Second Form Science; Domb; 10/12/2012 Name: _________________________________

Sexual Reproduction in Flowering Plants: PLANT REPRODUCTION AND PLANT PARENTING

Scientific Questions

Look for patterns in nature, and always ask “WHY?” Why do some plants produce flowers and fruits? Why do some plants produce flowers with no petals while others produce flowers with beautiful petals? And what about the different petal colors and shapes? Why do plants produce yummy fruits? And why are some fruits not so yummy? Always ask “WHY?”

Carrying Capacity

As you’ve learned, populations of organisms can grow at exponential rates, but available resources limit the number of individuals in a population (Darwin’s Postulate #1; Carrying Capacity). Similarly, remember the trophic pyramids? One trophic level can only support a limited number of organisms at the next trophic level. All populations are constrained (in terms of numbers) by their access to energy at the lower trophic level. Only so many individuals can grow, maintain and reproduce themselves at each trophic level because of the limitation of available resources.

With that thought in mind, individuals that accomplish their tasks (of growth, maintenance and reproduction) most efficiently – with the least amount of energy – will grow, survive and produce more babies than those that require more resources to accomplish the same tasks. These more efficient individuals pass on more genes than less efficient individuals; they will be “selected for”.

Strategies

So, why would any organism – plant or animal - put its valuable energy resources into what appear to be extravagant, expensive (requiring lots of energy) structures, like flowers and fruits? Why would a plant put its precious glucose it produced in photosynthesis into nectar, petals or sweet fruit?

Perhaps these plant strategies – these adaptations - pay off. Perhaps those organisms that put their precious resources into the construction of flowers and fruits pass on more genes than those that do not, in certain environments.

We will be exploring these plant strategies – these adaptations – in the following pages and in the following activities.

Adaptation: a feature that has become prevalent in a population because of a selective advantage conveyed by that feature in the improvement in some function.

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PART 1: FINDING A PARTNER

Sexual Reproduction

In sexual reproduction, a new individual is produced by the combining of material from two parents. In sexually reproducing plants, as in animals, a sperm moves towards an egg. Fertilization occurs when the egg and sperm unite to start the development of the

offspring. All living things that reproduce sexually take some features from each parent. Next year’s flowers will resemble this year’s flowers because they inherit features from both of their parents.

The flower is the structure that makes sexual reproduction in flowering plants possible.

Flower anatomy

The stamen – contains the male part of the flower. It produces pollen, a yellow powdery substance. Pollen is produced in the top of the stamen, in a structure called the anther. The female part of the flower includes the stigma, style and ovary. The top of the carpel is called the stigma. When a pollen grain reaches the carpel, it sticks to the surface of the stigma and makes its way down through the style, allowing delivery of the sperm down to the ovary. This is the enlarged part of the carpel where the female sex

cells (eggs or ovules) are produced.

The transfer of the pollen from an anther to a stigma is called pollination.

Most plants have strategies to guard against self-fertilization, and strategies to get their pollen to other plants.

Grasses, oaks, pines, and willows are all wind-pollinated plants. They have simplified flowers without petals that release lots of pollen for the wind to carry.

Wind-pollinated species produce lightweight, small and smooth pollen grains.

Some plants produce flowers with petals; these are animal-pollinated plants. The petals of the flower, and sometimes the scent of the flower, attract insects, birds or mammals that carry the pollen from one plant to another. These pollinators are rewarded with nectar (and/or an edible – non-reproductive form of pollen), which they consume. As they are dining, the real pollen sticks to their faces or bodies. When they finish dining at one flower, they move off to another flower, and in the process drop the pollen from one flower onto the stigma of another flower.

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Pollinators

The color and shape of the petals are clues to which animal the plant is trying to attract. Animal pollinated species have sticky or barbed pollen grains.

Why would flowers that are designed to attract nocturnal pollinators (animals that are active at night) most often be white?

The flowers of Carrion plants (image on left) smell strongly of rotting meat. What kind of insect do you think pollinates carrion flowers? Why?

Can you think of any other ways pollen could be transferred from plant to plant?

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PART 2: THE BABY

Once pollen from one flower makes its way to the egg of another flower, fertilization occurs. A fertilized egg then develops into a SEED (the new baby!).

All seeds are babies! Acorns are babies! Oak trees produce many babies!

The problem for the parent plant is that its seeds – its babies:

1) need food to grow;

2) are vulnerable to the elements; and

3) often have a better chance of survival away from the parent plant. Parents take up a lot of light, water and other resources. Babies will have a better chance of survival where there is less competition from other plants.

So parent plants do the following:

1) deposit stored chemical potential energy (lots of glucose in the form of starch = endosperm) around the embryo (the fertilized egg that has begun to divide and grow);

2) construct a protective covering (seed coat);

3) develop ways to move the babies away from the parent plant.

Seed dispersal

Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. As a result, plants have evolved many ways to disperse their offspring by dispersing their seeds. A seed must somehow "arrive" at a location and be there at a time favorable for germination and growth.

By wind

Dandelion seeds (left) can be carried long distances by the wind.

A milkweed seed pod opens up when the seeds are mature and the seeds have a wing that aids in wind dispersal.

SO, WHAT IS A FRUIT?4

Embryo = Seed leaves

+ shoot and root

A fruit is extra packaging for the seed(s). Some fruit are very fancy and expensive for plants to make; others are simple and relatively cheap. This packaging helps seeds survive and sometimes the fruit helps the seed(s) move away from the parent plant.

By water

Some plants produce buoyant seeds that can float in rivers to the oceans and wash up on beaches. Others produce buoyant fruits around their seeds that can survive water journeys.

By animals

Seeds with fruits that consist of barbs or hooks are useful because they attach to animal fur or feathers, and then drop off later.

Seeds with fruit that consist of a fleshy covering (e.g. apple, cherry, juniper) are eaten by animals (birds, mammals, reptiles, fish) which then disperse these seeds in their droppings.

Nuts (e.g. acorns, hazelnut, walnut) are fruits which consist of a hard outer wall that cover the seed coat. Nuts are usually packed with a great deal of storable food resources for the seed. This is why they are so attractive to animals that store their food, like squirrels. Plants that produce nuts typically produce a great number of them per season so that some of the nuts that are stored by animals like squirrels are stored some distance from the parent plant, and some will escape being eaten if the animal forgets them.

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Diagram of the anatomy of an acorn: A.)Cupule B.) Pericarp (fruit wall) C.) Seed coat (testa) D.) Cotyledons (2) E.) Plumule F.)Radicle G.) Remains of style. Together D., E., and F. make up the embryo.

Prelab Questions

1) What is a flower? A seed? A fruit?

2) What are some roles fruit play in insuring seed dispersal?

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In lab

Several types of seeds have been soaking overnight. Choose 3 types and carefully dissect them. Draw your observations in the space below, noting the type of seed and labeling the seed coat, endosperm, and embryo for each.

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Consider the seeds and fruit below (provided in lab), and pictured as flowers. For each, make a few observations about the structure and infer how that can be useful to the embryo. Be sure to note which portion is the seed and which portion is the fruit for each.

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Milkweed pod releasing seeds

Mango fruit, seed exposed

Raspberry fruit

Horse chestnuts

Coconut fruit and seed (they float!)

Postlab Questions

Explain the importance of the endosperm to the embryo, making reference to potential and kinetic energy in your answer.

If you could follow every seed released from a single plant, chances are high the placement of those seeds would resemble the diagram of a seed shadow (at right – this is not a literal shadow, it’s a metaphorical one). However, not every seed would germinate and, of those that did, not every embryo would grow into a mature plant.

Describe the graph in words.

Why does a seed shadow take this shape?

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distance from parent plant

Which of the seeds pictured in the diagram below do you predict will have the best chances of producing mature plants? Explain.

Name three different seed dispersal strategies plants use, and describe the adaptations that allow the seeds to disperse in these ways.

Mistletoe plants are parasitic on other plants, that is, they grow on another plant’s branch and drive their roots into the branch to get nourishment. Consider the following:

The mistletoebird (Dicaeum hirundinaceum) of Australia is dependent on the fruits of mistletoe for its survival. Though quite common, mistletoebirds are inconspicuous and rarely seen by untrained observers. They are effective dispersal agents for mistletoe because, although the fruits are eaten, the seed and its sticky coating pass through the bird undigested. Seeds pass out in the droppings and some are deposited on tree branches where their sticky coating helps them to attach and grow. Mistletoebirds are credited with perching sideways on branches thus facilitating placement of the seed. In all, 33 bird species have been

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recorded feeding on mistletoe berries but the behavior and digestive process of most do not aid mistletoe dispersal.1

What does the mistletoe bird get from the mistletoe plant?

What does the plant get from the bird?

What kinds of traits have been selected for in the mistletoe plant population?

1 http://www.dpi.vic.gov.au/dpi/nreninf.nsf/childdocs/-9599F8E44B161F63CA256BC800079622-738D5F5AA8EE28A2CA256BC800090A63-6F1B33D2E88612BE4A256DEA002933B4-B2A6433E6844F1D2CA256BCF00088823?open

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