Botany Lesson Thirteen Flowers and Food

Transcription

1 Botany Lesson Thirteen Flowers and Food In this unit, we will: Identify flower parts Distinguish between monocot and dicot flowers Investigate how plants reproduce Learn about seeds and dispersal mechanisms Discover ways that plants spread without seeds Dissect a flower You will need: A large monocot flower, such as a tiger lily or tulip A sharp knife, such as a dissection knife A magnifying glass Food from Beauty Isn t it wonderful that what we find breathtaking, intoxicating, and absolutely delightful to our senses is not only a feast for our eyes and noses, but for our mouths* as well? God didn t have to fill our world with such beauty, but it is a testament to His love that He chose to do this for us. All of our food supply is dependent on plants, but without flowers (and the pollinators they attract), most of the plants would have died out long ago and the world would be devoid of life. All of our fruits, nuts, grains, and many of our spices depend on the reproductive capacity of plants. In a future unit, we will be discussing the peril of many of our favorite foods because of disease striking our bee populations. Before we can talk about how plants reproduce, you need to understand the basic structure of flowers. While there is some variation in the size of the ovary and the number of stamen, the basic internal structure of a flower is the same. Keep in mind that members of the Asteraceae/Compositae (Daisy) family have hundreds of tiny flowers arranged in a Fibonacci spiral that looks like a fuzzy button surrounded by colorful rays that look like petals. Each individual flower has its own ovary, pistil, and stamen. Take a look at the diagram on the next page.

2 *Students and Parents Note: There are some plants, however, that are very dangerous and poisonous even deadly. Never eat a plant without a parent s permission. Diagram of flower parts Because flowers are more visible than roots, flowers and leaves are most often used to identify plants and determine whether they are monocots or dicots. Monocot flowers mostly follow the rule of three: their petals, sepals, stamens, and compound pistil usually are in multiples of three. Dicot flowers mostly follow the rule of four or five: their petals, sepals, stamens and compound pistil are usually in multiples of four or five. Please record this information in your Monocot/Dicot worksheet from previous units. Plant Reproduction The female part of the flower is the pistil, which consists of the ovary (housing the ovules), style, and stigma. The male part of the flower is the stamen, consisting of the filament and the anther, which has the pollen grains. It can be confusing remembering which name goes to which part, so I came up with a little pneumonic device: Stay men! This lady s got a pistil! You may already know that insects land on

3 the flowers and the pollen sticks to their legs, hitchhiking to another flower, where it rubs off on the stigma of the pistil. Once on the pistil, the pollen grain sprouts open, forming a pollen tube that travels down to the ovary where it unites with an ovule and becomes a seed. The ovary swells and becomes fleshy, encasing the seed or seeds with edible fruit. That s the basic story, but where does the pollen come from? Where do the ova come from? What about seeds that are not encased in edible fruit? Are all plants dependent on insects for pollination? First of all, the pistil and its parts are formed from a special leaf, called a carpel, which is fused along the edges. Try doing this yourself. Take a simple leaf and roll it so that the two edges meet. You will find that it forms a kind of funnel shape. The ovary forms the larger end of this funnel and inside, there are placental ridges where the ovules are formed. If you ve ever eaten cantaloupe and noticed how the seeds are formed in several lines through the center of the fruit, then you have noticed the placental ridges. Each ovule starts as a mound of tissue on the placental ridge, which grows after several steps into an egg and supportive tissue structures. These events start while the flower is still a bud. And while the flower is still a bud, the anthers are forming from differentiated tissue of the carpel. The anthers contain pollen sacs where the pollen grains are formed. The pollen grains consist of two parts, the vegetative cell and the sperm cell, which are formed from successive cell divisions and differentiation. The pollen sacs of the anthers rupture when the flower bud opens, releasing the pollen grains to be united with ovules. Many times, pollen grains are carried to other flowers by insects, birds, or animals, but sometimes flowers selfpollinate because some of the pollen grains fall on the flower s own stigma. Many trees and grasses spread their pollen by being blown by the wind instead of relying on insects and other animals.

4 On the next page is a diagram of how ovules are formed and fertilized. Below that is a diagram of the parts of monocot and dicot seeds. As you can see, the monocot seeds have one cotyledon and the dicot seeds have two cotyledons, hence the names. In the monocot seed, the endosperm stores food and the cotyledon directs the usage of the food. The coleoptile and coleorhiza cover the shoot and root, respectively, as they emerge from the seed. In the dicot seed, the cotyledons store the food and eventually emerge above ground as they shrink. The hypocotyl is the initial stem as the shoot emerges. Diagram of fertilization and seed parts Spreading Seeds After seeds form, plants use a variety of methods to spread their seeds around. One method is the expulsion method whereby the seeds are forcibly ejected from the fruit at maturity. Examples of this are milkweed pods and members of the legume family. Dandelions and maple trees disperse their seeds

5 using the wind which blows the seeds to new locations. Some fruits, such as the mangrove or coconut, are carried by water to new locations. Finally, many plants are spread by animals. Animals assist in plant propagation by eating the fruits and the seeds are dropped on the ground in the animal s waste. Animals also bury seeds for later consumption, but many of these are never eaten. Many seeds, such as cockleburs, also hitch a ride on animals by attaching themselves to their feathers or fur. But, can plants spread without seeds? No seeds necessary? There are four main types of asexual propagation of plants, or reproducing plants without going through the seed cycle. A fifth type, micropropagation, will be discussed in the last unit, which will cover genetic engineering of plants along with a few other things. The main types are: divisions, cutting, layering, and grafting. Please choose one of these propagation methods and write a description of what it is and how it is done. If possible, include an example of a plant species propagated with your chosen method. Here is one place to start: Activity We are going to dissect a large flower to see if we can identify the major parts of a flower. We will identify the following: sepals, petals, filament, anthers, ovary (maybe even the placental ridge!), style, and stigma. You will need: a large flower, such as a lily, a magnifying glass, and a sharp knife. 1. You will be able to identify the filaments and anthers, sepals and petals without doing any cutting. You may be able to see the style and stigma also. Gently separate the petals and see how many stamen you have. Is your flower a monocot or dicot? 2. Look at the anthers under your magnifying glass and draw a picture of the pollen grains. 3. Look at the stigma under your magnifying glass and identify whether it is compound or not. If it is, how many parts does it have? 4. Now, pull off one or two petals so that you can get to the ovary.

6 5. With a parent s permission or assistance with the knife, slice down the center of the ovary to reveal the interior. Under magnification, you should be able to see the two placental ridges with tiny ovules attached.