Sunflowers in full bloom are topped with cheery blossoms that follow the sun through the sky during the day and reposition themselves at night to start the process all over again. There are many variations of sunflowers, but all are warmly colored--from yellow through orange and into deep red--as though they have been kissed by the sun. Some sunflowers grow to be only a few feet high, while others are bred to grow over 15 feet and have heads that can be as large as 12 inches in diameter.
Germination is the stage at which water is soaked into the seed shell and starts the growth process that allows it to come out of dormancy. The first appearance of the plant out of its shell is the main root.
Emergence and Expansion
The main root grows and pushes the seed up through the surface of the soil by developing a seed stem. As the seed stem (hypocrotyl) grows, it uncurls and forms two seed leaves (cotyledons). These leaves open as the stem straightens out and the plant stands up straight, topped by the leaves. At this stage, many plants look similar--the seed leaves are simple oval- or teardrop-shaped leaves with no real distinct markings.
During the vegetative growth period, the sunflower leaves begin to grow and form distinctive leaves. The leafstalks begin to appear and define the leaves' inner sections. The leaves take shape and grow in pairs at right angles along the main plant stalk. The plant continues to grow to approximately three-quarters of its mature height.
Growth of the Flower Bud
As the plant begins to arrive at maturity, the flower bud (inflorescence) begins to form at the top of the stalk and appears to be wrapped in leaves. When it is viewed from the top, the bud seems to have a five-pointed star in the center. The bud ball grows larger as the bud is pushed up higher, out of the rosette of leaves that encased it. These leaves continue to grow as the stem below the flower bud grows to about 2 inches in length. The bud is still wrapped in its bud scales. At this point the bud grows, while remaining flat at the end of the stalk, to a size of about 3 inches in diameter. The bud scales begin to open and reveal the petals (ray florets) that are forming underneath.
Flowering, Pollination and Seed Production
As more bud scales pull back away from the flower, the bud continues to grow and spread out in size. The yellow petals emerge from the bud, growing quickly and opening the bud more with each day. As the bud opens and the ray florets form, the bud begins to turn on its stem to grow vertically when it is opened. The anthers of the three outermost rings of the the ray florets begin to let out pollen, and the stigmata begins to develop. The sunflower now looks like the famous flower, with large petals around a large center. As the flower continues to mature, the three outer rings lose their anthers and form stigmata, ready to be pollinated. As the plant continues to grow, three more rings form their stigmata and the process continues until all of the rings have been fully formed. Seeds can be seen producing within the outer rows and working their way into the center, three rows at a time. At this point the seeds are a light-gray color and soft. As the seed production continues into the center of the flower, the outside seeds darken and the ray begins to lose its florets. Most of the petals fall off and the plant is left with a barren seed-producing ball.
The seed ball continues to grow in size as each seed matures. The back of the head is still green but slowly begins to fade to yellow as the seeds mature and seed moisture drops to about 50 percent. By the time the seed moisture drops to 40 percent, the back of the head is pale yellow and the budscales are losing their color. As the seeds continue to mature, their shells get harder and turn darker. As the process continues, the seed moisture declines until it has reached no more than 10 percent, the stalk has turned yellow and then brown and the entire plant has dried up. If the seeds drop to the ground at this point, they will lie dormant until next season, until they begin to germinate and start the process all over again.