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?

• + + + +

• 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 plants- dicots 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 1- Roots 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 mesophyll-

where 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 days- the 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:

PhotoperiodismPlant Type Example Flowering

RequirementsFlowers 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 night shorter than a certain number of hours (e.g. 8 hours)

Late spring to early summer

Concept Check 2

1. Why is there no endodermis in the stems?2. What is the difference between cork

cambium and vascular cambium3. Auxin and gibberellins are associated

with which type of trophism4. What is responsible for a plants response

to the change in length of days5. 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 cohesion-tension 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