nerve activates contraction - st. johns county school...

• It has been said that an oak is an acorn’s way of

making more acorns.

• In a Darwinian view of life, the fitness of an

organism is measured only by its ability to replace

itself with healthy, fertile offspring.

• Sexual reproduction is not the sole means by which

flowering plants reproduce.

• Many species can also reproduce asexually, creating

offspring that are genetically identical to themselves.

Introduction

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Fig. 38.1

• Flowers, the reproductive shoots of the angiosperm

sporophyte, are typically composed of four whorls of

highly modified leaves.

2. Flowers are specialized shoots bearing

the reproductive organs of the angiosperm

sporophyte


• The four kinds of floral organs are the sepals,

petals, stamens, and carpals.

• Their site of attachment to the stem is the receptacle.


Fig. 38.2

Know these structures and tissues for the

EOC

• Roots

• Stems

• Leaves

• Flowers

• Fruits

• Cones

• Seeds

And these

• Meristem

• Cambium

• Ground tissue

• Dermal tissue

• Vascular tissue

• Xylem

• Phloem

And these

Stomata

• Guard cells

• Stamen: anther, filament, sperm

• Pistil: style, stigma, ovary, egg

• Sepal, petals

• The stamens and carpels of flowers contain

sporangia, within which the spores and then

gametophytes develop.

• The male gametophytes are sperm-producing structures

called pollen grains, which form within the pollen sacs

of anthers.

• The female gametophytes are egg-producing structures

called embryo sacs, which form within the ovules in

ovaries.


• The male gametophyte begins its develop within the

sporangia (pollen sacs) of the anther.

• The female gametophyte begins to develop within

the ovules of the ovary

3. Male and female gametophytes develop

within anthers and ovaries, respectively:

Pollination brings them together


• The development of angiosperm gametophytes involves

meiosis

and mitosis.


Fig. 38.4

• The male gametophyte begins its development

within the sporangia (pollen sacs) of the anther.

• Within the sporangia are microsporocytes, each of

which will from four haploid microspores through

meiosis.

• Each microspore can eventually give rise to a haploid

male gametophyte.


• A microspore divides once by mitosis and

produces a generative cell and a tube cell.

• The generative cell will eventually form sperm.

• The tube cell, enclosing the generative cell, produces

the pollen tube, which delivers sperm to the egg.

• This two-celled structure is

encased in a thick, ornate,

distinctive, and resistant wall.

• This is a pollen grain, an

immature male gametophyte.


Fig. 38.5

• A pollen grain becomes a mature gametophyte

when the generative cell divides by mitosis to form

two sperm cells.

• In most species, this occurs after the pollen grain lands

on the stigma of the carpel and the pollen tube begins to

form.


• Ovules, each containing a single sporangium, form

within the chambers of the ovary.

• One cell in the sporangium of each ovule, the

megasporocyte, grows and then goes through meiosis,

producing four haploid megaspores.

• In many angiosperms, only one megaspore survives.

• This megaspore divides by mitosis three times,

resulting in one cell with eight haploid nuclei.

• Membranes partition this mass into a multicellular

female gametophyte - the egg sac.


• At one end of the egg sac, two synergid cells flank

the egg cell, or female gametophyte.

• The synergids function in the attraction and guidance of

the pollen tube.

• At the other end of the egg sac are three antipodal

cells of unknown function.

• The other two nuclei, the polar nuclei, share the

cytoplasm of the large central cell of the embryo

sac.

• The ovule now consists of the embryo sac and the

surrounding integuments (from the sporophyte).


• Pollination, which brings male and female

gametophytes together, is the first step in the chain

of events that leads to fertilization.

• Some plants, such as grasses and many trees, release

large quantities of pollen on the wind to compensate for

the randomness of this dispersal mechanism.

• At certain times of the year, the air is loaded with

pollen, as anyone plagued by pollen allergies can

attest.

• Most angiosperms interact with insects or other animals

that transfer pollen directly between flowers.


• After landing on a receptive stigma, the pollen grain

absorbs moisture and germinates, producing a pollen

tube that extends down the style toward the ovary.

• The generative cells divides by mitosis to produce two

sperm, the male gametophyte.

• Directed by a chemical attractant, possibly calcium, the tip

of the pollen tube enters the ovary, probes through the

micropyle (a gap in the integuments of the ovule), and

discharges two sperm within the embryo sac.

5. Double fertilization gives rise to the

zygote and endosperm


• Both sperm fuse with nuclei in the embryo sac.

• One sperm fertilizes the egg to form the zygote.

• The other sperm combines with the two polar nuclei to

form a triploid nucleus in the central cell.

• This large cell will give rise to the endosperm, a food-

storing tissue of the seed.



Fig. 38.9

• The union of two sperm cells with different nuclei

of the embryo sac is termed double fertilization.

• Double fertilization is also present in a few

gymnosperms, probably via independent evolution.

• Double fertilization ensures that the endosperm will

develop only in ovules where the egg has been

fertilized.

• This prevents angiosperms from squandering nutrients

in eggs that lack an embryo.


• After double fertilization, the ovule develops into a

seed, and the ovary develops into a fruit enclosing

the seed(s).

• As the embryo develops, the seed stockpiles proteins, oils,

and starch.

• Initially, these nutrients are stored in the endosperm, but

later in seed development in many species, the storage

function is taken over by the swelling storage leaves

(cotyledons) of the embryo itself.

6. The ovule develops into a seed containing

an embryo and a supply of nutrients


Wrap your mind around this

• When a plant has babies, they look very different.

• An oak tree, for instance, has pollen grains and

embryo sacs for babies.

• Pollen grains and embryo sacs have oak trees for

babies.

• Confusing?

• Endosperm development usually precedes embryo

development.

• After double fertilization, the triploid nucleus of the

ovule’s central cell divides, forming a multinucleate

“supercell” having a milky consistency.

• It becomes multicellular when cytokinesis partitions the

cytoplasm between nuclei and cell walls form and the

endosperm becomes solid.

• Coconut “milk” is an example of liquid endosperm

and coconut “meat” is an example of solid

endosperm.


• As the seeds are developing from ovules, the ovary

of the flower is developing into a fruit, which

protects the enclosed seeds and aids in their dispersal

by wind or animals.

• Pollination triggers hormonal changes that cause the ovary

to begin its transformation into a fruit.

• If a flower has not been pollinated, fruit usually does not

develop, and the entire flower withers and falls away.

7. The ovary develops into a fruit adapted

for seed dispersal


• By selectively breeding plants, humans have

capitalized on the production of edible fruits.

• The apples, oranges, and other fruits in grocery stores

are exaggerated versions of much smaller natural

varieties of fleshy fruits.

• The staple foods for humans are the dry, wind-dispersed

fruits of grasses, which are harvested while still on the

parent plant.

• The cereal grains of wheat, rice, maize, and other

grasses are easily mistaken for seeds, but each is

actually a fruit with a dry pericarp that adheres

tightly to the seed coat of the single seed within.


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