Angiosperms are the second kind of true seed-bearing plants called spermatophytes, and they are also known as “flowering plants.” If you were to imagine a flower, any type of flower you can think of, you are almost certainly envisioning an angiosperm. Even though gymnosperms also technically have flowers, the angiosperms have a much broader diversity of more obvious flowering structures. And although angiosperm plants demonstrate an incredible diversity of flowers, the basic structure of a flower remains the same regardless of species. We went over flower structure in a previous tutorial, but let’s briefly review.
The small green leaf-like structures between the flower and the stem are called sepals. The sepals cover the flower and protect it when it’s still a bud before it blooms, and after a flower opens, the sepals help provide the flower with structure and stability. The petals are usually the most colorful parts of a flower, and also often the most specialized structures. Petals are often brightly colored to attract animals that facilitate the process of pollination, or the movement of pollen between flowers, an evolutionary strategy that is largely responsible for the reproductive success of this plant group.
Instead of relying on wind to bring pollen to the right place by chance, angiosperms became able to manipulate other organisms into carrying out highly targeted delivery. These pollinating animals can include insects, birds, and bats. Many flower petals even have stripes that act as a “runway” to direct pollinating insects to their centers, and some of these runways are only visible in the UV spectrum, which is visible to most insects. Other flower petals are shaped in such a way so as to attract or exclude certain pollinators, like these tube-shaped trumpet flowers that can only be pollinated by moths with very long tongues. But the sepals and petals are just the structures surrounding the important reproductive parts of the flower.
While gymnosperms have separate male and female cones, most angiosperms have both male and female reproductive organs in the same flower structure. In this situation, the male reproductive structures are called stamens, and the female reproductive structures are called pistils. The stamen, or male reproductive structure in a flower, consists of two parts: the anther and the filament. The filament supports the anther, or pollen head. The anther is where pollen is produced and released, and the filament is often long enough that the anther sits above the top of the pistil to allow for self-fertilization when pollen falls down off of the anthers and onto the top of the pistil, although most angiosperms employ mechanisms to favor cross-pollination to enhance genetic variability.
The pistil, which is the female reproductive structure in a flower, also consists of several parts: the ovary, style, and stigma. The stigma is the top of the pistil, which is often very sticky in order to collect pollen deposited upon it by either the wind, pollinating animals, or gravity. The style is a long, tube-like structure that supports the stigma. When the flower is pollinated, the pollen tube grows down through the style. While the pollen tube is growing, the generative cell in the pollen splits into two sperm cells. At the bottom of the style is the ovary, where the female gametophytes grow, which will become the egg cells.
Now unlike fertilization in gymnosperms, both of these sperm cells play an important role in angiosperms, and because of this we refer to the process of fertilization in angiosperms as double fertilization. The way this works is that one sperm cell fertilizes the egg, which generates the diploid zygote that will become the new sporophyte, just as we would expect. But the second sperm cell will fuse with two extra cell nuclei that accompany the egg cell. This triploid structure produces something called the endosperm, which will become part of the resulting seed, providing nutrition for the zygote as it grows into an embryo.
After double fertilization generates the zygote and endosperm, the ovary surrounding the zygote swells into a protective coating called a pericarp, which most of us know of as a fruit. This is a major difference from the naked seeds of gymnosperms. Angiosperm fruits come in a huge variety of shapes and sizes, and the shape is usually specialized for the kind of seed dispersal a plant relies on. For instance, bright red berries will attract the attention of birds, who will then ingest the berries and carry the seeds for miles before depositing them with their fecal material. Other angiosperm pericarps become tough nuts that squirrels will bury in the ground. Some angiosperm fruits even grow with lots of hooks on the outside, like this burdock, which can hitch a ride on the fur of passing animals. The main thing these different fruits have in common is that they protect the seeds inside them, while helping move the seed to a new location.
Flowering plants are the most diverse group of plants in the world, with more than 250,000 species estimated to exist. Angiosperms can grow as trees, shrubs, or herbaceous plants, and many scientists would also consider them to be the most successful group of plants on Earth, given the incredible strategies they employ in their reproduction. And with that, we’ve covered all the different groups of plants, so now let’s move forward and talk more generally about how plants respond to their environment.