Plants
Anatomy of Plants
• The anatomy of a plant in its
most simplistic form can be
divided into the roots and the
shoots
– Roots are portions of the plant
that are below the ground
– Shoots are the portion of the
plants that are above the ground
• The roots work their way
through the terrain, working as
an anchor to keep the plants in
place.
• In addition, the roots work as
gatherers, absorbing the water
and nutrients vital to a plants
survival
Tissue System
• There are three plant
tissue systems to
know:
– Ground
– Vascular
– dermal
Ground Tissue
• The ground tissue, that makes up most of
the body of the plant, is found between the
dermal and vascular systems and is
subdivided into three cell types:
– Collenchyma cells
– Parenchyma cells
– Sclerenchyma cells
Collenchyma Cells
• Live cells that provide
flexible and
mechanical support
– Often found in stems
and leaves
Parenchyma Cells
• The most prominent of
the three types, with
many functions
• When found in leaves
are called mesophyll
cells
– Allow CO2 and O2 to
diffuse through
intercellular spaces
• (owing to the presence of
large vacuoles, these cells
play a role in storage and
secretion for plants
Sclerenchyma Cells
• Protect seeds and support the plant
Vascular Tissue
• Plant vascular tissue comes up often on
the AP exam. There are two characters you
need to be familiar with… can you guess
what they are?
Xylem
• This structure has multiple functions
– It is a support structure that strengthens the
plant
– Functions as a passageway for the transport of
water and minerals from the soil
Sad fact: 
most
xylem cells
are dead
Xylem
• Xylem can be divided into two categories:
– Vessel elements and tracheid cells
• They both function in the passage of water, but
vessel elements move water more efficiently
because of structural differences
Phloem
• This structure also functions as a
“highway” for plants
– Assisting in the movement of sugars from one
place to another
Phloem
• Unlike the xylem, the functionally mature
cells of a phloem, sieve-tub elements, are
alive and well
Hmmmmm….?
• What are some ways you can remember
the differences between the xylem and
phloem????
I wonder who
is sooooo
smart?
Dermal Tissue
• Dermal tissue provides the protective
outer coating for plants
• It is the skin, or epidermis
– This coating attempts to keep the bad guys
(infectious agents) out, and the good guys
(water and nutrients) in
Dermal Tissue
• Within the epidermis are called guard cells
• What do you think they do?
– They control the opening and closing of gaps
called stomata that are vital to the process of
photosynthesis
Photosynthesis.. What’s that?!?
• What is the formula for photosynthesis?
•
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
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• Who can summarize the process of
photosynthesis?
Roots
• How do plants get their nutrients?
• Through the hard work of roots, whose
tips absorb nourishment for the plant
(minerals and water) via root hairs
Roots
• Most of the water and minerals are
absorbed by plants at the root tips, which
have root hairs extending from their
surface
– These hairs create a larger surface area for
absorption
Roots
• A root is not just a root, for not all root
structures are the same
• There are two types of angiosperm plantsdicots and monocots
– Dicots are known for having a taproot system
– Monocots are associated with fibrous roots
Taproot System
• The taproot system (e.g. carrot) branches
in a way similar to the human lungs;
– The roots start as on thick root on entrance to
the ground
– then divide into smaller and smaller branches
called lateral roots underneath the surface
which serve to hold the plant in place
Fibrous Roots
• These roots provide plants with a very
strong anchor in the ground without going
very deep into the soil
What type of
environments would
these roots be
helpful??
Root System Summary
• Dicots  taproot  thick entry root 
division into smaller branches
• Monocots  fibrous roots  shallow entry
into ground  strong anchor effect
Root Structure
• Lets take a look at the structure of a root
moving from outside to inside
• The root is lined by the epidermis
– Cells that give rise to root hairs (that plants
must thank for their ability to absorb water
and nutrients)
Root Structure
• Moving further in, we come to the cortex,
– the majority of the root that functions as a
starch storage receptacle
• Why would plants need starch? What type
of organic compound does it fall under?
Root Structure
• The inner most layer of the cortex is
composed of a cylinder of cells known as
the endodermis
– These cells are important to the plant because
walls between these cells create an obstacle
known as the, casparian strip, which blocks
water from passing
• This is one of the mechanisms by which plants
control the flow of water
Root Structure
• Moving in through the endodermis, we
come to the vascular cylinder, which is
composed of a collection of cells known as
the pericycle
– The lateral roots of the plant are made from
the pericycle and hold the vascular tissue of
the root
Root Growth
• Plants grow as long as they are alive as a
result of the presence of meristemic cells
• Early on in the life of a plant, after a seed
matures, it sits and waits until the time is
right for germination
Root Growth
• At the point of germination, water is
absorbed by the embryo, which begins to
grow again
• When large enough, it busts through the
seed coat, beginning the journey into
plant-hood
Root Growth
• At the start of the journey, the growth is
concentrated in the actively dividing cells
of the apical meristem
• Growth in this region leads to an increase
in the length of a plant: primary plant
growth
Root Growth
• Later on growth
occurs in cells known
as the lateral
meristems, which
extend all the way
through the plant
• This growth that leads
to an increase in the
width of a plant and is
known as secondary
plant growth
Regions of Growth
• Root cap: protective structure that keeps roots
from being damaged during push through soil
• Zone of cell division: section of root where cells are
actively dividing
• Zone of elongation: next section up along the root,
where cells absorb H2O and increase in length to
make the plant longer
• Zone of maturation: section of root past the zone
of elongation where the cells differentiate to their
finalized form (phloem, xylem, parenchyma,
epidermal, etc).
Plants Concept Check 1Roots and Shoots
1. Which structure acts as a passageway for the
transport of minerals and water from the soil and
a support structure?
2. What are the gaps underneath the leaves called
that are vital to the process of photosynthesis?
3. What structure is responsible for controlling the
flow of water?
4. What is responsible for making the lateral roots?
5. Describe in 5 sentences or MORE the process of
photosynthesis.
The Shoot System
• Now that we have discussed roots- the
parts of plants that are in the ground- lets
take a look at the shoots (leaves and
stems) the parts of the plant that are out of
the ground
Structure of a Leaf
• Leaves are protected by
the waxy cuticle of the
epidermis, which
functions to decrease
the transpiration rate.
– What’s that?
• Inside the epidermis
lies the ground tissue of
the leaf, the mesophyll,
which is involved in the
ever-so-important
process of
photosynthesis.
Structure of a Leaf
• There are two
important layers to the
mesophyll
– The palisade mesophyllwhere most of the
photosynthesis of the
leaf occurs, where there
are MANY chloroplasts
– The spongy mesophyll- a
bit farther in where cells
provide CO2 to the cells
performing
photosynthesis
Structures of a Leaf
• Stomata are important structures to
successful photosynthesis
– Controlled by guard cells that line the walls of
the epidermis
Structures of a Leaf
• Extending a bit further inside the leaf, we
find the xylem
– What does the xylem bring?
• And the phloem
– What does the phloem do?
Structures of a Leaf
• In C4 plants, a second type of cell called a
bundle sheath cell surrounds the vascular
tissue to make the use of CO2 more
efficient and allow the stomata to remain
closed during the hot day time hours,
• These cells prevent excessive transpiration
Structure of Stems
• Again, lets travel from the outside in and
discuss the basic structure
• The epidermis of the stem provides
protection and is covered by cutin, a waxy
protective coat
• The cortex of a stem contains parenchyma,
collenchyma, and schlerenchyma cells.
– What are the characteristics of each of these
cells?
Structure of Stems
• You’ll notice that there is no endodermis
in the stem
• This is because this portion of the plant is
not involved in the absorption of water
• As a result, the next structure moving
inward is the vascular cylinder and once
again the xylem and phloem
Structure of Stems
• A term to know is the
vascular cambium
– It extends along the
entire length of the plant
and gives rise to
secondary xylem and
phloem
• Over time, the stem of a
plant will increase in
width because of the
secondary xylem
produced each year
Structure of Stems
• Another term to know is the cork
cambium– Produces a thick cover for stems and roots
– This covering replaces the epidermis when it
dries up and falls off the stem during
secondary growth, forming a protective
barrier against infection and physical damage
Structure of Stems
• The growth of plants is not a continuous
process in seasonal environments
• There are periods of dormancy in between
phases of growth
Structures of Stems
• Rings of a stem
• These rings produced
each year are a
window into the past,
and give insight into
the amount of rain a
tree has encountered
in a given year
• The wider the ring,
the more water is saw
Plant Hormones
• Hormones perform the same general
function for plants that they do for
humans
– They are signals that can travel long distances
to affect the actions of another cell
• There are 5 main plant hormones you will
need to know; abscisic acid, auxin,
cytokinins, ethylene, gibberellins
Plant Hormones
• Abscisic Acid
– The “babysitter” hormone
– It makes sure that seeds do not germinate too
early, inhibits cell growth, and stimulates the
closing of the stomata to make sure that the
plant maintains enough water
Plant Hormones
• Auxin
– A hormone that performs several functions
• Leads to the elongation of stems
• Plays a role in phototrophism and gravitropism
Plant Hormones
• Cytokinins
– Hormones that promote cell division and leaf
enlargement
– Seem to slow down the aging of leaves
– Supermarkets use synthetic cytokinins to keep
their veggies fresh
Plant Hormones
• Ethylene
– This hormone initiates fruit ripening
– Causes flowers and leaves to drop from trees
(associated with aging)
Plant Hormones
• Gibberellins
– A hormone group that assists in stem
elongation
– When you think gibberellins, think “grow”
– It induces the growth of dormant seeds, buds,
and flowers
Plant Tropism
• A tropism is growth that occurs in
response to an environmental stimulus
such as sunlight or gravity
• The three tropisms you should familiarize
yourself with are gravitropism,
phototropism, and thimotropism
Plant Tropism
• Gravitropism
– This is a plants growth response to
gravitational force
– Two of the hormones that play a role in this
movement are auxin and gibberellins
Plant Tropism
• Gravitropism
– A plant placed on its side will show
gravitropism growth in which the cells on the
upward facing side will not grow as much as
those on the downward side
– It is believed that the relative concentrations
of these hormones in the various areas of the
plant are responsible for this imbalanced
growth of the plant
Plant Tropism
• Phototropism
– This is a plants growth response to light
– Auxin is the hormone in charge here, working
in the zone of elongation
Plant Tropism
• Phototropism
– When a plant receives light on all sides, auxin is
distributed equally around the zone of elongation
and growth is even
– When one half of the plant is in the sun, and the
other is in the shade, auxin (almost as if it feels
bad for the shady portion) focuses on the darker
side
– This leads to unequal growth of the stem with the
side receiving less light growing faster- causing
the movement of the plant toward the light source
Plant Tropism
• Thigmotropism
– This is a plants growth
response to contact
– One example involves
vines, which wind
around objects with
which they make
contact as they grow
Photoperiodism
• Like all of us, plants have a biological clock
that maintains a circadian rhythm
– A physiological cycle that occurs in time
increments that are roughly equivalent to the
length of a day
Photoperidism
• The month of June has the longest daysthe most sunlight. The month of December
have the shortest days- least sunlight
• How is it that plants, which are so
dependent on light, able to survive
through these varying conditions?
• This is thanks to photoperidism
– The response by a plant to change in the
length of days
Photoperiodism
• One commonly discussed example of
photoperiodism involves flowering plants
(angiosperms)
– A hormone known as florign is thought to assist
in the blooming of flowers
– An important pigment to the process of flowering
is phytochrome, which is involved in the
production of florigne
• Because plants differ in the conditions
required for flowering to occur, different
amounts of florigen are needed to initate this
process from plant to plant
Photoperiodism
• One interesting application of
photoperiodism involves the distinction
bewteen short day plants and long day
plants which flower only if certain
requirements are met:
Photoperiodism
Plant Type
Example
Flowering
Requirements
Flowers During
Short Day plants
Poinsettias
Exposure to a
night longer than
a certain number
of hours (e.g. 10
hours)
End of summer to
end of winter
Long Day Plants
Spinach
Exposure to a
Late spring to
night shorter than early summer
a certain number
of hours (e.g. 8
hours)
Concept Check 2
1. Why is there no endodermis in the stems?
2. What is the difference between cork
cambium and vascular cambium
3. Auxin and gibberellins are associated
with which type of trophism
4. What is responsible for a plants response
to the change in length of days
5. What can you tell about the rings of a
plant?
Go with the Flow
• Osmosis drives the absorption of water
and minerals from the soil by the root tips
• Water then moves deeper into the root
until it reaches the endodermis
• Once there, because of the casparian strip,
it can only travel through the selective
endodermal cells that choose which
nutrients and minerals they let through to
the vascular cylinder beyond
Go with the Flow
• The casparian strip essentially lets only
those with a backstage pass through
– Potassium has a backstage pass and can go
into the vascular cylinder
– Sodium does not and gets deeeeenied
• Once the water gets to the xylem, it has
reached the H2O super highway and is
ready to go all over the plant
Go with the Flow
• There are few driving forces responsible
for the movement of a plants water supply
• The three main forces responsible are
osmosis, capillary action, and cohesiontension theory
– Of these three cohesion tension theory pulls
the most weight
Go with the Flow
• Osmosis
– The driving force that moves water from the
soil into xylem cells
Go with the Flow
• How in the world does the plant keep the
concentration gradient such that it promotes the
movement of water in the appropriate direction?
There are two contributing factors
– The water is constantly moving away from the root
tips creating the space for more water to enter and
– Osmosis is defined as the passive diffusion of water
down its concentration gradient across selectively
permeable membranes
• It flows from a region with a high water concentration to a
region with a low water concentration
• There is a higher mineral concentration inside the vascular
cylinder, which drives water into the xylem contained in this
cylinder by a force known as root pressure
Go with the Flow
• Capillary action
– The force of adhesion between water and a
passageway that pulls water up along the sides
• Along with osmosis, this mechanism is a
minor contributor to the movement of
water up the xylem due to the
counteracting force of gravity
Go with the Flow
• Cohesion-Tension Theory and
Transpiration– This is the major mover of water in the xylem
• Transpiration creates a negative pressure
in the leaves and xylem tissue due to
evaporative loss of water
Go with the Flow
• Water molecules display molecular
attraction (cohesion) for other water
molecules, in effect creating a single
united water molecule that runs the length
of the plant
• When water evaporates off the surface of
the leaf, the water is pulled through the
xylem toward leaves- transpiration is the
force pulling water through the plant.
Concept Check 3
• What is transpiration