Introduction to Botany

•       What is a plant? 

–      Lives on land

–      Doesn’t move under its own power

–      Produces food and energy from sunlight (photosynthesis)

–      multicellular

•       Excludes algae (live in water and often unicellular) and fungi (no photosynthesis) and bacteria (unicellular prokaryotes)

–      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.

that perform photosynthesis.

•       Plants have regular eukaryotic cells:

–      DNA enclosed in a nucleus,

–      cytoplasm with organelles like the mitochondria, lysosomes, and endoplasmic reticulum.  Proteins are synthesized by ribosomes in the cytoplasm.

–      cells are surrounded by a membrane that keeps the inside separated from the outside, with transporter proteins to let specific molecules in or out.

•       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

•       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

•       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

•       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

•       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.

•       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).  Called plastids.

•       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.

•       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.  Cells in other parts of the plant don’t divide. 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

•       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

•       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 and 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

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.

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.

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 photosynthesis:  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.

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.

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.

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).

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.

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.

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. 

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.

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

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.

Coevolution Examples

Fertilization

Once pollen has been deposited on the stigma, the process of fertilization occurs.

Fertilization occurs when a pollen nucleus fuses with an ovule nucleus to form a new diploid organism.

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. 

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.

Fruit Development

Fruits

Fruits develop from the wall of the ovary, the pericarp.  Fruits contain the seeds and are responsible for seed dispersal.

In some fruits, the receptacle also forms part of the fruit.

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.

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.

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.

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.

Plant evolution

Algae

Algae are water dwelling, photosynthetic eukaryotes.  They can be either unicellular or multicellular

Multicellular forms are called seaweed.

“blue-green algae” is an old and outmoded term for cyanobacteria, which are prokaryotes.

Algae are a mixed group: they are not all descended from a single common ancestor

Several groups of algae are the result of secondary endosymbiosis: they swallowed a green or a red algal cell that already had a chloroplast in it.

Plants vs algae:  The defining characteristic of plants is that they retain the female gamete (ovule) and the developing embryo in the mother’s body.  Algae release the female gametes into the water before they are fertilized.

More Algae

Types of algae:

Green.  Ancestors of plants

Red.  Different photosynthetic accessory pigments allow them to live at greater depths than green algae

Brown: kelp

Diatoms, dinoflagellates, euglenoids: all unicellular forms

Lichens are symbiotic organisms: a fungus and an algae live together, with the fungus providing protection and the ability to extract mineral nutrients, and the algae providing photosynthesis

Coral reefs are built by the skeletons of animals, but they often have algae symbionts in them

Plant Evolutionary Trends

Plants are thought to have evolved from the green algae

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. Protection and dispersal of reproductive structures.  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.

Major Plant Groups

We are going to 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.

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.

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.

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).

Ferns and club mosses dominated the Earth during the Paleozoic Era.

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.

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.

Gymnosperms

Gymnosperms were the first plants to have pollen grains and seeds.

Cones are the reproductive structures: male and female are separate.

Male cones produce pollen, which is carried to the female cones by the wind.

Each scale of a female cone contains a single ovule, which develops into a seed.

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.  During the Mesozoic Era (dinosaurs), cycads were the dominant type of plant.

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. 

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.