Flowering plants produce seeds through the process of pollination, the release of pollen (containing sperm cells) from stamens, and the transfer of pollen to the receptive surface (stigma) of the pistil (containing egg cells).
Goldenrod flowers (see my earlier posts) are clustered in flower heads. The flowers on the edge (ray flowers) have large petals and are solely female (no stamens). When a flower head opens, the first flowers to bloom are the ray flowers, which (presumably) provide nectar as a reward for any pollinating insect, and which can receive pollen.
The disk flowers in the middle of the head are both male and female. When the head first opens, the disk flowers are closed (see their blunt, closed ends in the photo above).
But later, the disk flowers open and the stamens thrust the anthers upward, exposing the pollen above the surface of the flower head (see photo below).
Many insects visit goldenrod flowers. I posted a one-minute video (Twitter and Facebook) of pollinator frenzy on a couple of goldenrod inflorescences. The video is full of wasps and bees, and goldenrods also attract beetles, ants, flies, butterflies, moths, and more. These insects get nectar and/or pollen as a reward (and some probably nibble on the tissues of the flowers). For some, the reward is immediate and short-term, fueling their activity during that day. For others, the reward is long-term, as they take the pollen and/or nectar back to their nest to share (for social insects) or to provision the nest for their young when they hatch.
Pollen is a durable, nutrient-rich food. Nectar is almost entirely sugar and water, an energy drink. A few species can evaporate the water to produce honey, a sugar solution so highly concentrated that it won’t spoil. I posted about honey earlier.
As the insects cling to and walk on the flowers, they are almost certain to pick up some sticky pollen on their bodies. They might ignore the pollen, or try to clean it off, or eat some of it, or gather some of it to take with them, but no matter how carefully they groom themselves, some pollen grains are likely to be scattered on their bodies. When they move on to other flowers, some of those grains will rub off on stigmas, the topmost part of the female portion of a flower. If the pollen from plant A rubs off on a stigma of plant A, that will be a dead end. Goldenrods are self-incompatible (they can’t fertilize their own flowers).
But pollinators on goldenrods are greedy, abundant, and highly mobile. They will bump into each other, knock each other off of the flowers, and cause each other to fly to other flowers, some of which will be on another plant. Or they will drink the flowers dry and fly a bit to find more flowers, sometimes on another plant.
If the pollen from plant A ends up on the stigmas of plant B, then fertilization can get going.
The process of fertilization in flowering plants has several steps, and some of them seem at least somewhat improbable. But all flowering plants do it, and there are thousands of species of flowering plants. As complex as the process seems, they make it work.
When a pollen grain contacts the stigma of a flower, the grain “germinates.” A tube, called the pollen tube, grows from the pollen grain, and two sperm cells, both haploid, venture down the tube.
The stigma is the topmost portion of the pistil, the female portion of a flower, and the tube penetrates the stigma, the style below the stigma, and enters the ovary below the stigma. Inside the ovaries there are ovules, each surrounded by layers of cells, and each containing an embryo sac, a small structure with eight cells or nuclei, all of which are haploid.
The pollen tube finds its way into the ovary and then into an ovule and embryo sac. One of the sperm cells will fuse with one of the female haploid cells, the egg cell, and that fusion produces a diploid zygote, the first cell of a new plant. The zygote begins to divide and produces an embryo, which will stop growing and become dormant within the ovule. The ovule will eventually become a seed.
The other sperm cell inside the embryo sac fuses with two of the female haploid nuclei to produce a triploid cell (I warned you that the process was weird), and this triploid cell divides by mitosis multiple times to produce endosperm within the seed. Some plants have almost no endosperm (orchids), and others have a lot (coconuts). Goldenrods have enough to serve as a nutrition source for the seed when it germinates. Goldenrod seeds are small and light, easily blown around to places at some distance from the parent plant. But they also have a bit of endosperm to help the new plant to grow, wherever it lands.
Each goldenrod flower has one ovary with one ovule, so each flower produces, at most, one seed. The ovary provides a covering around the seed, which is technically a fruit, a dry fruit called an achene. As the ovary wall dries out, small filaments remain attached to the top of each seed. These filaments (the pappus) provide wind resistance and help the seed disperse away from the parent plant sometime in the next several months. Most of the seeds will fail for one reason or another, but there is always a chance that some will find a place to grow once the next spring rolls around.
This whole sequence of events is sexual reproduction: flowers attract pollinators, pollen transfer leads to fertilization, fertilization leads to seeds and dry fruits, and dry fruits blow around in the wind. Each offspring has half of its genes from one parent and half of its genes from the other parent, and thus is genetically different from both them. Each unique seed then has a chance to colonize new habitats. It’s a risky business, but it has worked for an unfathomably long time.
If you feel lost in the details, take a look at the video on Twitter or Facebook. The frenetic pollinators can’t get enough of the goldenrod flowers. All that messing around leads to pollination, an essential part of producing another generation of goldenrods. And there it is, right by your house, along the road, or in whatever habitat your local goldenrods occupy. It won’t last forever, so take a look while it’s going on. If you miss it, it will all happen again next year. Nature is persistent.