|Reproduction of Flowering Plants|
|CCSTD Science Grade 7 5 f.|
As we said in our last Instruction, all living things reproduce. Some things reproduce sexually and others reproduce asexually, as you learned in our Genetics Lesson (Lesson 2).
When things reproduce sexually, male and female sex cells -- gametes (an egg and a sperm, for example) -- unite. This produces offspring that are like one parent in some ways and like the other parent in others.
But when things reproduce asexually, there are no sex cells involved. The parent organism simply splits into two or more identical "daughter" organisms (clones).
How Plants Reproduce
Many plants reproduce asexually. This includes food crops like onions, pineapple, potatoes and carrots. This is good for growers, since they can be sure that all of their plants will be exactly the same.
Other plants can reproduce either sexually or asexually. Blackberries, for example, can reproduce asexually by sending out "runners" that take root wherever they touch the ground. But they can also reproduce sexually with the help of their flowers.
Which brings us to the main subject of this Instruction – reproduction in flowering plants.
The scientific name for flowering plants is angiosperms. There are at least 260,000 living species of angiosperms. And they have been around for a long, long time. Fossils of flowering plants have been discovered that are over 130 million years old.
All flowering plants reproduce sexually. In fact, the flower is the plant's reproductive organ.
Some flowers -- called perfect flowers -- are hermaphrodite, which means that they contain both male and female reproductive organs within the same flower.
Some flowers -- called imperfect flowers -- contain only male reproductive organs or female reproductive organs.
Some plant species have both male and female flowers on the same plant, while others have only male flowers on some plants and only female flowers on others.
As with most things in nature, there are many variations. But the most common flowers are the so-called perfect flowers. So let's take a look at them.
The Perfect Flower (From Sepals to Ovules)
Let's start with the outside of the flower. The first things you see are probably its Sepals and Petals.
The Sepals form a ring of small leaf-like sections around the base of the flower. They are usually green and their job is to protect and support the flower.
The bright-colored Petals are there to attract the birds and insects who will do the important work of pollination (we'll tell you more about pollination in a minute). The Petals often smell good, too. (Some flowers depend on the wind for pollination, so their Petals are less colorful & fragrant since they don't need to attract pollinators).
The Petals form the plant's Corolla, while the Sepals form its Calyx.
Now let's peel back the Petals and look inside the Corolla.
What do you see?
First, you'll probably see some slender stalks with little grain-like things at the ends of them. The stalks are Filaments and the little things on the ends of them are Anthers. The Anther and the Filament are the male reproductive organs of the flower. Together, they are called the Stamen.
The plant's male sex cells, a powdery substance called Pollen, are formed inside the Anthers.
What else will you see inside the flower?
Well, deep in the center of the Corolla, you will see the plant's female reproductive organs. Each of these organs is called a Carpel -- and all the Carpels together are called the Pistil.
Each Carpel consists of a Stigma, a Style and an Ovary.
The Ovary is located at the base of the flower -- and this where the plant's female sex cells (called Ovules) are produced. The Style is a tube on top of the Ovary and the Stigma is the top part of the Style. The Stigma is where Pollen sticks during fertilization.
Obviously, for fertilization to take place, Pollen from the male part of a plant -- the Stamen -- must reach the Ovule deep inside the female part of a plant, the Carpel (Pistil).
Itís the plant equivalent of human sperm fertilizing a human egg. In plants, the process is called pollination.
In humans and other animals, the male and female can get together on their own. But it's not so easy for plants.
Although a few plants can pollinate themselves (self-pollination), most need to be pollinated by (or pollinate) other plants of the same species. This is called cross-pollination -- and plants need help to do it. And they get it -- from birds, bats, insects, rain and wind (but especially from insects).
Here's how it works.
Birds and Bees
Insects, birds, bats and other small mammals are attracted to plants by the bright colors of their Petals or by their scent -- which tells them that there is nectar inside.
Nectar is a sweet, sugary liquid produced by an organ inside the flower that we haven't told you about yet. This organ is called the Nectary and it is located at the base of the Petals below the Ovary. Nectar sometimes also collects in little pouches or sacks at other places within the flower.
Insects and birds love nectar -- especially bees, who turn it into honey in their hives.
But since this Nectar is hidden deep inside the plant, the bee must climb all the way down into the flower to get it. On the way there or back, the bee gets covered with Pollen (the plant's male sex cells) from the flower's Anther.
Hopefully, the bee will then deposit this Pollen on the Stigma (the tip of the female reproductive organ) in the next plant it goes into as it flies from plant to plant collecting nectar. Once deposited on the Stigma, the Pollen is transported down the Style to the plant's Ovary -- where it can come in contact with the female sex cell, the Ovule. The result is a fertilized (pollinated) Ovule, which develops first into a seed and then into a fruit.
But what about the plants that don't attract birds and bees?
They have to depend on the wind (or even on raindrops) for pollination.
Blowing in the Wind Wind-pollinated plants are much less "showy" than plants that have to attract birds and insects. Their flowers usually have small petals, no nectar and no scent. But they do have large Anthers that hang down outside their flowers. This makes it easy for them to disperse their Pollen.
Grasses and many food grains (including corn, wheat and rice) literally throw their Pollen to the wind when it comes time to reproduce. They release great clouds of Pollen in the hope that some of it will land where it is needed.
Fortunately, it often does.
Maple trees, oak trees and pine trees reproduce this way, too.
Seeds and Fruit
No matter how an Ovule gets fertilized, it immediately begins to divide into different cells. Each fertilized cell becomes a seed. Each seed contains both an embryo (developing plant) and a food source (called endosperm). The embryo and endosperm are covered by a tough, protective covering called a seed coat.
The Ovary of a plant can contain one seed or many seeds. An Ovary filled with one or more fertilized seeds is called a fruit. As seeds develop within them, the walls of the Ovaries of plants often become fleshy, fragrant and attractive to animals (like us).
An apple, for example, is the ripened Ovary of an apple tree -- with apple seeds in its core.
Other fruits include berries, tomatoes and melons (which contain many seeds) and peaches, apricots and cherries (which contain only one).
In nature, animals (like birds, deer and bear) come along and eat the fruit. The seeds in the fruit survive the animal's digestive process and eventually get deposited in its droppings. This can take place a long way from where the fruit was originally eaten.
This spreads the seeds far and wide so new plants can grow.
Experiments for Home and Classroom
These role-playing activities help students understand the difference between pollination by animals and pollination by the wind or other elements. They were designed for slightly younger students but could be helpful for anyone. Click: http://www.teachersnetwork.org/IMSL/ps1/flowerpower/pollination.htm
Bees are great pollinators. But not all bees are the same. In this interesting activity, students are invited to "identify the bumblebee" in their own location. Select "Help" to begin. Click: http://pick4.pick.uga.edu/mp/20q?guide=Bumblebees