Angiosperms are a major division of plant life, which make up the majority of all plants on Earth.
Angiosperm plants produce seeds encased in “fruits,” which include the fruits that you eat, but which also includes plants you might not think of as fruits, such as maple seeds, acorns, beans, wheat, rice, and corn.
Angiosperms are also known as “flowering plants” because flowers are a characteristic part of their reproductive structure – though again, you may not always recognize their flowers as the pretty, colorful petaled things you think of when you hear the word.
Angiosperms evolved between 250-200 million years ago. They quickly gained an advantage over the previously dominant plant type – gymnosperms – for two reasons.
Angiosperms’ use of flowers to reproduce made them more reproductively successful. While gymnosperms relied primarily on the wind to achieve sexual reproduction by transferring pollen – which contains the male reproductive cells for plants – into the ovaries of female plants, angiosperms used sweet-smelling, brightly-colored flowers and sugary nectar to attract insects and other animals.
This process of cooperation, whereby animals like bees pollinate flowers in exchange for nectar, made angiosperms more reproductively successful.
Angiosperms also began to encase their seeds in fruits, which both provided extra nourishment and protection for their offspring plants and created new ways to cooperate with animals. Many angiosperm’s fruits, like their flowers, were designed to attract animals to eat them.
In many cases the seeds would then pass safely through the animals’ digestive tracts, getting carried far from the parent plant in the process. The seeds would eventually be excreted in fecal matter, which, as an added bonus, is often very nutrient-rich for plants. This enabled angiosperms to spread far and wide.
Today angiosperms make up about 80% of all plant species on Earth.
Gymnosperms, which include pines, redwoods, gingko trees, and palm trees, still hold an important place in several ecosystems. But many species of gymnosperms that lived in prehistoric forests are now extinct, having been replaced by angiosperms.
Scientists define angiosperms as plants that have several unique anatomical structures. These include:
- Stamens, which produce the pollen grains that act like sperm for angiosperm plants. Pollen grains contain male genetic information, and can be combined with female genetic information in a plants’ ovaries.
Some angiosperms can fertilize their ovaries with their own pollen, or can reproduce without being fertilized at all. But plants – and organisms in general – that exchange genes through sexual reproduction tend to have more diverse offspring, which means their offspring are more likely to be able to weather disease, predation, and natural disasters.
- Pollen, the angiosperm male reproductive material, which is smaller than the male reproductive materials of gymnosperms.
This means that angiosperm male reproductive cells can reach female eggs faster and with higher success rates than gymnosperm reproductive cells.
- Flowers, which are structures that contain the male and female reproductive parts of an angiosperm – and which are often designed to attract insects and other animals that can perform cross-pollination between different plants.
- Carpels, which enclose the ovaries that are are found inside or just behind the plant’s flower. Ovaries can receive pollen grains and begin producing seeds and fruit more rapidly than gymnosperms can produce their own seeds.
If you watch a plant’s development carefully, you can see the base of the flower swell and develop into fruit after pollination. This is the process of the carpel, which surrounds the plant’s ovary, growing into a fruit around the developing seeds.
In many fruits, the woody “spot” on the bottom opposite the planet’s stem shows where the flower was once attached before the carpel grew into a fruit.
Examples of Angiosperms
Examples #1: Fruits
Fruit trees are perhaps the most obvious illustration of the angiosperm’s life cycle.
Fruit trees often show flowers, such as apple, cherry, and orange blossoms, before they bear fruit. These flowers are pollinated by bees or other animals, allowing fruit trees to exchange genetic material and keep their population diverse.
Once the flowers have served their purpose of attracting pollinators, they lose their petals, and the carpels at the base of the flower begin to swell. These carpels continue to grow until the fruit has reached full size, and may change color to better attract animals that might want to eat it.
When a tree’s fruit is eaten by birds or ground-dwelling animals, its seeds get a free ride to wherever that animal is going – and free fertilizer, in the form of the manure it will be excreted with.
Examples #2: Grains
It might seem strange to think of grasses as flowering plants, but they are indeed a member of the flowering plant family.
Grasses have moved away from their evolutionary origin of attracting animal pollinators with big, colorful flowers and fruit. Because grasses like wheat and rice often grow in large numbers very close together, they can rely on the wind to pollinate them, and to spread their seeds through the environment.
The versions of rice, corn, and wheat that humans eat have seeds that could be described as “freakishly large,” because we have been selectively breeding our domesticated crops to have the largest possible seeds for thousands of years.
As such, these domesticated plants often don’t produce well without humans, because their seeds are too large to be carried by the wind. However, as long as humans are around, we will plant lots and lots of them to feed ourselves!
In the wild, the seeds of grasses are much smaller and are easily spread by wind.
Examples #3: Vegetables
The vegetables that come to our dinner plates have also been selectively bred by humans for many generations to make them as big, and tasty, as possible. As such, it may surprise you to hear that broccoli, kale, and lettuce are all flowering plants!
Broccoli, kale, and lettuce that are to be eaten are typically harvested before they flower since flowers are not considered delicious by most humans. The tight, green buds that makeup broccoli plants are just that – tiny flower buds!
Farmers and gardeners will typically allow some of their green vegetables to flower and produce seeds so that they can plant them for next year’s harvest. But green vegetables meant to be eaten are usually picked before their flowers show.
Examples #4: Flowers
When it comes to flowers that were bred to be big and bright, your question might be “where on Earth does the fruit come in?”
The truth is that not all fruits look like the big, colorful, sweet fruits we think of when we hear the term. In fact, a “fruit” is any protective layer around a seed, and many plants’ “fruits” may just look like swollen seed pods.
Many flowers, including roses, lilies, and daffodils, produce swollen green seed pods where the flowers used to be after their petals have dropped. If you walk through a daffodil garden after the flowers have lost their petals, you may see the stems “nodding” as they become heavy with the weight of the developing fruit.
If you leave the seed pods on the stems long enough, they will eventually take a dried-out appearance. If you can shake the seed pod and hear dried seeds rattling around inside, that means that the seed’s maturation process has finished, and you can harvest the seeds to grow more daffodils next year.
The much-touted “rosehips” which are sometimes used in food or medicinal preparations are actually the fruit of the rose plant!