Lecture 24: Plant Reproduction & Development

Covers Chapter 44

• AS mentioned in Lecture 22, what separates plants from their ancestors is the multicellular embryo.

• Think of it as being very similar to humans….sperm meets egg, fertilization, development into a new organism AND, importantly, the new plant embryo develops ON the parent plant (attached to and dependent upon…just like us!)

Plant Sexual Life Cycle is complex

• Called alternation of generations: sexual life cycle alternates between diploid and haploid stages that give rise to each other– The diploid form is called a sporophyte (this is the

plant body that you see)– The haploid form is called a gametophyte (not a

separate plant, but a part of the cycle that involves only the reproductive parts of the plant)

Sporophyte/gametophyte are not names for the plants themselves, just the STAGE of the life cycle!

Plant Sexual Life Cycle

• Before we go over the process, let’s take a close look at the flower

Closeup on the flower*• Sexual reproductive structures of angiosperms

(they are actually modified leaves):– Sepals: green leaves at base of flower– Petals: usually colorful (ATTRACT POLLINATORS)– Sporangia: sex organs

• Stamen: male reproductive structure– Anther: area where pollen is produced– Filament: stalk that holds anther

• Carpel: female reproductive structure– Stigma: sticky structure at top of carpel– Style: tube connecting stigma and ovary– Ovary: bulb-like structure at bottom of style that contains one or

more eggs/ovules (Fruit is a mature ovary with seed inside!)

sepal

ovules

petalfilament

style

stigma

anther

ovary

carpel

stamen

(a) A representative dicot flower

A Complete Flower

Fig. 44-2

Male/female gametes*• Male gamete: SPERM/POLLEN/POLLEN GRAIN

• Hundreds of thousands in each anther• Each pollen grain is TWO sperm and a tube cell surrounded by a tough,

waterproof covering• One sperm will fertilize female egg, other will contribute to endosperm:

food storage tissue that nourishes the embryo.• Female gamete: egg/ovule

– Egg is the actual gamete– Egg ENCLOSED in a nutritive food layer (for future embryo) is called an OVULE– Each ovary can hold one or many ovules, depending on type of plant– Each ovule has numerous eggs in it.

How are gametes formed?

• Each diploid cell in ovule & anther will do meiosis FIRST, producing haploid spores.– Male: Each spore will then do one round of

mitosis, resulting in two gametes enclosed in a pollen grain

– Female: Three of four eggs from meiotic division will degenerate, leaving one egg. This egg will do 3 rounds of mitosis, and result will be one egg that will be fertilized and all others will contribute to endosperm.

Female gamete formation

Male gamete formation

Pollen Grains

Fig. 44-5

Pollinators & Plants

• Animals and insects help plants to reproduce• Pollinators will transport pollen from one plant to another.• Flowers have evolved to attract pollinators and pollinators

have co-evolved to be more effective at getting pollen!– Some flowers look like insects (to draw insects in)– Some flowers provide a place for insects to lay eggs– Some flowers have evolved structures to make it easier for

pollinators to access pollen – Pollinators have evolved body structures that allow best

access to pollen

“Pollinating” a Pollinator

Fig. 44-14

Sexual Deception Promotes Pollination

Fig. 44-16

The process!*• Sporophyte (diploid) plant grows and matures, developing

male AND female sporangia ON THE SAME PLANT.• Cells in anther and ovary perform meiosis (making spores)

and then spores do mitosis, resulting in pollen grains and ovule(s) with eggs inside.– THIS IS THE GAMETOPHYTE STAGE!

• Pollen grain (from same plant or perhaps another) lands on stigma.

• Tube cell in pollen grain burrows down the style (making a pollen tube) and merges with egg in the ovule.

• Result is diploid zygote.• Zygote develops into an embryo: a seed!

Pollination and Fertilization of a Flower

Fig. 44-8

pollen tube

tube cellnucleus

sperm

sperm

tube cell nucleus

ovary centralcell

egg

ovule

pollengrain

Pollination occurs whena pollen grain lands on the stigma of a carpel

1

A pollen tube grows down through the style of the carpel to the ovary; the tube cell nucleus travels at the tip of the tube, and the two sperm follow close behind

2

Doublefertilization:

One sperm fuses with the central cell

One sperm fuses with the egg cell

3

The process-continued*

• Seed contains embryo (diploid organism that is developing-but only so far) and endosperm- nutritive layer made of unfertilized eggs and extra sperm from pollen grain.

• Seed will be dropped from plant, blown by the wind, picked up by an animal, or become encased in the ovary (fruit) depending on plant type.

• Once seed “lands” in an area appropriate for growth, it will germinate.

• We have a new sporophyte!!

Angiosperm Life CycleCraig Savage You Tube

Seeds: Hooks & Spikes

• You Tube: Global Zoo

Seeds

• You tube: Global Zoo

Germination: details• Germination: embryo breaks out of seed and forms a seedling. This

happens after a dormant period– Dormant period keeps seeds from breaking out of ripe fruit (dormant

period ends after fruit rots/dries out)– Three criteria determine period of dormancy:*

• *Drying: some seeds must be dried out before they will germinate• *Cold: some seeds require exposure to cold for a period of time,

then warmer temps before germination• *Seed Coat Disruption: some seeds require the coat be disrupted

before germination

Fruits• Fruit is a ripe ovary with seeds inside.• Packaging the embryo in fruit allows embryo to get far away from the

mother plant…..better to spread DNA! Genetic diversity!• Only ripe fruit containing mature seeds are sweet and brightly colored• Some fruits:

– Explode and disperse seeds– Float in water to other areas– Float in the wind (really lightweight fruit)– Have evolved to be edible and attract herbivores (eat fruit, travel,

poop out seeds)

Some fruit have seeds on outside

Others have seeds on inside