Pollination
Flowers – we see them everywhere, from the grocery store (roses, carnations) to our front yards (dandelions, clover). They come in a variety of shapes, colors, sizes, and scents. The flower pictured here belongs to a white thistle. It can be tempting to think that the beauty we see in flowers is for our own enjoyment, but flowers evolved to house a reproductive system. That’s right – flowers are where babies are made (baby flowering plants, that is)! The outer parts of a flower, including the petals, attract pollinators like insects that facilitate fertilization, and the structures inside a flower produce gametes (a plant’s versions of eggs and sperm), which allow for the development of seeds.
Let’s take a closer look at the inner workings of a flower to see how it does the important work of reproduction in flowering plants.
Written by Kristen Short; Fort Wayne Community Schools; Indiana, USA
Background image by Jan Martinek; Charles University; Prague, Czech Republic
A peek inside a flower
Imagine if you could look through the petals of a flower to see what’s really going on in there, the way an X-ray can show doctors the bones inside your body. Using a technique called chemical tissue clearing, it is possible to look through the petals of the flower and see its internal structures. The pollen-producing part of the flower is the stamen, consisting of an anther (green) at the end of a long filament. This flower has six of them surrounding the center of the flower, where the female part, called the pistil, is located. At the pistil’s base is the ovary, which contains ovules that can be fertilized to form seeds. Fertilization is made possible when the genetic information from a pollen grain travels down pollen tubes (yellow) to meet the ovules.
As you explore further, you’ll see a more detailed look at how a flower works to form the seeds that the plant needs for reproduction.
Image by Jan Martinek, Charles University
Anthers make pollen
Here’s a closer look at the pollen-producing parts of the flower: the anthers and filaments. Each anther makes many tiny pollen grains, shown here in orange. Pollen (the powdery stuff that makes people sneeze in the spring) has one job: to fertilize the ovules within the pistil of another (or sometimes the same) flower.
Image by Jan Martinek, Charles University
Pollen travels
This orange anther used to be full of tiny yellow pollen grains, but now only a handful remain. Most pollen must travel to gain access to ovules during reproduction, and since it can’t move by itself, the flower relies on wind, insects like bees and flies, birds, or other animals to move pollen around. And plants typically produce way more pollen than they need because most of it never reaches another flower once it leaves the anther, ending up in animal fur, on the ground, or in human noses (achoo!).
Image by Jan Martinek, Charles University in Prague
Pollen lands on a stigma
Those pollen grains that do find another flower end up stuck to the flower’s pistil (blue). At the top of the pistil is the stigma, which catches pollen grains (orange) on its sticky surface. You’d probably never notice this part happening, but this movement of pollen from the anther to the stigma is called pollination, and it is essential for the reproduction of flowering plants.
Image by Jan Martinek, Charles University
The pollen grain grows a pollen tube
Pollen grains come in many shapes and sizes; this yellow/orange one, shaped like a spiky whiffle ball, has bound to the pink stigma of this flower. Once a pollen grain has stuck to a stigma, it initiates a sort of biochemical “conversation,” ensuring that the pollen and the stigma belong to the same species of plant. If the pollen and the stigma are compatible with each other, the pollen grain can start to grow a pollen tube.
Image by Igor Siwanowicz, HHMI's Janelia Research Campus
The pollen tube grows quickly
This closer view of a pollen grain (pink) attached to a stigma (light blue) shows the pollen tube (light blue) growing down into the style (darker blue/purple). The pollen tube is a tiny tube that grows for the sole purpose of allowing sperm contained within the pollen grain to travel toward the ovules to fertilize them, eventually forming seeds. It grows quickly, within hours or days, and guides sperm toward unfertilized ovules within the ovary of the plant.
Image by Igor Siwanowicz, HHMI's Janelia Research Campus
The ovules await fertilization in the ovary
The ovary of the plant, at the base of the pistil, contains unfertilized ovules (light green). When pollen (blue) lands on the stigma (green), the pollen tube from each pollen grain extends toward one of the unfertilized ovules, allowing sperm to find and fertilize the ovules to form seeds. As these seeds develop, the ovary enlarges and matures, forming a fruit. The next time you eat an apple or a cucumber, or any fruit with seeds inside it, remember that you are eating the ovary of a flowering plant!
Once fertilization has taken place and the ovary is maturing, the flower often shrivels and falls away, because its job of uniting sperm with eggs is done.
Image by Jan Martinek, Charles University
A seed germinates
Once seeds have matured in the ovary of a plant, they are released from the ovary and dispersed by animals, wind, water, etc. Seeds land and wait for the right conditions to germinate and grow into baby plants. Sometimes those conditions are available right away, if the seed lands somewhere with the right amount of light and water and the right temperature and soil quality. However, other times the seed lies in wait until it receives signals that conditions are right for growing. Then the seed starts to grow, breaking out of its tough seed coat as a tiny shoot extends upward toward the light. The seed contains some stored food that the tiny seedling can use for energy until the baby plant can make its own food through photosynthesis above ground.
Image by Jan Martinek, Charles University
Conclusion
The next time you see a flower, think about how much is going on in there – and see if you can identify all the structures explained here!
For suggestions on how to incorporate this journey into your teaching, see our “Implementation Suggestions.”