Chapter 24 lecture- Seeds

24–1 Reproduction With Cones and Flowers

Alternation of GenerationsAll plants have a life cycle in which a diploid

sporophyte generation alternates with a haploid gametophyte generation.

Male and female gametes of a gametophyte plant join and form a zygote that begins the next sporophyte generation.

In mosses & ferns, the two stages of the life cycle are distinct, independent plants.

In seed plants, gametophytes are found within tissues of the sporophyte plant. In gymnosperms, they are found inside cones.

In angiosperms, they are found inside flowers.

Cones and flowers represent two different methods of reproduction.

Alternation of Generations

Reproduction in gymnosperms takes place in cones, which are produced by a mature sporophyte plant.

Gymnosperms produce two types of cones: pollen cones and seed cones.

Life Cycle of Gymnosperms

Pollen Cones and Seed Cones

Pollen cones produce the male gametophytes, also called pollen grains.

Pollen grain (N)(male gametophytes)


Seed cones produce female gametophytes and are generally larger than pollen cones.

Female gametophytes develop in two ovules located near the base of each scale.


Within the ovules, meiosis produces haploid cells that grow and divide to produce female gametophytes.

Each gametophyte contains egg cells.


Pollination

The gymnosperm life cycle typically takes two years to complete.

The cycle begins as male cones release pollen grains.

Pollen grains are carried by the wind and reach female cones.


Meiosis

Fertilization


Male Cones

Mature sporophyte

Pollen cone

Pollen grain (N) (male gametophytes)


Female Cones

Mature sporophyte

Seed cone

Cone scale

Ovules

Diploid cell (2N)

Ovule

Four haploid cells (N)

Female gametophyte (N)


If pollen grains land on and enter an ovule, pollination occurs.

A pollen tube grows out of each pollen grain and releases sperm near an egg.

Egg cells

Discharged sperm nucleus

Pollen tube


Fertilization produces a diploid zygote which develops into a new sporophyte plant.

Zygote (2N)(new sporophyte)


The zygote grows into an embryo and is encased within what will develop into a seed.

Gametophyte tissue

Embryo (2N)

Seed coat(old sporophyte)

Seed


The seed is then dispersed by wind.

When conditions are favorable, the seed germinates and its embryo grows into a seedling.


Structure of Flowers

Flowers are reproductive organs

that have four kinds of specialized leaves: sepals, petals, stamens, and carpels.

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Sepals enclose the bud before it opens and protect the flower while it is developing.

Sepal


Petals are often brightly colored and are found just inside the sepals.

Petals attract insects and other pollinators to the flower.

Petal


The male parts of a flower consist of an anther and a filament, which together make up the stamen.

Filament

AntherStamen


An anther is an oval sac where meiosis takes place, producing pollen grains.

Anther


The filament is a long, thin stalk that supports an anther.

Filament


The innermost parts are carpels(pistils).

Carpel Style

Stigma

Ovary


Each carpel has a broad base forming an ovary where female gametophytes are produced.

Ovary

Ovule


The narrow stalk of the carpel is the style.

Style


At the top of the style is the stigma—a sticky portion where pollen grains frequently land.

Stigma


Ovary

Ovule

Carpel Style

Stigma

Ovary

Filament

AntherStamen

Parts of a Typical Flower

Sepal

Petal


A typical flower produces both male and female gametophytes.

In some plants, male and female gametophytes are produced in separate flowers on the same individual.


Life Cycle of Angiosperms

Reproduction in angiosperms takes place within the flower. Following pollination and fertilization, the seeds develop inside protective structures.


Each flower contains anthers and an ovary.

In anthers, cells undergo meiosis to make a haploid spore cell.

Spore nuclei undergo mitosis to make two haploid nuclei = pollen


The pollen grain usually stops growing until it is released from the anther and deposited on a stigma.

Pollen grains (N)(male gametophyte)

Stigma


In the ovule, a single diploid cell undergoes meiosis to produce four haploid cells that become the female gametophyte.

Haploid cell(N)

Ovule

Ovary (2N)


Only one of the four cells undergoes mitosis to produce eight nuclei, called the embryo sac.

Pollen tubeEgg cell

Sperm

Polar nuclei

Embryo sac (N)(female gametophyte)


The embryo sac is the female gametophyte.

One of the eight nuclei is the egg nucleus—the female gamete.

Pollen tube

Embryo sac (N)(female gametophyte)

Egg cell

Sperm

Polar nuclei


When fertilization takes place, this cell becomes the zygote that grows into a new sporophyte plant.

Endosperm(3N)

Zygote (2N)


Fertilization in Angiosperms


If a pollen grain lands on the stigma of a flower of the same species, it grows a pollen tube.


Pollen tubes

Ovule

The pollen tube grows into the style, reaches the ovary, and enters the ovule.


Pollen tubes

Ovule


One of the sperm nuclei fuses with the egg nucleus to produce a diploid zygote.

The zygote will grow into the new plant embryo.

Zygote (2N)

The other sperm nucleus fuses with two polar nuclei in the embryo sac to form a triploid (3N) cell = endosperm. This cell will grow into a food-rich tissue known as endosperm, which nourishes the seedling as it grows.

Endosperm(3N)

Zygote (2N)


Because two fertilization events take place between the male and female gametophytes, this

process is known as double fertilization.

1 makes zygote

1 makes endosperm


Pollination

Most gymnosperms and some angiosperms are wind pollinated, whereas most angiosperms are pollinated by animals.

Pollination

Wind pollination

•is less efficient than animal pollination

•relies on weather

Animal pollination

•plants have bright colors and sweet nectar to attract animals

•benefits both the plants and the animals that pollinate them

24-2 Seed Development and Germination



As angiosperm seeds mature, the ovary walls thicken to form a fruit that encloses the developing seeds.

Seed and Fruit Development


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A fruit is a ripened ovary that contains angiosperm seeds.

As seeds mature, the ovary walls thicken to form a fruit that encloses the developing seeds.

Seed and Fruit Development


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Seed Dispersal

Seeds are dispersed by

•Animals- typically in fleshy, nutritious fruits.

•Wind- flying seeds

•Water- floating seeds

Seed Dispersal

Seeds of many plants are eaten by animals.

These seeds are covered with tough coatings that protect them from digestive chemicals, allowing them to pass through an animal’s digestive system unharmed.

The seeds then sprout in the feces eliminated from the animal.

Seeds dispersed by wind or water are typically lightweight, allowing them to be carried in the air or to float on the surface of the water.





Seed Dispersal

Some seeds are encased in winglike structures that spin and twirl, helping them glide from their parent plants.



Seed Dispersal

A coconut is buoyant enough to float in seawater within its protective coating for many weeks.



Seed Dispersal

Tumbleweed plants break off at their roots and scatter their seeds as they are blown by the wind.



Seed Dormancy

•Many seeds will not grow when they first mature.

•Many seeds enter a period of dormancy, during which the embryo is alive but not growing.

•The length of dormancy varies in different plant species.

Seed Dormancy

Environmental factors such as temperature and moisture can cause a seed to end dormancy and germinate.

Seed dormancy can be adaptive in several ways:

•allows for long-distance dispersal

•allows seeds to germinate under ideal growth conditions

Seed Germination

•Seed germination is the early growth stage of the plant embryo.

•Seeds absorb water which causes food-storing tissues to swell and crack open the seed coat.

•The young root grows through the cracked seed coat.



Seed Germination

In most monocots, the single cotyledon remains underground.

The growing shoot emerges while protected by a sheath.

Seed Germination

Corn (monocot)

Young shoot

Germinating seed

Primary root

Foliage leaves

Seed Germination

In dicots, germination takes place in one of two ways.

•In some species, the cotyledons emerge above ground, protecting the stem and first foliage leaves.

•In other species, the cotyledons stay underground and provide a food source for the growing seedling.

Seed Germination

Cotyledons

Bean (dicot)

Germinating seed

Primary root

Young shoot Cotyledons

Seed coat

Foliage leaves

24-3 Plant Propagation and Agriculture

Vegetative Reproduction

Vegetative reproduction is a method of asexual reproduction used by flowering plants that enables a single plant to produce many offspring genetically identical to itself.



Vegetative reproduction includes the production of new plants from horizontal stems, from plantlets, and from underground roots.




Some angiosperms produce tiny plants, or plantlets, at the tips of elongated stems.



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Some plants grow horizontal stems, called stolons, that produce roots when they touch the ground.


Once the roots are well established, each stolon may be broken, forming a new independent plant.






Plant Propagation

In plant propagation, horticulturists make many identical copies of a plant or produce offspring from seedless plants.

Plant Propagation

Cuttings

•One of the simplest ways to reproduce plants vegetatively is by cuttings.

•A grower “cuts” a plant stem that includes buds containing meristematic tissue.

•That stem is then partially buried in soil or in a special rooting mixture.

•Some plants are treated with rooting powders to help them grow.

Plant Propagation

Grafting and Budding

•Grafting and budding are used to reproduce seedless plants and varieties of woody plants that do not produce strong root systems.

•A piece of stem or a lateral bud is cut from the parent plant and attached to another plant.

Plant Propagation

The cut piece is called the scion, and the plant to which it is attached is called the stock.

When stems are used as scions, the process is called grafting.

When buds are used as scions, the process is called budding.

Agriculture

Agriculture is the systematic cultivation of plants.

Agriculture

Worldwide Patterns of Agriculture

•Most of the people of the world depend on a few crop plants, such as wheat, rice, and corn, for the bulk of their food supply.

•Roughly 80 percent of all U.S. cropland is used to grow wheat, corn, soybeans, and hay.

Agriculture

Changes in Agriculture

•The efficiency of agriculture has been improved through:

improvements in farming techniques

the selective breeding of crop plants

•Selective breeding allows only organisms with certain traits to produce the next generation.

Agriculture

Improvements in farming techniques have contributed to dramatic improvements in crop yields.

Some of the most important techniques have been the use of pesticides and fertilizers.

Education

Chapter 24 lecture- Seeds