I have been asked to explain in as simple a way as possible, how Cannas produce seed. Let’s start by identifying the parts of the flower that are involved.
In the picture left, the central upright petal is the style, the female part of the flower. At the tip of the style is the stigma. The stigma is slightly sticky, and it is where the pollen must end up if it to fertilise the female ovaries, which are located below the style and out of the picture. The stickiness is caused by a sugar-type solution, which serves to both secure the pollen and also provide it with the energy to grow a pollen tube down to the ovaries.
To the left of the style is another petal called the stamen, and attached to the stamen is the anther, the sack where the pollen grows, becoming fertile at the same time as the flower opens.
The pollen in the picture has already been squeezed out of the anther, the pressures involved with the flower opening have the effect of squeezing the pollen onto the style. The cultivar in the picture is now waiting for a pollinator to arrive, it may be a bee, or a hummingbird, whichever, it is attracted to the flower by the prospect of drinking nectar, a sugary solution, produced in the base area where the petals all join together in a twirl, called the tube. But remember, there is not such a thing as a free meal! In return, the Canna expects to be pollinated.
The pollinator pushes its head down into the folds of the petals and drinks from the nectar, in the process it will attach the pollen you see in the picture to its body or bill, and then transfer it onto the stigma as it backs out again, a self-fertilisation. Alternatively, if it had visited another Canna prior to this one, it will have pollen attached to it from that one, and on its way down to the nectar that might be transferred onto the style, so performing a cross fertilisation.
In wild species, the anther is situated a little higher than the one in the picture and the pollen is automatically squeezed onto the stigma, that is what we mean when we say a species is self-pollinating.
Finally, at the bottom of the picture you can see the only real petal in the picture, although this article has referred to petals elsewhere, they are really staminodia, but more on that another time.