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* Water has been replaced by wind and animals as a means for spreading gametes.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) Sepals2) Petals3) Stamens:

-anther, filament4) 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!