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Plants - General
the plant and animal kingdoms are the most diverse and successful of all the
kingdoms of life on earth
they show a much more complex level of organization
both are comprised of many cells
in most plants and animals the cells have specialized into
various tissues and the tissues into organs for greater efficiency
Today, range in size from microscopic pondweeds to giant sequoias:
one of world’s most massive organism:
“general Sherman’ (giant sequoia in California)
272’ tall [81-5M] 79’ girth (>25’ dia)
tallest life form:
coastal redwood = 385’ tall[117M]
some of oldest organisms on earth:
Sequoias are 2000-3000 years old
bristlecone pines one >4900 years old
all land based animal life is dependent on plants as beginning of food chains
probably the most important and fundamental value of plants is least
understood by us:
 they are vital to our ecosystem
1. primary producers, basis of land food chain
2. plants also produce oxygen which all animals need to
breath
humans use plants for:
food,
heat,
antibiotics,
oils,
rubber,
poisons,
shelter,
furniture,
chemicals,
lubricant
latexes,
etc
clothing,
paper,
dyes,
waxes
resins,
esthetic benefits
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recreation (camping, backpacking, hiking)
house plants
flowers
Major Characteristics of Plants
1. autotrophs (photosynthesis)
while diverse in form almost all plants are similar in function:
use the sun’s energy to make organic food from
simple inorganic nutrients; nitrogen, phosphorus, CO2, etc.
photosynthesis to make food
CO2 + H2O + sunlight  sugars + O2
don’t usually need organic foods
a few are parasitic, cannot do photosynthesis
a few are carnivorous, usually grow in nitrogen poor soil
 use nitrogen in proteins of animals they catch
2. aerobic respiration to extract energy from that food
sugars + O2  CO2 + H2O + energy
3. plants produce several distinctive or unusual chemicals:
cellulose, lignin, secondary plant compounds
used for support, defense, communication, coordination of plant
activities, waste disposal
4. most plants store excess foods as starch
5. plant cells contain characteristic structures:
a.
b.
c.
d.
eukaryotes
surrounded by cell walls containing cellulose
large vacuole in center of cell for storage
plastids
other plastids containing various pigments
plastids to store starch and other materials
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6. all plants are multicellular
7. most have true tissues
tissues = groups of specialized cells performing common function
Animals are the only other kingdom with true tissues and
true organs
3 main plant tissues:
dermal: cover outer surface of all organs
vascular: plumbing system – moves water,
minerals, sugars, hormones throughout the plant
ground: does most of the “work” of a plant
eg. photosynthesis, storage,
8. most plants have simple organs
organs are groups of tissues working together to perform a
common function
all plant organs contain all 3 different plant tissues
Vegetative Organs
such as roots, rhozoids, stems, leaves, fronds
Reproductive Organs
Plants reproduce both sexually and asexually
asexual reproductive organs:
sporangia, capsules, sori
sexual reproductive organs:
antheridia, archegonia, cones, stamens,
pistils (carpels), and fruits.
unlike most animals:
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sexual organs are usually temporary structures
appear as plant reaches maturity
in perennials, may form each year
9. plants are nonmotile
subjects plants to dictates of nature
must be able to withstand fluctuations in
light, temperature, water availability, space & nutrients
10. Plants have simple responses to environmental stimuli
plants are nonmotile but do show some simple movements:
sunflowers and others turn leaves or flowers to
track sun during day
sensitive plants close their leaves when touched
vines and tendrils wrap around supports
insectivorous plants close leaves to trap insects
also, slower, more permanent, changes in orientations =tropisms
eg. phototrophism (light)
eg. geotropism (gravity)
eg. thigmotropism (touch)
11. plants have two different kinds of growth:
a. primary growth (herbaceous)
all plants
herbaceous plants have only primary growth
annuals – live for a single season
b. secondary growth (woody)
perennials
some vascular plants show woody (trees & shrubs)
related to the length of their life cycle:
a. annuals: live 1 year only, herbaceous
eg. beans, grasses, many wildflowers
b. biennials: require two growing seasons to
complete their life cycles;
most are herbaceous
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eg carrots, cabbage, beets, etc
c. perennials: live >2 years
all woody plants are perennials
sometimes above ground part dies back in
winter, sometimes just roots are woody
and the roots remain dormant
12. most plants show a distinctive alternation of
generations (sexual and asexual)
Plants reproduce both sexually and asexually
plants (& some algae and fungi) have a life cycle which consists of two
different forms
 alternation of generations
both forms reproduce, usually 1 sexually and the other asexually
in plants:
asexual form consists of a spore forming sporophyte generation
sexual form consists of a gamete forming gametophyte generation
Origins of Plants
plants are multicellular, photosynthetic, mainly terrestrial
only plants and fungi are primarily terrestrial organisms
 only a few are truly aquatic (but mostly freshwater)
 these two kingdoms have a long history of symbiosis
the earliest true plant fossils that we have are ~400 M Y old
true plants probably arose ~500 MY ago as
Advantages of moving from water to land:
1. plenty of light
2. CO2 more easily available than in water
3. no competition for minerals or space (at first at least)
Disadvantages of moving from water to land:
1. obtaining & holding onto water
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water now becomes a limiting factor
need to find ways to get it and to store it
also outer surface must become more water tight
2. as outer surface becomes less permeable still
need a way to get gasses in and out
3. need effective support to counteract counteract gravity
most organisms in water are naturally buoyant or have
floats or gas sacs to lift them to the surface
4. must be able to withstand greater extremes
in temperature, weather, etc
must develop ways survive freezing,
drought, extreme heat, etc
5. reproduction and dispersal on land requires
some way to get sex cells together and some way to disperse
offspring
in water gametes and zygotes are released directly into the
water
Plant Ancestors
all plants appear to have arisen from green algae living in freshwaters
Evidence for green algal ancestor to plants:
1. most green algae are found in freshwaters;
including swamps and marshes where some of the earliest plant
fossils first appear
2. many green algae today can live on land:
some are found in moist soil
some on forest floor
some on tree trunks
3. both green algae and plant cells:
a. are multicellular
b. main pigment is chlorophyll a
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c. store starch in plastids rather than free in
cytoplasm
d. have cellulose in cell wall
e. have peroxisomes (protective enzymes for
photorespiration)
f. cytoplasm divides by cell plate
4. primitive plants are still tied to water
plants originated more than once:
mosses not directly related to other plants
other plants ferns, conifers, flowering plants
many of the distinctive characteristics of plants evolved to solve the problems
for living on land:
1. obtaining water
land animals are motile and can seek it out
plants developed roots to search it out
2. retaining water
land animals have waterproof skin
plants developed waxy cuticle covering exposed areas (leaves
and stems)
some desert plants can absorb and store large quantities of water
in succulent stems or leave
 some only need a drink once a year or so
3. Plants also needed effective way to store nutrients and energy
animals seek out their food
plants use roots to grow (like fungal hyphae) into nutritious
soil
plants also store food in specialized structures such as
rhizomes, tubers, storage roots, etc
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4. plants must still be able to do gas exchange through this
waterproofing
 have pores (stomata)for gas exchange
5. plants must counteract gravity
 need to hold leaves toward the sunlight
cellulose cell walls offer enough support for smaller plants
later they developed special support tissues
6. plants needed to develop ways to withstand severe changes in
temperature
eg. cold – resistant seeds, dormancy, protected
underground parts
eg. heat – evaporative cooling (transpiration)
7. reproduction and dispersal require special adaptations
getting gametes together
dispersal of offspring
overcoming these limitations favored multicellularity
and specialization of layers of cells into tissues
and tissues into specialized organs
Plant Cells
animals have 100’s of different kinds of cells
plants have relatively few kinds
plant cells share many features with most eukaryotic cells:
nucleus
cell membrane
mitochondria
ribosomes
additional structures typical of most plant cells
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a. plastids:
chloroplasts – chlorophyll for photosynthesis
chromoplasts – pigments, fruits, flowers
amyloplasts – colorless, store starch, tubers
b. central vacuole – storage; contains “cell sap”
c. cellulose cell wall – protection, support, porous
Plant Organs
probably the most recognizable feature of plants are their simple organs
organ = groups of tissues arranged in clearly defined structures or parts
 isolated portions of plant tissues
Vegetative Organs
such as roots, rhozoids, stems, leaves, fronds
almost all plants live on land
have developed three major parts:
a. an underground portion that anchors the
plant and in some absorbs water and nutrients = root
b. some kind of supporting structure =stem
c. a flat structure used for most photosynthesis =leaf
Reproductive Organs
such as antheridia, archegonia, sporangia,
cones, flowers, stamens, pistils (carpels), and fruits.
Vegetative Plant Organs
Roots
are the part of the plant normally found below ground.
They are nonphotosynthetic and therefore not green
usually highly branched
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 often more extensive than above ground part of plant
roots grow rapidly
soil conditions greatly affect extent of root growth
Functions:
1. Anchor plant
They also help to anchor and to support the
above ground parts of the plant body.
usually in soil
in epiphytes, anchor them to other plants
in parasitic plants anchor them to host
2. in most plants roots are a food gathering organ
[like hyphae of fungi]
absorb minerals and water from soil
taproots extend deep to water tables
shallow fibrous roots are better at collecting rainwater
3. storage
surplus carbohydrates produced by leaves are
sent to roots for protected storage
some desert plants store scarce water in taproots
whatever conditions the above ground part of
plant is subjected to the roots most often survive
even in winter when whole above ground plant
dies back, roots live and resprout the next spring
4. transport
must be able to send these to rest of plant body
Root Structures
three basic patterns of root growth
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1. taproots
1 main root
many smaller lateral branches
eg. dandelion
most mature trees begin with a taproot but at
maturity have large, shallow, lateral roots instead
2. fibrous roots
several to many roots of same size that
develop from bottom end of stem
smaller lateral roots branch from them
eg. onions, crabgrass, monocots
3. adventitious roots
any roots that arise from stems or leaves
often modified for various additional functions
eg attachment, reproduction, etc
root cap
tip of each growing root is protected by a root cap
 covers and protects rapidly dividing and
growing cells of root (apical meristem)
also directs tip of growing root downward when cap is removed
roots grow randomly until they grow new root cap
root hairs
epidermal cells behind root cap and growing zone
produce long threadlike extensions =root hairs
short lived, single cell extensions of epidermal cells of root
form continually in area of cell maturation close to root tip
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very thin cuticle
greatly increase surface area for absorption
 up to 90% of root absorption occurs through root hairs
removing plant from soil destroys most of these root hairs
 decreases plants ability to absorb water
 need to keep transplants moist
since plants lack any kind of respiratory system each organ must be able to
carry out gas exchange on its own
like all plant organs roots need O2
good soil normally has lots of air spaces to allow gas exchange with
roots
eg. wont grow well if covered by concrete
eg. soil with too much clay (Austin) must be aerated with
compost and sand for most plants to grow well
eg. soil aerators sold at home stores
Stems
in plants with true stems, there is no sharp line of demarcation between roots
and stems
the stem is typically above ground and the roots, below ground
but many variations exist.
stems connect the nutrient and water gathering organs
(=roots) with the food synthesizing organs (=leaves)
General Stem Functions:
1. support
leaves and reproductive organs
2. Internal transport:
Conducting water and minerals from the root to other parts
of the plant.
Conducting food, which is manufactured in the leaves by
photosynthesis, to all other parts of the plant
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External Features of Stems
leaves and buds branch off at specific locations on the stem= nodes
distances between nodes = internodes
at nodes are leaves, buds or branches
new stems, leaves or reproductive organs develop from buds
terminal bud = tip of stem
axillary bud = at nodes
Stems can be herbaceous or woody:
herbaceous: annual or biennial - grow 1 or 2 years then die
woody: perennial - grow >2 yrs
A. herbaceous
dermal tissue is epidermis
stem is usually green
a few herbaceous plants lack a distinct stem
B. woody stems:
hard, thick, long lived
stem is not green
dermal tissue is periderm
 outer surface rough and covered by corky bark
outer surface of periderm often contains raised areas
= lenticels  for gas exchange
bulk of stem consists of tough woody tissue
lots of cellulose and lignin
woody plants are either:
trees = branching relatively high above ground
shrubs = branches at or near ground
since woody stems are perennial: they have some embryonic tissues
that can grow into new plant tissues and organs
Leaves
leaves are the most variable part of a plant
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can be round, needle-like, scale-like, cylindrical, heart-shaped, fan shaped
an entire set of terminology is used to describe
shape
margins
vein patterns
ways of attaching to stem
vary in size:
raffia palm  20 M (65’) long
duckweed  1/32nd of an inch
Victoria water lily pads up to 12’ circumference
leaves
can be deciduous or evergreen
can be simple or compound
can be alternate, opposite, whorled
Function of Leaves:
1. Carry out most photosynthesis
for most plants
herbaceous stems also do photosynthesis
2. Gas Exchange
leaves have pores (=stomata) that
take in CO2 for photosynthesis
release water vapor and O2
3. Absorption
all leaves can absorb some water and minerals
in some plants leaves absorb most of the
nutrients and water
eg. epiphytes
eg. pond plants
Leaf Structure
most leave composed of two parts petiole & blade
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at base of petioleoften with 1 or 2 stipules and axillary bud
vascular tissue can be seen as leaf veins also help support the blade
epidermis contains pores (=stomata) for gas exchange
stomata protected by two guard cells
stomata generally
open during day,
closed at night to conserve water
usually on lower surface of leaf but some floating
plants have them on the upper surface
evaporation of water from stomata also helps to cool plant
epidermis may also contain outgrowths = trichomes (=leaf hairs)
eg. in plants in salty environment often have
trichomes on leaves that remove excess salt (excretion)
eg. some have protective function
stinging nettle – contain poisons that inhibit herbivory
between the two layers of epidermis is 1 or 2 layers of ground tissue
(=mesophyll)
 photosynthesis
 gas exchange
Reproductive Plant Organs
in addition to vegetative organs plants typically produce reproductive organs
unlike most animals:
sexual organs are usually temporary structures
appear as plant reaches maturity
in perennials, may form each year
reproductive organs can be produced for asexual
reproduction or sexual reproduction
organs for asexual reproduction
produce asexual spores
capsule
sori
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organs for sexual reproduction
produce egg and sperm
antheridium and archegonium
cones (sexual cones)
flowers and fruits
specific kinds of reproductive organs are characteristic of each major plant
group and will be discussed with each group
Plant Tissues
plant cells are arranged into tissues
Plant cells are grouped together in various ways to form three major kinds of
plant tissues:
1. Dermal Tissue
2. Vascular Tissue
3. Ground Tissue
plant organs are much simpler than animal organs
each plant organ (roots, stem, leaf) has extensions of all three tissues
but they work together to perform specific functions
as plant grows the spatial relationships between these
systems may change but they remain continuously connected
1. Dermal Tissues
a. usually the epidermis ( mostly parenchyma cells)
single layer of tightly packed cells
covers and protects the young plant
is the “skin” of a plant
Functions of dermal tissue:
physical protection
water conservation
 protect & repair damaged area
absorbs water and minerals
generally no chloroplasts
transparent: allows light to penetrate into
interior photosynthetic cells
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dermal tissue on leaves and herbaceous stems secretes waxy protective
cuticle to restrict water loss
 thicker in drier habitats; thinner in wet habitats
epidermis contains pores (=stomata) for gas exchange:
stomata in epidermis protected by two guard cells
stomata generally open during day, closed at night to conserve
water
evaporation of water from stomata also helps to cool plant
during drought, stomata may remain closed during day
in many desert plants, stomata are closed during day and open at night
epidermis may also contain outgrowths = trichomes
eg. in plants in salty environment often have
trichomes on leaves that remove excess salt (excretion)
eg. some have protective function
stinging nettle – contain poisons that inhibit herbivory
epidermis of roots often has root hairs that greatly increase the surface area
of the roots for absorption of water and mineral
b. periderm replaces epidermis in stems and roots of woody plants
periderm is a complex tissue of cork and cork parenchyma cells
consists mostly of cork (=a kind of parenchyma)
the outermost protective layer on stems & roots
2. Vascular Tissues
the plumbing of vascular plants
mainly interconnected xylem (sclerenchyma) and phloem (parenchyma) tissues
for support and transport
xylem:
made up of dead cells that are joined together to form long tubes
or straws
moves water and nutrients from roots up stem to leaves:
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phloem: moves sugars, hormones and organic materials throughout
plant for use or storage or signaling
3. Ground Tissues
bulk of young plant
most of space between dermal and vascular tissue systems
not just a filler does most of the plants’ metabolic work such as
photosynthesis
used mainly for storage
some support in herbaceous plants or organs
gas exchange
in roots and stems:
between epidermis and vascular bundles = cortex
inside of vascular tissue to core of stem = pith
Location of Tissues in Roots, Stems & Leaves
Root Cross Section
the three tissues are arranged in distinct layers
1. dermal layer
lines outside of root
single layer of tightly packed epidermal cells
2. vascular tissue
usually in center of root
3. ground tissue
between the dermal and vascular tissue
used by some plants as major food storage area
Leaf Cross Section
1. Dermal Tissue
secretes waxy protective cuticle to restrict water loss
 thicker in drier habitats; thinner in wet habitats
generally no chloroplasts
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transparent: allows light to penetrate into
interior photosynthetic cells
contains pores (=stomata) for gas exchange
stomata in epidermis protected by two guard cells
2. Vascular Tissue
in the form of leaf veins
usually easily visible from outside of leaf
help support leaf
3. Ground Tissue
between the two layers of epidermis is 1 or 2 layers of ground tissue
(=mesophyll)
 photosynthesis
 gas exchange
Cross Section of herbaceous stem
are soft and green
little or no woody tissue
usually short lived: characteristic of annual plants
little or no growth in diameter
1. Epidermis on outside
outer surface has stomata in thin epidermis for gas exchange
may also have trichomes
2. Vascular Tissue
arranged as a circle of bundles just inside of the epidermis
or as bundles scattered throughout the interior of the stem
3. Ground Tissue
fills in the spaces between vascular tissue and epidermis
between epidermis and vascular bundles = cortex
inside of vascular tissue to core of stem = pith
Internal Anatomy of Woody Stems
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Conifers and flowering plants are the only major plant
groups that have woody, perennial species
Plants that live more than one year, ie. perennials,
produce secondary growth each year.
In woody plants the stem grows in width from cells produced by a layer of
cambium
 a layer of embryonic (meristematic) cells
In woody plants there are two major layers of embryonic cells called the
cambium, that produces this secondary growth each year.
1. Vascular Cambium
One layer of cambium is found in the vascular
bundles of the stem between the xylem and phloem.
Each year this cambium produces new layers of xylem and phloem
cells.
Xylem grows much faster than phloem and
 virtually all of the “wood” of a tree is dead xylem cells
growth rings are due to differences in cell size and thickness of
cell wall:
spring: fastest growth, plenty of water and nutrients
cells are larger with thinner walls=springwood
late summer: less water fewer nutrients available
cells are small and thicker walled= summerwood
Differences in the size of the cells produced throughout the
growing season produce the familiar “growth rings” in the
wood.
growth rings can also retain info on yearly climate and
other factors that affect growth
dendrochronology  can date back to ~9000 yrs
eg. Mesa verde
Cliff Palace constructed in 1073
Pueblo Bonito 919-1130
the oldest cells are closer to the center of the trunk
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Each year the cambium lays down a new layer of xylem
the amount of old wood closest to the center expands as new cells
are laid down in the vascular cambium
these older cells are often darker and are called heartwood.
 heartwood no longer functions in transport
vs sapwood (still used for transport)
the arrangement of different cell types in the secondary tissue
results in the distinctive characteristics of each kind of wood:
eg oak, maple, pine, etc
the layers of phloem are much thinner and become part of the
bark
The phloem, on the “outer-side” of the cambium,
the cortex and the periderm make up the bark of a tree.
2. Cork Cambium
Another layer of cambium is located between the cortex and
epidermis, this produces periderm
makes up most of the bark of woody plants.
The bark replaces the epidermis for protection
of the stem in large woody plants.
the bark contains cork cells
dead at maturity
heavily waterproofed cell walls
protects against injury, mild fires,temperature extremes, water
loss
variations in wood structure:
1. in moist humid tropics trees do not produce annual growth rings
environmental conditions determine the presence or absence of
rings
eg. those growing in areas with pronounced wet and dry
seasons show rings
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but may be more than one ring per year
2. in Panama canal zone is “the valley of square trees”
a species of cottonwood
trunks are roughly square in cs;
growth rings are also square
believed to be due to local growing conditions
3. softwood vs hardwood
 conifer woods are generally softer than wood of flowering
plants
hardwoods are mostly flowering plants
wood is typically produced in the stem (=trunk) but major roots of trees also
usually have wood, bark and annual rings
today >4500 products are made from wood
the unique properties of wood depend on its chemical composition:
eg. cellulose
secondary plant tissues have very thick layers of
cellulose around cell
produce strong wood and fibers
the strength of the wood and fibers is directly related
to how much cellulose they contain
cellulose is very resilient
 difficult to decompose
very few organisms have developed the ability to
break it down (need cellulase enzyme)
some bacteria, many fungi, a few protozoans and
a few insects (silverfish)
eg. lignin
also found with cellulose in secondary cell walls
even more rigid and resistent to decay
even fewer organisms can break it down
 fungi
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in pulping industry its difficult to remove
must be removed completely or paper will yellow
and lose quality
need strong chemicals to do this
 expensive and dangerous to environment
eg. pectin
often found in association with cellulose
used as a cement to glue cell walls together
extracted and used for making jellies and jams
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