Plant Reproduction
What Are the Processes of Pollination and Fertilization in Plants?
Pollination and Fertilization
The movement of pollenglossary term (opens in a new window) from an anther to a stigma is called pollination. Once a pollen grain reaches a stigma of a flower of the same species, it develops a pollen tube. This tube grows down the style to the ovary. Sperm inside the pollen grain travel down this tube and fertilize the egg inside the ovary.
Pollination
Pollen can be transferred from an anther to a stigma in the same flower or to a stigma in another flower on the same plant. This is called self-pollination because the plant is pollinating itself. Alternatively, pollen can be transferred from the anther of one flower to the stigma of a flower on another plant of the same species. This is called cross-pollination, or outcrossing. Self-pollination ensures that a flower is pollinated but, after fertilization, results in offspring that are less genetically diverse than offspring that are the product of cross-pollination. Many species of plants go to great lengths to ensure cross-pollination will occur and use self-pollination only as a last resort to ensure fertilization.
Most plants are monoecious. In these species, individuals carry flowers that have both carpels and stamens. Some species of plants have individuals that are different sexes. Plants that use this strategy are dioecious. Male plants grow flowers with stamens but no carpels. Females produce female flowers with carpels but no stamens. These dioecious species have no option but to cross-pollinate. Other species prevent self-pollination by having their ovules and pollen mature at different times on the same individual. Many species have chemical inhibitors that prevent pollen tubes from forming when pollen and stigma come from the same individual.
Cross-pollination is accomplished in a variety of ways. Pollen can be carried from one plant to another by animals, or by physical agents such as wind and water. Most plants that are animal pollinated have co-evolved a mutually beneficial relationship with their pollinator. This is an example of mutualism. The plant feeds the pollinator. The pollinator pollinates the plant. Plants have evolved flowers with bright petals, food sources (such as nectar or ample pollen), and scent to attract their specific pollinators. On visiting the flower, the pollinator inadvertently picks up pollen and deposits some of it on the stigma of the next flower it visits. Most pollinators are insects. Many other invertebrates also pollinate flowers. Some vertebrates, most commonly birds and bats, also pollinate flowers.
Wind pollination is less targeted than animal pollination. Wind-pollinated plants rely on producing large amounts of pollen, a small amount of which will be blown to the stigma of a plant of the same species. These plants do not need petals, nectaries, or scent. Instead, they typically have hairy stigmas that hang outside the flower and catch windborne pollen. The table below compares structures in wind- and animal-pollinated flowers.
|
Wind Pollinated |
Animal Pollinated |
|
Small flowers, often green |
Large brightly colored flowers |
|
Don’t produce nectar |
Produce nectar to attract pollinators |
|
Not scented |
Scented |
|
Stamens hang outside the flower to catch the wind. |
Stamens are positioned inside the flower. |
|
Pollen grains are smooth and light—easily transported by wind. |
Pollen grains are spiky or sticky—to stick to animal pollinators. |
|
Produce large amounts of pollen |
Produce less pollen |
|
Stigmas are large and often hairy and hang outside the flower. |
Stigmas are small and are usually inside the flower. |
A few aquatic plants are water pollinated. They have characteristics that are similar to wind-pollinated flowers.
Teacher Note: Practice
In this item, students analyze data to identify the design or characteristics of the components of different types of flowering plants. They compare a theoretical class’s observational data to archival data about plants that are pollinated by wind versus by animals. Extend this item by having students make their own observations about characteristics of flowering plants as a field study, or in the classroom using flower samples or appropriate images. Have students draw conclusions about each plant’s pollination method.
| Wind-Pollinated | Animal-Pollinated |
| Small flowers, often green | Large, brightly colored flowers |
| Don’t produce nectar | Produce nectar to attract pollinators |
| Not scented | Scented |
| Stamens hang outside the flower to catch the wind. | Stamens are positioned inside the flower. |
| Pollen grains are smooth and light—easily transported by wind. | Pollen grains are spiky or sticky to stick to animal pollinators. |
| Produce large amounts of pollen | Produce less pollen |
| Stigmas are large and typically hairy and hang outside the flower. | Stigmas are small and typically inside the flower. |
| Specimen | Observations | Pollination Method |
| Specimen 1 | Flowers have a strong fragrance and are about 2 inches across the corolla. | |
| Specimen 2 | Flowers are one-half inch in size, white, and have a fuzzy appearance. No flying insects were observed. | |
| Specimen 3 | Fifteen bees visited plant over 30 minutes. Flowers are yellow. | |
| Specimen 4 | Flowers have prominent stamens and large, pink petals. Pollen is sticky to the touch and adheres to fingers. | |
| Specimen 5 | Yellow clouds emanating from plant. No apparent flowers, but brown cones are present. |
Fertilization
For fertilization to take place, the sperm in the pollen grain must fuse with the ovum in the ovule. Upon landing on the stigma of a flower of the same species, the pollen grain germinates. The tube cell in the pollen grain grows into a long microscopic tube called the pollen tube. The tube grows down through the style. It is attracted toward the ovule by chemicals secreted for this purpose. The pollen tube enters the ovule through a small hole called the micropyle. The two sperm in the pollen grain migrate down the tube. One fertilizes the egg. The other fuses with other cells in the female gametophyteglossary term (opens in a new window). This will later develop into a food store called the endospermglossary term (opens in a new window). Fertilization results in the first stage of the sporophyteglossary term (opens in a new window) generation. The ovule will develop into a seedglossary term (opens in a new window). The ovary surrounding the seeds develops into a fruitglossary term (opens in a new window).
Teacher Note: Misconception
Students may think that when a single plant gives rise to offspring, asexual reproduction has occurred in every instance. However, even though only one parent plant is involved when the sperm and egg of a single flower fertilize, sexual reproduction has still occurred.
Seed and Fruit Development
The fruit is the part of a plant that develops from a fertilized ovary. Fruits contain the seeds. Fruits are how most flowering plants protect and disperse seeds.
In flowering plants, seeds begin to develop after the ovule has been fertilized. The cells of the zygote differentiate to form a tough protective outer coat, called the testaglossary term (opens in a new window), that surrounds the seed. An embryo sporophyteglossary term (opens in a new window) plant forms. The embryo consists of the embryonic shoot called the plumuleglossary term (opens in a new window), the embryonic root called the radicleglossary term (opens in a new window), and one or two cotyledons. Cotyledons may act as the first seed leaves of a plant and as a food store for the seed. The testa also surrounds a food store called the endospermglossary term (opens in a new window). The endosperm is part of the seed, but is not part of the embryo. The endosperm forms when the second sperm from the pollen grain enters and fertilizes the polar nuclei in the embryo sac of the ovule. The endosperm is rich in starch. The part of the wheat or maize seed used to make flour is the endosperm. When seeds develop to a mature stage, the plant removes most of the water from them. Plant tissues normally contain about 75 percent water. Seeds usually consist of less than 20 percent water. In this dehydrated condition, the embryo enters dormancy. In this state, the seed can resist severe conditions such as extreme cold or dryness.
As the ovules develop into seeds, the ovary develops into a fruit. This process varies depending on the type of fruit produced. Often, a number of ovules will develop into one fruit, as in raspberries and strawberries. In fruits such as berries, the ovary becomes fleshy and ripens. In some fruits, such as nuts, the ovary hardens. Often the ovary will fuse with other parts of the plant to create accessory or false fruits. For example, apple ovaries (the core) join with the receptacle of the apple flower that forms a fleshy edible layer. Fruits take on a wide variety of forms that depend in part on their strategies for dispersal.
The process of fruit development and ripening is controlled by plant hormones. When a fruit ripens, starch is turned into sugars and the walls of the cells in the fruit soften. Both changes make the fruit palatable to the organisms involved in dispersal. These changes are controlled in part by the gas ethylene. Ethylene is a gaseous hormone. Fruit growers use ethylene-producing compounds to accelerate the ripening of fruit. They may also use methods that reduce the concentration of ethylene to slow down the ripening of fruit to prevent spoilage. Ethylene plays a role in other parts of the plant, particularly meristems. Ethylene production is controlled by another plant hormone, auxinglossary term (opens in a new window) (IAA, indole-3-acetic acid).
