If you don't understand terms like sepal and stamen, you may want to consult our Standard Blossom Page as you read through the following.
In canna flowers the bright "petals" are actually stamens modified to look like petals. Such modified stamens are often called staminodia. Atop the green, roundish ovary arise three small, usually green sepals, hidden by one of the petals in the image at the left, but visible in the next picture. There are also three petals, which can be green or colored.
The stamens modified to look like petals usually number five, or sometimes fewer. They are more or less united at their bases into a tube arising from atop the ovary. Of the five stamens, three are much enlarged, looking like big, splashy petals, like the one labeled in the above picture as a modified stamen.
Of the two remaining modified stamens, one is narrower than the three big ones and is curled back so that it forms the flower's "lip." The last modified stamen is still narrower and more or less coiled, and bearing one fertile anther (the pollen-producing part of the stamen) on its side, thus proving that it really is a modified stamen. At the right I have removed the four largest modified stamens leaving only the small one on which you can see the anther. The sepals are also more clearly visible in this photo.
Notice that a flat, bright yellow, slender appendage rising straight up is attached to the right side of the anther-bearing stamen at the right. That appendage is the flower's style, at the top of which you can barely see the stigma. In some canna flowers the style instead of being flat is club-shaped. Whatever the style's shape, pollen lands on the stigma, sending tubes carrying the male sex germ down through the style, to the ovary at the bottom (it's a classic inferior ovary). At the left is a close-up so you can see the anther and stigma better.
In the cross-section through a canna ovary at the right you can see how neatly the ovules are arranged. In nature, ovules develop into seeds. However, the genes of cannas have been so screwed up by humans that typically the ovules don't develop into viable seeds.