodes.
 Nodes
are where leaves are attached and
internodes are the sections in between.
Shoot Systems
 Stems
have a terminal bud at the tip and
this is where elongation takes place,
enabling the stem to reach upwards
towards the light.
 If
the tip of the stem is eaten or shaded,
however, axillary buds (buds on the side)
will begin to grow.
Shoot Systems
 Gardeners
 By
shape plants by pruning them.
removing terminal buds a bushier plant
can be produced or by removing lateral
flower buds a single large flower can be
Shoot Systems
 Stems
have been greatly modified in many
plants to perform a variety of functions.
 Rhizomes,
bulbs, tubers, and stolons are
all modified stems although they are often
mistaken for roots.
Modified stems
Bulbs: vertical shoots that grow underground.
The “flesh” of a bulb (e.g. an onion) consists of
leaves modified for food storage.
Stolons and rhizomes: are stems that grow on
(stolons) or just under (rhizomes) the soil
surface. New plantlets form periodically along
the length of these stems (asexual
Modified stems
 Tubers:
are enlarged ends of rhizomes
specialized for storing food (e.g. potato).
 The
“eyes” of a tuber are axillary buds.
 Leaves
are the main photosynthetic organ
of plants, although green stems also
perform photosynthesis.
 Leaves
vary in form, but usually have a flat
blade and a stalk (petiole) that joins the
leaf to the stem.
 Leaves
are generally flat to maximize the
area exposed to the sun and minimize the
distance gases must be transported to and
from photosynthesizing cells.
 However,
in many cases leaves have been
substantially modified by natural selection
to perform other functions.
Modified leaves
Tendrils of climbing plants such as clematis are often
modified leaves.
Spines of cacti are modified leaves (most
photosynthesis being carried out by the fleshy stem.
Some leaves are modifed as storage leaves to store
Some leaves called bracts look like petals (e.g. in
dogwoods) being brightly colored and enlarged to attract
pollinators to the flowers they surround.
Some leaves produce plantlets that drop off the plant
and take root in the soil.
Plant vascular system
 Plants
contain two vascular systems that
transport water, minerals, and sugars
around the plant.
 Xylem
transports water and dissolved
minerals from the roots into the shoots.
 Phloem
transports sugars from the leaves
to where they are needed in the plant.
Plant vascular system
 Xylem
cells are dead at functional maturity
and form thin elongated tubes that water
moves through.
 Phloem
cells are alive.
Plant secondary growth
Primary growth is stem elongation, secondary
growth refers to the thickening of woody plants
over time.
Xylem and phloem cells are both produced by a
plant tissue called vascular cambium that is
located under the bark.
This cambium produces xylem cells on the
inside and phloem on its outside.
Plant secondary growth
 As
the plant grows older inner xylem
tissue forms the heartwood of the tree.
This tissue no longer transports liquid.
 Xylem
cells have thick lignified walls (lignin
is a complex cross-linked polymer) that
provide structural support for the plant.
 The outer (more recently produced) xylem
is called sapwood and this carries liquid.
Plant secondary growth
 On
the outside of the cambium layer
phloem is produced. Phloem is produced
more slowly than xylem and older phloem
is sloughed off the tree so it does not
accumulate as xylem does.
Plant secondary growth
 Because
the outer layer of phloem is
essential to transportation, a tree that is
“ringed” by grazers (i.e., has its outer bark
removed around the circumference of the
plant) will die.
 In
contrast, a tree may be hollowed out
and still survive.