Flowering plants are the largest group of plants on Earth. More than 300,000 species of them have been named. They grow as herbs, as grasses, as vines and as trees.
The group has a scientific name: angiosperms. Pines, firs, cycads and ginkgo are not angiosperms. They are gymnosperms, and there are only about 1,200 known species of them all together.
Why did one group end up with so many more species than the other? The answer lies in a handful of features that flowering plants have and other plants do not. This article takes those features one at a time.
What Makes a Flowering Plant Different
A flowering plant is not defined by its size or its shape. It is defined by how it makes its seeds. Its seeds form inside a case. That case is the ovary, and it sits inside a flower.
The cones of a pine work differently. A pine seed sits out in the open on the scale of a cone. Nothing wraps around it. The name gymnosperm means exactly that: a naked seed.
How Many Flowering Plants Are There?
Counts this size are harder to make than they sound. Botanists disagree about where one species ends and the next begins, so the number keeps moving as new plants are described and old names are combined.
A 2022 study in the journal Biodiversity Science went around the problem by counting order by order, using several global databases at once. It is the source of the rough comparison below, which is the striking part of the whole story.
| Flowering plants | Cone-bearing plants | |
| Seeds | Inside an ovary, which ripens into a fruit | Exposed on the scales of a cone |
| Known species | About 300,000 or more | About 1,200 |
| Double fertilization | Yes | No |
| Water-conducting cells | Vessels and tracheids | Mostly tracheids only |
Flowering plants and cone-bearing plants compared. One group has a few thousand species; the other has a few hundred thousand.
Flowers: The Part That Gives the Group Its Name
A flower is a set of leaves folded into a job. Some of its parts protect the bud. Others make pollen. Others hold the eggs.
The petals and the sepals are the showy outer parts. Inside them sits the stamen, which makes pollen in a sac called the anther. At the center is the carpel. The carpel carries the stigma at its tip, the style below it, and the ovary at its base.
The ovary is the important part for this article. It holds small bodies called ovules, and each ovule can become a seed. Because the ovules sit inside the ovary, a flowering plant carries its seeds in an enclosed case from the very start.
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| The main parts of a mature flower, labelled. The stamens make pollen and the carpel holds the ovules inside its ovary. Image: Mariana Ruiz LadyofHats, public domain, via Wikimedia Commons |
Matching a Flower to Its Pollinator
Flowers come in shapes that suit the animals that visit them. A long tubular flower can be reached by a hummingbird's beak. A flat, open flower gives a bee somewhere to land.
These pairings are often described as syndromes, which makes them sound stricter than they are. Red tubular flowers are the textbook hummingbird flower, yet a US Forest Service study found generalist bees pollinating red-flowered Penstemon eatonii and setting heavy seed when no hummingbirds were around.
So a flower may look built for one visitor and still be served by others. The shape is a tendency, not a lock.
What Happens Inside the Ovary
Double Fertilization
Fertilization in a flowering plant is unusual, because it happens twice inside the same ovule. That is what the name double fertilization describes.
Pollen lands on the stigma and grows a tube down through the style. Two sperm cells travel down the tube. The tube reaches the ovule, and the two sperm go to work.
The first sperm fuses with the egg cell. That makes a zygote with two sets of chromosomes, and the zygote grows into the embryo.
The second sperm does something quite different. It fuses with two polar nuclei that sit together near the center of the ovule. The result is one nucleus with three sets of chromosomes. Biologists call it triploid.
That triploid nucleus divides again and again. It becomes the endosperm, the tissue that stores food for the seedling. Its stores include starch, sugars, fats and proteins.
Double fertilization is found in flowering plants and nowhere else. It is the clearest single feature that separates the angiosperms from every other group of plants.
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| A flower's ovules being reached by sperm cells carried in a pollen tube. Two sperm enter each ovule, and each one has a different job. Image: Chiswick Chap, CC BY-SA 4.0, via Wikimedia Commons |
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| The life cycle of a flowering plant, from flower and fertilization through seed and germination to a new plant. Image: Mariana Ruiz LadyofHats, public domain, via Wikimedia Commons |
When There Is No Endosperm
Endosperm does not always survive in the mature seed. Orchids and some other plants with very small seeds hold almost none at all.
In those seeds the stored food sits elsewhere. In beans, lettuce and peanuts it sits in the cotyledons, the seed leaves. In a few plants it sits in a tissue built from the nucellus, called the perisperm.
The food is still there. Only the container has changed.
A Fruit Is a Ripened Ovary
Once the ovule is fertilized, the ovary changes. It ripens. The ripened ovary, together with what it holds, is the fruit.
That definition comes from botany rather than from the kitchen. The New York Botanical Garden states it plainly. The ovary encloses and protects the ovules from the youngest stage of the flower until they become seeds. The fruit then helps to spread them.
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| One nectarine, photographed at intervals from the bud stage to a ripe fruit, about seven and a half months later. Image: jjron, CC BY-SA 3.0, via Wikimedia Commons |
Is a Tomato a Fruit?
Under that botanical definition, yes. A tomato is a fruit, and so are an eggplant, a green bean and an okra pod. A walnut in its shell is a fruit. So are the grains we eat: corn, wheat and rice.
A strawberry is a special case. The red fleshy part is not ovary tissue at all, which makes it an accessory fruit. The little seed-like specks on its surface are the real fruits. Each one is a small dry fruit.
An oak acorn is a fruit too. It sits in a cup that is not ovary tissue, so the cup is not part of the fruit.
None of this is merely wordplay. It shows how firmly the seed case belongs to the flowering plants. A pine has no fruit because it has nothing to ripen into one.
A Better Plumbing System
A tall tree has to lift water from its roots to leaves that may be thirty metres up. Two tissues in the stem do that work.
Xylem carries water and dissolved minerals upward from the roots. Phloem carries sugars away from the leaves to wherever the plant needs them. The leaves that make the sugar are the source, and the roots, storage organs, flowers and fruits that use it are the sink.
Tracheids and Vessels
Water moves through dead, hollow cells. There are two kinds. The first, the tracheid, is a long narrow cell with tapering ends. It has pits in its walls, which let water pass sideways into the next cell.
The second kind, the vessel element, is wider and shorter. Vessel elements stack end to end and the walls between them dissolve. The result is a continuous tube, and the gap in the wall is called a perforation.
Tracheids occur in almost every vascular plant. Vessels are found almost exclusively in the flowering plants. A few gymnosperms have them too, among them Ephedra, Gnetum and Welwitschia.
Wide tubes carry more water than narrow ones. That difference in plumbing is one reason flowering plants can support large leaves and fast growth in warm, wet places.
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| A labelled cross-section of a flax stem. The xylem and phloem sit among the pith, cortex and outer tissues. Image: SuperManu, public domain, via Wikimedia Commons |
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| A long section through a squash stem at 400 times magnification. The walls of the xylem vessels carry rings, spirals and blocks of thickening. Image: Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons |
Pollinators, Fossils and the Family Tree
A plant that cannot move has to find another way to send its pollen to a mate. Most flowering plants hire an animal to do it.
The figure is remarkably high. A global survey published in National Science Review reported that 90 percent of flowering plant species depend on animals for pollination. Among genera the share was 88 percent, and among families it was 64 to 68 percent.
The visitors include bees, flies, butterflies, moths, beetles, birds and bats. A smaller number of species skip animals entirely and use wind or water instead.
The Oldest Evidence
Flowering plants appear in the fossil record about 125 million years ago, during the Early Cretaceous. They spread quickly after that.
A study of fossil pollen from Minnesota pushed the partnership with insects back to 96 million years. The pollen came in clumps of five to a hundred grains, which is how insect-carried pollen travels today. Pollen carried by wind or water is usually a single grain, and smaller.
Clumping is not just a shape. Grains that stick together have more surface texture, so they cling to the animal that is carrying them.
Where Amborella Sits
Not every flowering plant fits the mold of an apple tree or a rose bush. One species sits alone at the base of the family tree.
Amborella trichopoda is a shrub that grows in the forest understory of New Caledonia, an island in the Pacific. It is the only living species in its genus and in its order.
Plant systematists treat it as the probable sister to all the other living flowering plants. That makes it the closest thing we have to a witness of what early flowers were like.
Its flowers are small and its parts are separate rather than fused. It is also dioecious, which means male and female flowers grow on different plants. The photograph below shows male flowers.
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| Male flowers of Amborella trichopoda, the single species of its order and the probable sister to all other living flowering plants. Image: Scott Zona, CC BY 2.0, via Wikimedia Commons |
The next branches down the tree are the water lilies and their relatives in Nymphaeales, followed by Austrobaileyales. Together with Amborella these make up the ANA grade.
Everything further out is a gymnosperm. Those plants reproduce by a single fertilization, their seeds are left uncovered, and most of them rely on tracheids alone to move water. They are a successful group, but they are a much smaller one.
That is the whole picture in one sentence. A case around the seed, a second fertilization, and a partnership with animals are enough to explain why the flowering plants took over.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Angiosperm | A flowering plant. Its seeds form inside an ovary that ripens into a fruit. |
| Gymnosperm | A seed plant whose seeds stay exposed, usually on the scales of a cone. Pines and cycads are gymnosperms. |
| Flower | The reproductive structure of a flowering plant, holding the parts that make pollen and the parts that hold the ovules. |
| Ovule | The structure inside an ovary that becomes a seed after fertilization. |
| Ovary | The part of the flower that encloses the ovules. After fertilization it ripens into the fruit. |
| Stamen | The part of the flower that makes pollen, with the anther at its tip. |
| Carpel | The female part of the flower: stigma, style and ovary together. |
| Pollination | The transfer of pollen from an anther to a stigma. |
| Double fertilization | The pair of fusion events in a flowering plant: one sperm makes the embryo, the other makes the endosperm. |
| Polar nuclei | Two nuclei near the center of the ovule. Together they fuse with a sperm cell. |
| Triploid | Having three sets of chromosomes. The endosperm nucleus is normally triploid. |
| Endosperm | The food-storing tissue inside a seed, formed from the triploid nucleus. |
| Cotyledon | A seed leaf. In beans, lettuce and peanuts the stored food sits in the cotyledons instead of in endosperm. |
| Xylem | The tissue that carries water and dissolved minerals upward from the roots. |
| Phloem | The tissue that carries sugars from the leaves to the parts that need them. |
| Vessel element | A short wide water-conducting cell. Stacked end to end, vessel elements form tubes. |
| Tracheid | A long narrow water-conducting cell with pits in its walls but no perforations. Found in almost all vascular plants. |
| Accessory fruit | A fruit in which part of the flesh comes from tissue other than the ovary, as in a strawberry. |
Key terms used in this article, defined for quick reference.
Key Takeaways
- Flowering plants, or angiosperms, are the largest group of plants, with more than 300,000 known species.
- Seeds form inside an ovary, and the ripened ovary is the fruit, so the seed is wrapped from the start.
- Double fertilization happens only in flowering plants: one sperm makes the embryo and the other makes the endosperm.
- The endosperm nucleus is normally triploid, and the tissue it builds stores starch, sugars, fats and proteins.
- Water climbs through tracheids and vessels in the xylem, while sugars travel in the phloem from source to sink.
- About 90 percent of flowering plant species rely on animals to move their pollen.
- Amborella trichopoda, a lone species from New Caledonia, is the probable sister to all other living flowering plants.
- Cone-bearing plants share the same basic plan, but they have about 1,200 species and no fruit.
Frequently Asked Questions
What makes a plant an angiosperm?
An angiosperm makes its seeds inside an ovary, and that ovary ripens into a fruit. A gymnosperm leaves its seeds exposed, usually on the scales of a cone.
Why is double fertilization unique to flowering plants?
No other group brings two sperm into the ovule so that one makes the embryo and the other makes its food supply. Gymnosperms use a single fertilization, and the food around their embryo comes from the female gametophyte instead.
Is a tomato a fruit or a vegetable?
By the botanical definition it is a fruit, because it is a ripened ovary with its seeds inside. Cucumbers, green beans, okra and the grains are fruits by the same test. The word vegetable has no strict botanical meaning.
Do all flowering plants need animals to pollinate them?
No. About 90 percent of species rely on animals, but the rest use wind or water. Grasses, oaks and maples are wind-pollinated.
What is the difference between a vessel and a tracheid?
Both are dead, hollow cells that carry water. A tracheid is long and narrow, and water passes sideways through pits in its walls. Vessel elements are shorter and wider, and they stack end to end with holes in the walls between them.
Key references
• Britannica: Double fertilization
• Britannica: Gymnosperm
• OpenStax Biology 2e: Pollination and Fertilization
• OpenStax Biology 2e: Transport of Water and Solutes in Plants
• The New York Botanical Garden: What Is a Fruit? (Lawrence M. Kelly)
• Botany One: Amborella trichopoda helps reveal features of the first flowering plants (Charlie Scutt)
• The Global Plant Council: 90% of flowering plant species found to rely on animal pollinators
• Florida Museum: 96-million-year-old fossil pollen sheds light on early pollinators
• Qian, Zhang & Zhao: How many known vascular plant species are there in the world? (Biodiversity Science, 2022)
• University of British Columbia BIOL/APBI 210: Xylem Tissue
• Cane & Dunne: Generalist bees pollinate red-flowered Penstemon eatonii (US Forest Service Treesearch)
Sources: the Britannica articles on double fertilization and on gymnosperms, two OpenStax Biology 2e chapters (pollination and fertilization; transport of water and solutes), the New York Botanical Garden on what a fruit is, a Botany One article by Charlie Scutt of the CNRS on Amborella, the Global Plant Council report of the National Science Review survey of pollination modes, a Florida Museum study of 96-million-year-old fossil pollen, the 2022 Biodiversity Science tally of known vascular plant species, the University of British Columbia BIOL 210 notes on xylem tissue, and a US Forest Service study of bee pollination in Penstemon eatonii. Each was opened and read on 16 October 2026. The mechanism of double fertilization, the ploidy of the endosperm and the cases where it does not persist, the definition of a fruit and the accessory-fruit example, the difference between vessels and tracheids, the pollinator share of 90 percent, the fossil dating of flowering plants and of clumped pollen, the position of Amborella and the naked-seed contrast with gymnosperms all follow those sources. The species comparison in the table is drawn from the order-by-order counts in the Biodiversity Science study and is stated as an approximation, because the totals differ between databases. Figures are from Wikimedia Commons under the license named in each caption. This article is written for education.
