2. Botany: the Study of Plants
• What is a plant?
– Lives on land
– Doesn’t move under its own power
– Produces food and energy from sunlight (photosynthesis)
– multicellular
– embryo develops inside the mother's body
• Excludes algae (live in water) and fungi (no photosynthesis)
and bacteria (unicellular)
– All of these have been traditionally part of botany, in which
plants were defined as anything that wasn’t an animal.
• However, some plants don’t photosynthesize (parasites) or
live in the water. They are considered plants because they
are descended from legitimate photosynthesizing, land-
dwelling plants.
3. Plant Cells
• All living things are made of cells.
• Plant cells are very similar to animal cells:
• plants and animals are both eukaryotes (as
opposed to prokaryotes, which are more
primitive single celled things like bacteria and
archaea)
– eukaryote means the cell's DNA is enclosed in
a nucleus
• all cells are surrounded by a cell membrane,
which keeps the inside separated from the
outside.
• The cytoplasm is everything between the cell
membrane and the nucleus. It contains lots of
useful organelles which do all the metabolic
activity and chemical changes needed to keep
the cell alive.
4. Plant Cells vs. Animal Cells
• Plant cells are different from animal cells in several ways:
– Rigid cell wall (cell wall is OUTSIDE the cell membrane. It is the box that contains
the cell.)
– Central vacuole that stores water
– Chloroplasts that perform photosynthesis
• Chloroplasts were originally free-living photosynthetic bacteria that got
swallowed by a primitive eukaryotic cell and developed a mutually beneficial
symbiotic relationship inside the cell (endosymbiont theory)
5. Cell Walls and Vacuoles
• All cells have to deal with osmotic pressure.
– There is a higher concentration of particles inside the cell than outside: in other words,
there is less water in the cell than outside
– So, water is trying to diffuse into the cell, going down its concentration gradient
– This would cause an unprotected cell to swell up and burst.
– The cell wall acts as a rigid box to prevent the cell from bursting.
• On the other hand, if the plant isn’t getting enough water (or if the plant is put in a
high salt solution), the water supply in the central vacuole moves into the
cytoplasm.
– This causes the cell to shrink away from the cell wall.
– The plant wilts
6. Cell Walls
• The cell wall is mostly made of cellulose.
– Cellulose is a molecule made of many glucose
sugar molecules linked in long chains
– Starch is also made of many glucose units, but
the linkages between the glucoses is different in
cellulose and starch. This gives them different
chemical properties.
– Notably, almost all organisms can easily digest
starch, but very few can digest cellulose.
• Mostly just some types of bacteria and
protists
– Cellulose is probably the most common organic
compound on Earth.
• In cells needed for support or water
conduction, the cell wall is thickened and
strengthened by lignin, a complex organic
compound that is even harder to digest than
cellulose.
7. Photosynthesis
• Photosynthesis uses energy from light
to convert carbon dioxide (CO2) into
sugar.
• Occurs in the chloroplasts, which were
once free-living bacteria that got
swallowed up by endosymbiosis.
– In other parts of the plant, chloroplasts
get used for storage of food or other
pigments (like in flowers)
• Two parts to photosynthesis: light
reactions (occur only in the light) and the
Calvin cycle (occurs in both light and
dark).
– Light reactions: Light energy is captured
by chlorophyll and used to extract
electrons from water, which converts it
to oxygen.
– Calvin cycle: The high energy electrons
are used to convert carbon dioxide into
sugar. This is called carbon fixation.
8. Plant Tissues
• A tissue is a group of cells that
performs a specific function.
• Four basic types in plants:
meristems, dermal tissue, vascular
tissue, and ground tissue.
• Meristems are special regions
where cell division occurs.
Meristems produce all of the new
cells; once a cell leaves the
meristem, it can enlarge but not
divide.
– Apical meristem: at the tip of the
plant shoots and at the tip of the
roots. This is where growth occurs,
producing new leaves, branches,
flowers, etc.
– Lateral meristem: in the stems of
woody plants: they produce lateral
growth. Also called cambium
layers.
9. Vascular Tissue
• Two basic types: xylem and phloem
• Xylem conducts water and mineral nutrients up from the roots.
– Xylem cells are dead and hollowed out.
– Wood is made of xylem, but even non-woody plants have xylem.
– Water is pulled up by transpiration: water molecules evaporating from the leaves pull other
water molecules up the tubes, because water molecules stick together.
• Phloem cells carry organic matter (mostly sugar) from the leaves to other parts of the
plant.
– Unlike xylem, phloem cells are alive.
– The cells are connected by many pores, so material flows easily between the cells.
– Flow of material in both directions
10. More Tissues
• Dermal tissue is the outer covering (the skin) of the
plant.
– Secretes waxes that make up the waterproof cuticle.
– Stomata: openings in the leaves to let gases in ant out.
Stomata open and close under different conditions.
– Hairs on leaves, shoots, and roots
• Ground tissue is all the rest of the cells in the plant.
Photosynthesis, food storage, support, fibers.
11. The Plant Body
• The basic parts: roots, shoots, leaves,
flowers, fruits.
– Most photosynthesis occurs in the
leaves. Photosynthesis produces sugar
(sucrose), which is used to feed the rest
of the plant.
– Water and mineral nutrients come from
the soil: they are absorbed into the
plant by the roots.
– Stems hold the leaves and flowers up in
the air: off the ground, above things
that might block the sun, away from
predators and decay organisms. Stems
contain the plumbing that carries
nutrients to different parts of the plant.
– Flowers are the reproductive structures,
which produce the plant equivalents of
sperm and egg.
– Fruits hold the seeds (products of
reproduction) and provide nutrients
and a means of dispersing the seeds to
new locations.
12. Leaves
• Leaves are the main site of
photosynthesis.
• Photosynthesis mostly occurs in
the layer of cells just below the
epidermis. (palisade layer)
• The sugars are then transported to
other parts of the plant through
the vascular system.
– The spongy tissue below the
palisade layer carries the sugar
(dissolved in water) to the veins of
the leaf, which are part of the
vascular system.
– Monocot leaves have parallel leaf
veins, while dicot leaves have a
net-like vein pattern.
• Leaves are coated with a waxy
layer called the cuticle. The leaf
epidermis cells secrete the cuticle,
which helps prevent the leaf from
drying out.
13. Stomata in the Leaves
• Photosynthesis needs CO2 from the
atmosphere, which comes in through
the stomata. Transpiration needs water
vapor to evaporate out through the
stomata
• Stomata are located on the underside of
the leaves.
• Stomata can open and close: need them
open to admit carbon dioxide, but not
so much as to dry out the plant.
• C4 and CAM metabolism: Some plants
(notably grasses and succulents like
cactus) have developed a fancy
mechanism that allows CO2 to enter the
stomata and be temporarily fixed at
night when it is cool. During the day,
the stomata are closed and the plant
does the rest of photosynthesis on the
stored CO2.
14. Stems
• In the stem, the xylem and phloem cells are organized
into vascular bundles.
• In monocots (grasses, lilies, orchids), the vascular
bundles are scattered throughout the stem.
• In non-woody dicots, the vascular bundles form a ring,
with the xylem cells towards the inside and the phloem
cells on the outside.
• In woody dicots (trees and shrubs), the stem grows
larger by adding new xylem and phloem cells.
– The new cells are made by a cambium layer between the
xylem and phloem. At different times of the year,
different sizes of xylem cell are produced, creating an
annual growth ring.
– Wood is xylem cells with their cell walls thickened with
lignin. The inner areas of a tree’s trunk (the heartwood)
no longer functions, but the outer part (sapwood)
conducts water up from the roots.
– The bark is produced by a second cambium layer, the cork
cambium, which is outside the phloem layer.
15. Roots
• The roots anchor the plant to the
ground. They also take in water
and minerals from the soil.
– Water and minerals are then
conducted to the rest of the plant
through the xylem
– The leaves supply sugar to the root
cells through the phloem.
• Two main types: fibrous roots ( a
tangle of small roots) and taproots
(a single main root)
– Fibrous roots are common in the
grasses
– Taproots are often enlarged for
food storage: things like carrots
and turnips.
16. Flowers
• Flowers are the defining
characteristic of the angiosperms
(the flowering plants). They are the
reproductive organs of the plant.
• Flowers consist of 4 whorls of organs:
sepals, petals, stamens, and carpels.
– Carpels used to be called pistils.
• The four whorls of the flower are
inserted into a receptacle, which is
the tip of the flower stem.
• Different plant groups have
characteristic numbers of these
parts: monocot flower parts come in
3’s, while dicot flower parts come in
4’s (especially the mustard family)
and 5’s (like roses and apples).
17. Four Whorls
• The sepals are the outermost whorl. They are the protective covering for
the unopened flower bud. Usually sepals are green and leaf-like.
– However, sometimes the sepals are colored: in lilies there are 3 sepals and
petals that are almost identical.
• The petals are the next whorl in. They are the part that are often
conspicuously colored, used to attractive animal pollinators like bees,
birds, and bats.
– The petals are not always symmetical, and sometimes they are fused to each
other and to the sepals.
• The stamens are the male reproductive organs. The most important parts
of the stamens are the anthers, which release the pollen grains. Pollen is
the plant equivalent of sperm cells.
• The carpels are the female reproductive organs. The most important part
of the carpel are the ovaries, which hold the ovules. The ovule is the plant
equivalent of the egg cell. After the ovules are fertilized, the ovary
develops into a fruit. Another important carpel structure: the stigma, the
sticky part where the pollen lands.
18. More Flowers
• Some flowers are imperfect,
which means they contain only
male parts or only female parts.
Corn is a good example: the
tassel is the male flower: it
sheds pollen. The silks and ears
are the female parts: each corn
kernel started out as a single
ovule.
– Perfect flowers contain both
male and female parts. This is
the usual condition in plants.
• Some plants have male flowers
on one plant and females on
another: date palms, marijuana,
holly are example.
19. Meiosis and Fertilization
• All plants except the most primitive
ones (the bryophytes) are basically
diploid. This means that every cell
has two sets of chromosomes, one
from each parent, just like us.
• For reproduction to occur, the
plants must produce haploid cells,
the gametes. Gametes have one
copy of every chromosome.
• The male gamete (pollen) fertilizes
the female gamete (ovule) to
produce the zygote, the first cell of
the new individual. The zygote is
diploid.
• Meiosis is the cell division process
that creates haploid gametes from
the diploid plant cells.
20. Pollination
• Pollination is the process of getting the pollen to the
stigma of the female plant.
– Some plants allow self-fertilization: the male pollen
fertilizes the female ovule of the same plant. This is the
closest possible genetic cross, and it isn’t possible in
animals.
– Most of the time it is advantageous to have cross-
pollination: the pollen from one plant fertilizes the ovules
of another plant. This increases the genetic diversity of
the offspring, which means more will survive under
varying conditions.
• Wind pollination is how all gymnosperms are
pollinated. Also, angiosperms with small,
inconspicuous flowers. Grasses are a good example.
– The plant produces huge numbers of pollen grains, which
get blown off the anthers by the wind, and occasionally
end up on the female parts of another plant of the same
species. Not very efficient.
21. Animal Pollination
• Animal pollination is much more efficient than
wind pollination: the animal delivers the pollen
directly to the female.
• Bees, butterflies, wasps, birds, bats
– very ancient plants like magnolia are pollinated
by beetles. Bees hadn't evolved when these
flowers first appeared.
• Plants attract animals by supplying them with
food. Nectar is a sugary liquid secreted by
glands at the base of the flower: the animal
eats it. Animals also eat the pollen. However,
some pollen gets on the animal and gets
carried to the next flower, where is gets
deposited on the stigma.
– So, the animal isn't pollinating just to be helpful.
The animal is feeding, and pollination is just an
accidental byproduct.
• Plants also supply guiding signals: flower color,
pattern, scent
22. Co-evolution
• Animal pollination is a major evolutionary innovation in the angiosperms, and the
plants and animals have modified each other through the process of co-evolution to
make it more efficient for both.
– Natural selection for mutations in the plant that make it more attractive to a pollinator: the
mutant plants are fertilized more frequently than the original plants.
– Natural selection for mutations in the animal to make it more efficient at finding the proper
plant and extracting the nectar.
• Some examples:
– flowers with long throats are pollinated by hummingbirds with long beaks.
– Rotting meat smell attracts fly pollinators.
– Orchid flowers look enough like the pollinating wasp that the wasps try to mate with them.
• Bees don't see the color red, but they do see blue and UV. Bee-pollinated flowers are
usually blue or purple, and often have patterns visible in the UV range.
• Butterflies can see red and all other colors, but have a poor sense of smell. They also
need a wide perch to land on. Butterfly-pollinated flowers are large and bright, with
little scent.
• Moths are nocturnal and have a good sense of smell. Moth-pollinated flowers are
white so they can be seen at night, and have a strong scent.
24. Fertilization
• Once pollen has been deposited on the stigma, the process of fertilization occurs.
• Angiosperms (flowering plants) have a unique process called double fertilization. Found in
all angiosperms but no other organisms.
• The pollen grain grows a long tube down the carpel until it reaches an ovule in the ovary.
•Two sperm nuclei then enter the
ovule.
•One sperm nucleus fuses with
an ovule nucleus to form the
zygote, the first cell of the new
individual. The zygote starts
dividing and becomes an
embryo. This is the equivalent of
fertilization in animals.
•The other sperm nucleus fuses
with two other ovule nuclei to
form the endosperm, which is a
nutritive tissue for the developing
seed. Most of the food found in
grains like wheat, rice and maize
is the endosperm.
25. Post-Fertilization
• The embryo develops into a seed.
– Seeds are multicellular, fully formed, miniature plants that are in a dormant
state. This allows them to survive winter or other bad conditions, and then to
quickly turn into functioning plants when conditions improve.
– In contrast, lower plants have single-celled spores instead of seeds. Spores
can only survive briefly, and it takes a long time to get from a single cell to a
large mature organism.
• Sepals, petals, and stamens wither away.
• The ovary increases in size and becomes a fruit, which contains the seeds.
– Fruits are a mechanism for seed dispersal.
– As with pollination, some fruits use animals for dispersal, and other fruits use
the wind.
27. Fruits
• Fruits develop from the wall of the
ovary, the pericarp. Fruits contain
the seeds and are responsible for
seed dispersal.
• Lots of types of fruit, we are going
to stick with a simple classification
scheme. First, we classify by the
number of ovaries that make up
the fruit:
– Most fruits are simple fruits: the
product of a single ovary, which
can contain one or many seeds
– There are also aggregate and
multiple fruits, which develop
from one flower that has many
carpels, or from the fusion of the
ovaries form many flowers:
raspberries and pineapples for
example.
28. More Fruit Classification
• Second, three categories of fruit
appearance:
– Fleshy: what we think of as fruit: a
soft, juicy layer surrounding the
seeds. This layer causes an animal to
eat the fruit and carry the seeds to
new locations in its digestive system,
depositing the seeds with a load of
fertilizer.
– Dry: the pericarp is either tough and
woody or thin and papery.
• Some dry fruit is dehiscent, which
means it splits open to release
the seeds, like pea pods or
milkweed or poppy
• Other dry fruit is indehiscent,
meaning that the seeds stay
inside the fruit, like the winged
seeds of maple trees and cereal
grains.
29. Seeds
• Seeds develop from the fertilized ovule. Their DNA comes
from the pollen (father) as well as from the ovule (mother).
– In contrast, the fruit’s DNA is strictly from the maternal plant.
• Inside the seed, the plant has both a root and a shoot.
• Seeds contain a food source as well as the embryo. Until
photosynthesis gets started, the new plant needs to live on
stored food.
• The cotyledons are the first leaves of the new plant. They
are fully formed in the seed. The cotyledons unfold when
the seed germinates.
• Major difference between monocots and dicots:
– Monocots have a single cotyledon (which is what “monocot”
means). They use endosperm (the other product of double
fertilization) as food.
– Dicots have two cotyledon leaves. Before the seed in fully
formed, the dicot cotyledons absorb the nutrients from the
endosperm, so dicot seeds use food stored in the cotyledons,
not the endosperm.
30. Seed Germination
• Seeds need proper conditions of moisture, oxygen, and temperature to germinate.
– Some seeds will only germinate if they have been through a cold spell, or if they have had their
seed coats injured by fire or abrasion.
– Seeds of the Tambalacoque tree on the island of Mauritius (in the Indian Ocean) apparently only
germinated when passed through the digestive system of the dodo (which is extinct). Turkeys
work as an adequate substitute. (this story may not be 100% true)
• The dry seed imbibes water, and the root sprouts, followed by the shoot.
• Once the shoot breaks through the surface of the ground, it is exposed to light, which
allows it to develop chlorophyll and start photosynthesis.
31. Legal Fruits
• Botanically, a fruit is an ovary that has ripened after fertilization.
• However, in 1883 a 10% duty was placed on all vegetables being imported
into the US.
• John Nix, an imported from New Jersey, argued that he shouldn’t have to
pay the duty on tomatoes, because botanists consider them fruits.
• The case went all the way to the Supreme Court (which means at least 3
separate courts examined the question). In 1893, the Court ruled that for
legal purposes, tomatoes were a vegetable, not a fruit.
• Based on popular usage: vegetables (including tomatoes) are eaten at
dinner, while fruits are sweet and are eaten at dessert.
• Tomatoes are the state vegetable of New Jersey. Ohio considers
tomatoes to be the state fruit. In Arkansas, tomatoes are both the state
vegetable and the state fruit (indecisive).
33. Plant Evolutionary Trends
• Plants are thought to have evolved from the green algae, which live in the
water.
• By moving onto the land, plants had to deal with 2 big issues: gravity ( or
lack of buoyancy) and dryness.
• Major trends:
– 1. development of roots, shoots, vascular system. Roots needs to absorb
nutrients, not just hold onto the surface. Shoots need to support
photosynthetic system off the ground. Vascular system to transport materials
between parts of the plant. Waxy cuticle on the leaves to prevent desiccation.
– 2. increasing the diploid phase of the life cycle, and decreasing the haploid
phase. Diploid gives a backup copy of each gene, as a defense against random
mutations. Allows a larger, more complex body.
– 3. Seed and pollen protection and dispersal. Development of very different
male and female gametes, so only one type needs to be dispersed in the
environment. The pollen (male gametes) needs to be protected from
desiccation, and needs to find the female gametes successfully. Seeds also
need to be protected from harsh conditions and to disperse to new locations.
– 4. Flowers and fruits used to attract animals to help spread pollen and
offspring.
34. Major Plant Groups
• We are going to briefly
examine several groups that
show these trends:
– 1. bryophytes: non-vascular
plants including liverworts
and mosses
– 2. seedless vascular plants
such as ferns and horsetails
– 3. gymnosperms, which have
seeds and a vascular system,
such as the conifers
– 4. angiosperms, the flowering
plants that dominate the
world today.
35.
36. Bryophytes
• The bryophytes include the mosses,
liverworts, and hornworts. They are short
plants mostly growing in wet environments.
• Bryophytes have a waxy cuticle on their
leaves to prevent desiccation.
• Bryophytes have no internal vascular system.
• Bryophytes spend most of their lives as
haploids: the body of the moss plant is
haploid.
• The only diploid structure is a stalk and spore
capsule, which grow out of the haploid plant
body.
• Peat moss is used to help soil hold water. It
can also be used as fireplace fuel when it is
dried. Peat bogs are very acidic, which allows
plants like cranberries and blueberries to
grow.
– Also, the acidic conditions preserve animal
bodies—several humans who lived up to 5000
years ago have been dug out of peat bogs.
37. Bryophyte Life Cycle
• The haploid gametophyte plant
bodies are either male or female.
Each produces a different kind of
gamete (eggs or sperm) at the tip of
the plant body.
• The sperm are motile: they swim
through drops of water (rain or dew)
to reach the eggs. The eggs are
encased within the female
gametophyte’s body.
• After fertilization, the diploid
sporophyte grows as a stalk out of
the female gametophyte’s body.
• After the diploid sporophyte
matures, the cells in it undergo
meiosis, forming haploid spores.
• The haploid spores disperse in the
wind, and go on to form new
gametophyte plants.
38. Seedless Vascular
Plants
• The seedless vascular plants include
ferns and horsetails.
• A vascular system to distribute
nutrients throughout the plant allows
them to grow tall. Some ferns grow
up to 80 feet tall, and some extinct
horsetails were also tree-sized.
• Being seedless means that the diploid
sporophyte grows out of the
fertilized egg, attached to the
gametophyte.
• The diploid sporophyte is much larger
than the haploid gametophyte stage:
most of what you see in these plants
is the sporophyte.
• The sperm have flagella and swim to
the eggs through drops of water (just
like the bryophytes).
39. Fern Life Cycle
• The main plant body in the diploid
sporophyte. Specialized structures
on the underside of the leaves
develop, and inside them meiosis
occurs.
• The haploid meiotic products are
released as spores, which are
dispersed to new locations and
germinate into gametophytes.
• The haploid gametophytes are quite
small, a few millimeters in diameter.
They contain structures that produce
sperm and eggs.
• The sperm swim to the eggs and
fertilize them
• The fertilized eggs are diploid, and
they grow into the sporophyte plant
body.
40. Seeds and Pollen
• A major development in plant
evolution was the development
of pollen grains and seeds.
• Pollen grains are the male
gametophyte packaged in a hard
coat that allows it to reach the
female without having to swim
through water. This is a large
advantage on dry land.
• Seeds are diploid sporophyte
embryos, packaged to survive a
period of dormancy and bad
environmental conditions. Seeds
develop from the fertilized egg.
They are multicellular: small
plants that need very little
growth to live independently.
41. Gymnosperms
• Gymnosperms were the first plants
to have pollen grains and seeds.
• Gymnosperm means “naked seed”:
their seeds develop on the outside
of the plant, instead of inside an
ovary as in the flowering plants.
• The most important gymnosperms
today are the conifers: pines,
redwoods, cedars, etc. All are
woody plants with needles or scales
as leaves.
• Conifers are our main source of
wood and paper.
• Ginkos and cycads are other
gymnosperms.
– Cycads were the dominant plant
type in the Mesozoic era
42. Angiosperms
• Angiosperms are flowering plants.
Most of the plants we see are
angiosperms.
• Unlike the other plant groups,
angiosperms are often fertilized with
the aid of animals: insects, birds,
bats, that carry the pollen from one
plant to another. The plants and
their pollinators have co-evolved in a
symbiotic relationship.
• Flowers produce the visual signals
and the scents that pollinators use to
find the plants. Flowers secrete
nectar which is eaten by the
pollinators. The pollen is carried
from flower to flower on the body of
the pollinator, as a consequence of
its going into the flower in search of
nectar.
• Some angiosperms have wind-
dispersed pollen. Flowers on these
plants are usually small and
inconspicuous.
43. Angiosperm Life Cycle
• Most of the angiosperm’s life is the
diploid sporophyte stage.
• The male gametophyte is the pollen
grain; the female gametophyte is the
ovule.
• Angiosperms have double
fertilization: 2 sperm fertilize
different cells in the ovule, producing
the diploid embryo and the triploid
endosperm.
• The embryo develops into a seed, a
small immature plant, which goes
into a dormant phase.
• The seed germinates, putting our a
root and a shoot. The shoot turns
green and starts photosynthesis
when light hits it.
44. Angiosperm Groups
• Flowering plants used to be split
into 2 groups: monocots and
dicots.
• More recently it has become
clear that several groups split
off from the main evolutionary
lineage before the monocots
did.
• Now, we can divide the
angiosperms into 3 main
groups: the basal angiosperms,
the monocots, and the
eudicots.
--basal angiosperms are not a single
unified group. We are just
throwing them together for
convenience.
45. Basal Angiosperms
• Most basal, meaning the earliest to split off from
the main lineage: Amborella. A group of shrubs
growing on the island of New Caledonia in the
Pacific Ocean east of Australia.
• Magnolia and relatives is the largest group of basal
angiosperms. Several useful ones: nutmeg, bay
laurel, cinnamon, avocado, black pepper.
• Water lilies are another group of basal
angiosperms.
46. Monocots
• Monocots are a very large group.
– One cotyledon leaf. The cotyledons are the leaves
found in the seeds that push up above the soil
when the seed imbibes water and starts to grow.
– Parallel leaf veins
– Flower parts in groups of 3
– Scattered vascular bundles. Means there are no
woody monocots.
• Main groups: grasses, lilies, orchids, palms,
onions.
47. Eudicots
• The largest group of plants today.
• Many groups, mostly of interest only to botanists.
• We will often speak of plant families. A few
examples:
– Nightshade family: tomato, potato, tobacco,
capsicum pepper
– Rose family: apples, cherries, strawberries
– Legume family: peas, beans
