The angiosperms, or flowering plants, have evolved fascinating ways to reproduce themselves. This video takes you through the life cycle of a familiar flowering plant, an apple tree.
Let’s start with the apple tree itself. In spring, it is awash with pink flowers, which mostly emerge before the leaves do. Those flowers become a huge target for pollinators such as mason bees and other insects, which feverishly are gathering nectar and pollen from flowers to feed their awakening colonies.
Let’s now focus in on the flowers. Here, you can see a fully open flower (ready to receive pollinators), and above it, two flower buds that have not yet opened. Many flowering plants stagger the times at which they produce mature flowers, so that later flowers will receive pollinators that the earlier ones may have missed.
Now, we’ll take a much closer look at the flower itself.
You’ll probably first notice the colorful pink petals that surround the reproductive part of the flower. Petals often attract pollinators, and some have patterns, visible only in the ultraviolet that only insects can see, that actually guide the pollinator toward the center of the flower. Aptly, these are called “nectar guides.” You’ll also see a couple of leaf-like, green sepals beneath the petals. Sepals often surround the developing flower bud, protecting the delicate petals as they first emerge.
Here’s the central part of the reproductive plant, the ovary, which will eventually become the fruit we know as “apple.”
At the top of the ovary, you’ll see a couple of long tubes. These are called the styles, and the very tops of the styles are stigmas. Stigmas are the receptive parts of the styles, which will receive the pollen grains that insects or wind deposit on them.
Okay, here’s where the pollen comes in. The circles highlight the yellow anthers, which are structures that bear pollen (this contains the sperm, which carry the male genes of a plant). When the pollen is mature, the anthers release it. These grains (hugely more magnified in this drawing!) are making their way toward the stigma of the flower. Often pollinators will gather it, either for food, or unwittingly. Some plants, like grasses, release their pollen directly into the air, where it floats around and will passively land on a receptive stigma. Many plant species have evolved ways to ensure that a flower will not self-pollinate, either by staggering the times at which the pollen matures, or by holding male and female structures on separate flowers, or male and female flowers on separate plants. This discourages inbreeding and increases the genetic diversity of a population.
Once the pollen grain has landed on the stigma, it sends out a pollen tube, which grows downward through the style, until it reaches the ovary, where it can fertilize the egg. At that point, a new plant has been born!
When the ovary has been fertilized, an embryo plant forms. The ovary tissues often begin to expand at this time. They are the “maternal” tissues that will provide the flesh and starch of the apple that we consume. The sepals fall off as the apple matures.
That embryo will now fill the seed, as this cross-section of an apple seed shows. The green embryo on the right is nourished by the starch and protein-filled endosperm that surrounds it in the seed.
When the apple is ripe, it may fall to the ground to rot or be eaten by animals, exposing the seed. Animals feeding on the fruit may deposit the undigested seed far away from the maternal tree. In New England, the seed will likely become dormant for a while (to over-winter), waiting to germinate until it receives cues of light and warmth in the following spring.
Now, the seed is sprouting. As the embryo thrusts above the ground, it puts up its first set of leaves, called the cotyledons. In grasses and other plants we term “monocots” or “monocotyledons,”, only one cotyledon is sent up; in other species, two cotyledons are produced…these species are classified as the “dicotyledons.” The cotyledons provide a head-start to the seedling, giving it the energy for early growth.
Now the seedling is growing vigorously, putting out leaves and growing taller. It’s also putting down roots into the soil, gathering up nutrients that will fuel its growth.
After a few years, that seedling will become a sapling – strengthening its stem with wood, and producing a lot of leaves. The apple is a woody plant, of course. Other species that are herbaceous (with no wood) will simply be growing their green tissues, perhaps producing younger plants attached to the mother plant by rhizomes or runners. Annual plants will only pop up for a single year and then die back, placing their fortunes into their seeds: next year’s generation.
We hope you’ve gotten an idea of the life stages a very common plant goes through, and that you’ll gain an appreciation of how a plant faces its own challenges at each life stage. Now go eat an apple!