Flowering Plants: The birds and the bees and a little sneeze
By Sarah Sharman, PhD, Science writer
Spring has sprung and so too have my allergies. If you live in an area with a lot of greenspace, you might also be feeling the woes of seasonal allergies—itchy eyes, runny nose, sneezing. And even if you are not allergic, you probably notice a light dusting of yellow powder on your car or porch during pollen season.
Although pollen causes seasonal allergy misery in many humans, its spread helps fertilize trees and flowering plants. Let’s find out how that pesky yellow stuff floating around leads to the production of new plants.
There are two types of pollen-producing plants, angiosperms and gymnosperms. Here we are going to focus on angiosperms, which represent the largest and most diverse plant group within the plant kingdom. Angiosperms are flowering plants that often produce fruits. Not all flowering plants produce what you may consider a traditional flower—many cactuses, grasses and crop plants produce flowers.
Similarly, the fruit of flowering plants does not always fit our grocery store idea of a fruit being a sweet, edible food. Fruits, by definition, are structures that form from the ovary of the plant. Green beans, tomatoes and squash are all fruits that developed from the ovary of a plant that we do not traditionally think of as fruits.
Speaking of ovaries, let’s discuss the parts of a flowering plant. The flower itself is the reproductive structure of the plant that is composed of male and female organs. Flowering plants produce flowers that can be male, female, or both (called a hermaphrodite). Hermaphroditic plants contain both the male and female reproductive organs in one flower, and can often pollinate themselves. Some plant species have separate male and female sexes, where plants will only produce either male or female flowers.
Both male and female flowers have flower petals to attract pollinators and sepals to protect the flowering bud of young plants. The male reproductive organ is called the stamen, which consists of the anther and filament. The anther produces pollen. It sits atop the filament, which is a long stalk that supports the anther and holds it in place so pollinators can access it.
The female reproductive organ is called the pistil, which is made up of the stigma, style, and ovary. The stigma is a sticky structure that captures pollen from the air or from a pollinator. It is held up by the style, which also transports the pollen from the stigma to the ovary. Like in animals, the ovary is the location where fertilization and production of seeds occurs.
The image above shows the structures of a hermaphrodite flower (A) that can pollinate itself (a “selfer”), and both the female and male flowers in a dioecious plant with separate sexes that is forced to outcross (B).
Introducing your friendly neighborhood pollinator
In order for plant reproduction to occur, pollen must get from the anther of a male flower to the stigma of a female flower (or from the anther to the stigma of the same flower in the case of hermaphrodites, but we will expand on that later). Pollen is transported by wind, water, birds, insects, butterflies, bats and other animals that visit the flowers. The animals or insects that transfer pollen from one flower to another are called pollinators.
Pollinators visit the flowers to drink nectar or feed off the pollen. While they are drinking or eating, pollen grains from the flower attach to their bodies. Then pollinators travel from plant to plant transferring the pollen from their bodies onto other flowers. In hermaphroditic plants, pollinators can even knock pollen off the anther and it falls down onto the stigma in the same flower.
Perhaps one of the most well-known pollinators are bees. They carry pollen on their hind legs in a pollen basket. A honey bee can fill its leg baskets with about 15 mg of pollen in a single foraging trip.
Steps of plant reproduction
Flowering plants reproduce in two ways—through self-pollination or cross-fertilization. Self-pollination occurs in plants that are hermaphrodites, while cross-fertilization occurs when pollen is carried from the anther of one flower to the stigma of a different flower.
In both instances, once the pollen lands on the stigma, the journey to fertilization begins. Pollen grains consist of two types of cells, a tube cell and a generative cell. The tube cell helps the pollen grain burrow down a tube in the style to the ovary. Once inside the ovary, the generative cell divides to form two sperm cells.
One of the sperm cells joins with an ovule within the ovary to produce a fertilized seed. The other sperm cell joins with two cells called polar nuclei which turn into an endosperm that serves as a source of nutrients for the developing baby plant.After fertilization, the ovary matures into a fruit that protects the developing seeds. Fruit also serves as a vector for transporting the seeds from the parent plant to another location. Non edible fruits are carried on animal fur or by wind or water. Edible fruit seeds are transported by animals that eat them. The mature seeds, if planted under the right conditions, will eventually grow into mature flowering plants..
Why are scientists studying plant reproduction?
It seems like we know all we need to know about plant reproduction, right? Wrong. Over 90% of the flowering plants are hermaphroditic and can often reproduce with themselves. About 6% of flowering plants, including asparagus, kiwifruit and strawberries, evolved to have separate male and female plants and reproduce by cross-fertilization. The sex of dioecious plants is important to breeders and consumers. For example, brewers only use the female hop cones for producing beers, male asparagus plants live longer in fields, and hermaphroditic papaya fruits taste better than female fruits.
Alex Harkess, PhD, is an Assistant Professor in the Department of Crop, Soil, and Environmental Science at Auburn University and a Faculty Investigator at the HudsonAlpha Institute for Biotechnology. His research tries to pinpoint the genetic changes that make a plant male, female, or hermaphrodite. Determining these genetic changes is valuable to breeders and growers in order to produce a more robust, durable crop.
To hear more about Dr. Harkess’s research in asparagus, duckweed and more, watch this episode of Genomics and Java.