Weed sunflowers are common in North America growing along roadsides and in vacant fields. There are sixty-seven species and many more varieties.
The sunflower is not a single flower at all, but a whole bouquet. Examine it closely and you will find that there are perhaps a hundred or more small flowers packed together in a structure known as a head. On the outside of the head is a series of greenish bracts called phyllaries (little leaves). Next to the phyllaries are the ray flowers (sometimes called petals by the uninitiated). Normally ray flowers produce seeds, but the ray of the sunflower is sterile and serves only to attract insects.
The flowers in the center of the head, called disk flowers, are smaller and quite different in shape from the ray flowers. They are "perfect" flowers, meaning that they have both male and female producing parts. To prevent inbreeding, the pollen producing structure (the anther) forms a tube around the style of the pistil. The pollen is shed to the inside of this tube, and as the style grows, it pushes the pollen out the top. The pollen is then ready to be picked up by any insects that happen to visit the flower.
Once the style has pushed the pollen out by its piston-like action, it splits open, exposing the stigmatic surface, which thus far has remained virgin but is now ready to receive pollen. Any insect coming from another flower will brush against the stigma depositing pollen and thus bringing about cross-pollination. Each disk flower that is pollinated will produce one "seed" called an achene.
One of the chief virtues of sunflowers is the ease with which they can be grown. Nearly all species, once successful germination has been accomplished, will do well in a variety of garden soils.