NOTES: CH 38 –

Plant Reproduction

*Modifications in reproduction were key

adaptations enabling plants to spread

into a variety of terrestrial habitats.

* Water has been replaced by

wind and animals as a means

for spreading gametes.

* Embryos are protected in

SEEDS.

• Vegetative (asexual)

reproduction is used for

propagation of some plants

in a variety of environments.

**Our focus in this chapter:

SEXUAL REPRODUCTION!

GAMETOPHYTE: reproductive structure

of plants

-where haploid GAMETES (n) are produced

(sperm and egg cells)

-gametes fuse to form a ZYGOTE (2n) that

develops into a multicellular plant

…ALTERNATION OF

GENERATIONS!

FLOWERS:

-the reproductive structure of angiosperms

-four sets of modified leaves:

1) Sepals

2) Petals

3) Stamens:

-anther, filament

4) Carpel (a.k.a. pistil):

-stigma, style, ovary

FLOWERS:

STAMENS and CARPELS (PISTILS) are

the reproductive parts of the flower.

FLOWERS:

Female gametes develop in EMBRYO

SACS, which contain the EGG CELLS

this occurs at the base of the carpel /pistil,

inside the OVARY

Male gametes develop in the stamen

inside POLLEN GRAINS

this occurs at the stamen tips within

chambers of the ANTHERS

FLOWER VARIATIONS:

Complete flower = flower with sepals, petals, stamens, carpels

Incomplete flower = flower missing one or more of these parts

(e.g. most grasses do not have petals on their flowers)

FLOWER VARIATIONS:

Perfect flower = flower having both stamens and carpels (a.k.a. bisexual)

Imperfect flower = flower that is either STAMINATE (male) or CARPELLATE (female) – a unisex flower. (a.k.a. unisexual)

Monoecious = “one house”; plants with

either perfect flowers or with both

staminate (male) and carpellate (female)

flowers on the same plant

(ex: star lily, rose, dandelion)

Dioecious = “two houses”;

plants having staminate and

carpellate flowers on separate

plants (ex: willow, juniper)

Development of the Male

Gametophyte (POLLEN):

POLLEN GRAIN = the immature male

gametophyte; develops within the anthers

of stamens in an angiosperm

extremely durable;

tough coat to

prevent

biodegradation

At the time of pollination:A pollen gain consists of 2 cells (both

haploid):

-GENERATIVE CELL: divides to form 2

sperm cells

-TUBE CELL: produces the pollen tube;

contains the generative cell

Development of the Female

Gametophyte (EMBRYO SAC)

● Female

gametes are

contained in

the

EMBRYO

SAC

Embryo Sac:

egg cell (OVUM) at one end

ovum flanked by 2 syngerids (help to attract sperm cells)

two nuclei in center (“polar nuclei”)

POLLINATION:

-occurs when wind- or animal-born pollen

released from the anthers lands on the

STIGMA at the tip of the carpel

POLLINATIONPOLLINATION = the placement of pollen

onto the stigma of a carpel

Methods of Pollination / spread of pollen:

• wind

• animals / insects

• self-pollinate

(most plants

cross-pollinate)

To prevent self-pollination (and thus

ensuring sperm and egg from different

plants meet), some plants:

-have stamens and carpels that mature at

different times;

-have structural arrangement of flower parts that

reduces the chance of pollen getting

transferred from stamen to carpel;

-are SELF-INCOMPATIBLE (a biochemical

block that prevents the pollen grain from

developing and fertilizing the egg)

SELF-INCOMPATIBILITY: the rejection of

pollen from the same, or closely related,

plant by the stigma

after POLLINATION…

-pollination is followed by this sequence:

1) a pollen tube grows from the pollen grain, down

the carpel, into the embryo sac;

2) sperm are discharged from the pollen grain

resulting in fertilization of the egg;

3) the zygote develops into an embryo;

4) as the embryo grows, the ovule surrounding it

develops into a SEED;

5) while seed formation is taking place, the entire

ovary is developing into a FRUIT, which will

contain one or more seeds.

DOUBLE FERTILIZATION:

When a compatible pollen grain lands on a

stigma of an angiosperm, double

fertilization occurs.

DOUBLE FERTILIZATION = the union of

two sperm cells with two cells of the

embryo sac

DOUBLE FERTILIZATION:

Steps:

1) pollen grain adheres to stigma, germinates, and extends a pollen tube toward the ovary;

2) generative cell divides (mitosis) to form 2 sperm;

3) directed by a chemical attractant, pollen tube enters and discharges its 2 sperm nuclei into the embryo sac;

4) 1 sperm unites with the egg ZYGOTE;

5) other sperm combines with the 2 polar nuclei to form a 3n (triploid) nucleus in the large central cell of the embryo sac.

this central cell will give rise to the ENDOSPERM (a food storing tissue for the seed/embryo)

**after double fertilization, each ovule develops into a seed and the ovary into a fruit surrounding the seed(s).

ENDOSPERM:

• rich in nutrients (which it provides to the developing embryo)

Structure of the mature seed:

in mature seeds, the embryo is quiescent

(dormant) until germination

• the seed dehydrates until its water content

is only 5-15% by weight.

• the embryo is surrounded by endosperm,

enlarged cotyledons, or both.

• the seed coat is formed from the outer

layers / integuments of the ovule.

Dicot seed structures:

-cotyledons =

embryonic leaves

-hypocotyl =

embryonic axis

below cotyledons

-radicle = embryonic

root

-epicotyl =

embryonic axis

above cotyledons

*Seeds are dispersed from the source plant when fruits are moved about by

the wind or animals.

*Seeds deposited in soil of the proper conditions (moisture, nutrients), will

GERMINATE.

FRUITS:-ovary develops into fruit (adapted for seed

dispersal)

*Fruits ripen about the time seeds are

becoming fully developed

Ripening fruits are characterized by:

1) fruit becomes softer as a result of enzymes digesting the cell wall components;

2) colors usually change and fruit becomes sweeter as organic acids or starch are converted to sugar;

3) these changes produce an edible fruit which entices animals to

feed, thus dispersing the seeds!