HL #15

Flowering plants can be found in a wide variety ofhabitats.

The structure of each type of plant is closely relatedto the amount of water available in the habitat.

XEROPHYTES

These are plants that are adapted to grow in very dryhabitats.

Cereus giganteus, the saguaro or giant cactus is anexample. Some of its adaptations are:

• Spines instead of leaves, to reduce transpiration.

• Thick stems containing water storage tissue.

• Very thick waxy cuticle.

• Vertical stems to absorb sunlight early & late in the day, but not at midday when light is most intense. • Very wide-spreading network of shallow roots to absorb water after rains.

• CAM physiology, which involves opening stomata during the cool nights instead of in the intense heat of the day.

HYDROPHYTES

These are plants that are adapted to grow either submerged in water or floating on the surface.

Victoria amazonica, the Amazon water lily is anexample. Some of its adaptations are:

• Air spaces in the leaf to provide buoyancy.

• Stomata in the upper epidermis of the leaf, which is in contact with the air, but not in the lower epidermis.

• Waxy cuticle on the upper surface of the leaf, but not on the lower surface, which is in contact with water.

• Small amounts of xylem in stems & leaves.

LEAF STRUCTURE & FUNCTION

The function of a leaf is to produce food for the plant byphotosynthesis.

Tissues of the leaf & their function:

The main part of the leaf is the leaf blade blade or lamina.lamina.

The blade has a large surface area to absorb sunlight,but is very thin – only about 0.3 mm.

The leaf has four thin tissue layers with veins at intervals.

• UPPER EPIDERMIS:UPPER EPIDERMIS: A single layer of cells covered by a thick waxy cuticle to prevent water loss.

• PALISADE MESOPHYLL: PALISADE MESOPHYLL: A layer of densely packed cylindrical cells with many chloroplasts found near the upper surface where the light intensity is highest.

• SPONGY MESOPHYLL: SPONGY MESOPHYLL: A layer of loosely packed cells with few chloroplasts. This layer provides the main gas exchange surface so must be found near the stomata in the lower epidermis.

• LOWER EPIDERMIS: LOWER EPIDERMIS: A single layer of cells with a thinner waxy layer.

On the lower epidermis are stoma stoma or stomata stomata – pores that allow CO2 for photosynthesis to diffuse in & O2 to diffuse out.

On either side of the stomata are guard cellsguard cells – a pair of cells thatopen or close the stomata & control the amount of transpiration.

In the veins you find vascular tissue called xylemxylem – which transports water & phloem phloem which transports food.

LEAVES & TRANSPIRATION

Photosynthesis depends on gas exchange over a moist surface.

Spongy mesophyll cell walls provide this surface.

Water often evaporates from the surface & is lost, in a process called transpiration transpiration.

TRANSPIRATION is the loss of water vapor from the is the loss of water vapor from the leaves & stem of a plant.leaves & stem of a plant.

TRANSPORT IN PHLOEMTRANSPORT IN PHLOEM

Sugars, amino acids & other organic compounds produced inphotosynthesis are transported out of the leaf by phloem tissue.phloem tissue.

The structure within phloem tissue that transport organic compounds are called sieve tubes.sieve tubes.

Columns of cells develop into sieve tubes by breaking down their nuclei & cytoplasm & making larger pores in their endwalls to allow a flow of sap.

The plasma membranes remain & have the important task ofpumping organic compounds into the sieve tube by a process into the sieve tube by a process called called active translocationactive translocation. .

Transport is thus an active process involving the use of ATP.Transport is thus an active process involving the use of ATP.

AA high solute conc. is created inside the sieve tubes of the leaf,which causes water to enter by osmosis.

This creates a high enough pressure to pump the sap insidethe sieve tube, containing dissolved organic compounds, toany part of the plant.

Sugars and amino acids are loaded into the phloem in parts of the plant called sourcessources and are translocated to sinkssinks where they are unloaded.

Examples of sourcesExamples of sources are parts of the plant where photosynthesisis occurring ( stems and leaves)

Examples of sinksExamples of sinks are roots, growing fruits and the developingseeds inside them.

Phloem also transports some spray chemicals if they areabsorbed into the leaf after being sprayed onto it.

The transport of any biochemical in phloem whether produced by the plant or not is called translocation.translocation.

ABIOTIC FACTORS AFFECTING THERATE OF TRANSPIRATION

Four external abiotic factors have an effect on the rate of transpiration .

1. LIGHT –LIGHT – guard cells close the stomata in darkness, so

transpiration is much greater in light.

Plants lose water vapor from their stems and leaves by transpiration.The rate of water loss varies depending on internal and externalconditions.

The main internal condition is whether the stomata are open or closed. The plant hormone abscisic acid causes the guard cells to close the stomata. Plants produce abscisic acid when they are suffering water stress.

2. TEMPERATURE TEMPERATURE – heat is needed for evaporation of water from the surface of spongy

mesophyll so as temperature rises the rate of transpiration rises.

Higher temperatures also increase the rate of diffusion through the air space in the spongy layer & reduce the relative humidity of the air outside the leaf.

3. HUMIDITY HUMIDITY – water diffuses out of the leaf when there is a conc. gradient between the air spaces inside the leaf & the air outside.

The air spaces are always nearly saturated.

The lower the humidity outside the leaf, the steeper the gradient & therefore the faster the rate of transpiration.

4. 4. WIND WIND – pockets of air saturated with water vapor tend to form near stomata in still air, which reduce the rate of transpiration.

Wind blows the saturated air away & so increases the rate of transpiration.

FOOD STORAGE IN PLANTS

Many perennial plants develop a food storage organ in whichfood is stored during a dormant season & then used in the nextgrowth season.

The food is transported to & from the organ in the phloem.

Potato tubers (swollen underground stems) are an example of a storage organ

STRUCTURE & FUNCTION OF STEMSSTRUCTURE & FUNCTION OF STEMS

Stems connect the leaves, roots & flowers of plants & transportmaterials between them using xylem & phloem tissue.

Stems support the aerial parts of terrestrial plants.

Xylem tissue also provides support, especially in woody stems.

Cell turgor pressure provides support.

Support is provided in several ways:

Some cells develop thick cellulose cell walls which strengthen the plant.

STRUCTURE & FUNCTION OF ROOTS

Roots absorb mineral ions & water from the soil.

They anchor the plant & are sometimes used for food storage.

The structure of root systems gives them a large surface areafor absorption.

Plants increase the surfaced area for absorption by branching.

Plants absorb potassium, phosphate, nitrate and other mineralsions from the soil. The concentration of these ions in the soil isusually much lower than inside root cells, so they are absorbedby active transport.

Root hair cells have mitochondria and protein pumps in their plasma membranes. Most roots only absorb mineral ions if they have a supply of oxygen, because they produce ATP for activetransport , by aerobic cell respiration.

There are three ways in which ions can move into a root:

Diffusion Diffusion of mineral ions

flow of waterflow of water carrying ions, when water drains through the soil.

fungal hyphaefungal hyphae absorb ions and then they move from the hyphae to the roots.

TRANSPORT OF WATER THROUGHTHE PLANT

Transpiration causes a flow of water from the roots, to thestem to the leaves. This is called transpiration stream.

The process starts with evaporationevaporation of water from the cellwalls of the spongy layer.

The water that evaporates is replaced with water fromxylem in the leaf.

The water is pulled out of the xylem & through pores in thespongy layer by capillary action.apillary action.

Low pressure or suction is created inside the xylem whenwater is pulled out. This is called transpiration pull.transpiration pull.

Xylem vessels contain long, unbroken columns of water &the transpiration pull is transmitted down through these columns of water to the roots.

Mature xylem is dead & the flow of water through them is passive.

The transmission of the transpiration pull through xylemvessels depends on the cohesion cohesion of water molecules, dueto hydrogen bonding.

Another process than can help water to move up in xylem vesselsis the adhesionadhesion of water to the wall of the vessel.

STRUCTURE OF XYLEM VESSELS

• No plasma membrane are present in mature xylem, so water can move in and out freely.

• Lumen of the xylem is filled with sap, as the cytoplasm & the nuclei of the original cells break down. …End walls also break down to form a continuous tube.

• Helical ring shaped thickenings of the cell wall are impregnated with lignin. This makes them hard, so that they can resist inward pressures.

• Pores in the outer cellulose cell wall conduct water out of the xylem & into cell walls of adjacent cells.

WATER UPTAKE BY ROOTS

• The cytoplasm of root cells usually has a much higher total solute conc. than water in the surrounding soil, as a result of active transport of mineral ions.

• Water therefore moves into root cells from the soil by osmosis.osmosis.

• Most of the water absorbed by roots is eventually drawn by transpiration pull into xylem vessels in the center of the root.

REPRODUCTION OF FLOWERING PLANTS

STRUCTURE & FUNCTION OF FLOWERSSTRUCTURE & FUNCTION OF FLOWERS

• Flowers are the structure used by flowering plants forsexual reproduction.

• Female gametes are contained in ovules in the ovaries ofthe flower.

• Pollen grains, produced by the anthers, contain the male gametes.

• A zygote is formed by the fusion of a male gamete with a female gamete inside the ovule. This process is called fertilization.fertilization.

Before fertilization, another process called pollination pollination mustoccur.

• Pollination Pollination is the transfer of pollen from an anther to a stigma.

• Pollen grains containing male gametes cannot move without help from an external agent. … Most plants use either wind or an animal for pollination.

Pollen grains germinate on the stigmastigma of the flower & a pollen tube containing the male gametes grows down thestylestyle to the ovary.ovary.

• The pollen tube delivers the male gametes to an ovule, which they fertilize.

• Fertilized ovaries develop into seeds.

• Ovaries containing fertilized ovules develop into fruitfruit.

• The function of the fruit is seed dispersal.seed dispersal.

FACTORS NEEDED FOR SEEDGERMINATION

Seeds will not germinate unless external conditionsare suitable.

• Water must be available to rehydrate the dry tissues of the seed.

• Oxygen must be available for aerobic cell respiration.

• Some seeds respire anaerobically if oxygen is not available, but ethanol produced in anaerobic respiration reaches toxic levels.

• Suitable temperatures are needed.

Germination involves enzyme activity & at very low & very high temperatures enzyme activity is to slow.

Some seeds remain dormant if temperatures are above or below particular levels, so that they only germinate during favorable times of the year.

METABOLIC EVENTS DURING GERMINATION

• The first stage in germination is the absorption of water & the rehydration of living cells in the seed.

This allows the seed to become metabolically active.

• Soon after absorbing water, a plant growth hormone called gibberlin gibberlin is produced in the cotyledons of the seed.

• Gibberlin stimulates the production of amylase, which catalyses the digestion of starch into maltose.

• Maltose is transported from the food stores to the growth regions of the seedling, including the embryo root & the embryo shoot.

• Maltose is converted into glucose, which is either used in aerobic cell respiration as a source of energy, or is used to synthesize cellulose or other substances needed for growth.

As soon as the leaves of the seedling have reached lightAs soon as the leaves of the seedling have reached light& have opened, photosynthesis can supply the & have opened, photosynthesis can supply the seedlings with food & the food stores of the seed areseedlings with food & the food stores of the seed areno longer needed.no longer needed.

Flowering plants are divided into two groups, accordingto the number of leaves the embryo has, inside the seed.

Monocots have one cotyledon (seed leaf)

Dicots have two cotyledons (seed leaves)

Monocots Dicots

• Parallel veins

• Vascular tissue spread randomly

• Flower parts in multiples of three

• Fibrous roots

• Leaves attach directly to the stalk

• Veins form net-like pattern

• Vascular tissue is in a ring

• Flower parts on multiples of four or five.

• Taproot (main root)

• Leaves are attached by short petioles to the stem

MODIFIED ROOTS, STEMS AND LEAVES

1. BULBS

In some monocot plants leaf bases grow to form anunderground organ called a bulb.

Plants use bulbs for food storage.

2. Stem tubers

In some dicot plants stems grow downward into the soil andinto stem tubers.

Plants use stem tubers for food storage.

3. Storage roots

Some roots become swollen with stores of food.

4. Tendrils

Tendrils are narrow outgrowths from leaves that rotatethrough the air until they touch a solid support to which they attach allowing the plant to climb upward.

GROWTH & DEVELOPMENT IN PLANTS

APICAL AND LATERAL MERISTEM

Plants have regions called meristemsmeristems where cells continue togrow and reproduce throughout the plants life.

Flowering plants have meristemsmeristems at the top and tip of theirroots and stems called apical meristem.apical meristem.

Growth in apical meristemsapical meristems allow roots and stems to growin length and also produces new leaves and shoots.

Many dicot plants also develop lateral meristems.lateral meristems.

Growth in lateral meristemlateral meristem makes roots and stems thicker.

Growth in the thickness of tree trunks is due to laterallateralmeristemmeristem, inside the bark

PHOTO PERIODIC CONTROL OF FLOWERING

Some plants only flower at the time of the year when days areshort and other plants only flower when days are long.

They are called short-day and long-day plants.

Experiments have shown that it is not the length of the day, butthe length of the night that is significant.

For example chrysanthemums are short-day plants and onlyflower when they receive a long continuous period of darkness.

AUXIN AND PHOTOTROPISM

Plants use hormones to control their growth and their development.

Auxin which acts as a growth promoter is an example of a planthormone.

It secretes hydrogen ions into cell walls which loosens connectionsbetween cellulose fibers, allowing for cell expansion

Auxin also controls phototropism which is directional growthof a plant in response to a light source.

The tips of shoots can detect the source of the brightest light.They also produce auxin.

One theory is that auxin is redistributed in the shoot tip from the side with the most light to the darker side. This promotesmore growth on the darker (shadier) side, causing the shootto bend towards the light.

PHYTOCHROME & PHOTOPERIODISM

Plants can measure the length of periods of dark to anaccuracy of a few minutes.

They do this by using a pigment in their leaves calledphytochromephytochrome which exists in two interconvertible forms.

One form is called Pr because it absorbs red light with a

wavelength 660 nm.

Pr is the inactive form of phytochrome.

When it absorbs red light it is rapidly converted into the activeform called Pfr.

This form can absorb far red light with a wavelength of 730 nm and is then rapidly converted back to Pr.

In normal daylight there is much more red light than far red Light so phytochrome exists in the active Pfr form.

In darkness Pfr reverts very slowly to Pr.

This gradual reversion process is probably how the lengthof the dark period is timed.Enough Pfr remains in long-day plants at the end of short

nights to stimulate flowering.

In short day plants Pfr presumably acts as an inhibitor of

flowering.