In other groups of flowering plants a different number is established, the number five being the most common. For example, in the common wild geranium the flower formula is as follows: sepals 5, petals 5, stamens 5 plus 5, carpels 5. In still other flowers the number four is established.
In many common flowers it will be noticed that no definite number is established, or that it is not completely established. For example, in the common wild rose there are 5 sepals and 5 petals, but an indefinite number of stamens and carpels; while in the water lily there is no definite number established, the sepals being usually 4, and the other parts indefinitely repeated.
In those flowers in which some number is definitely established, it often happens that one set may be reduced in number, and this is usually the carpel set. In the families of highest rank among flowering plants, such as the figworts, mints, and composites (sunflowers, asters, dandelions, etc.) the flower formula is sepals 5, petals 5, stamens 5, and carpels 2.
Another fact shown by the lily flower is that the different sets alternate with each other in position. The three petals do not stand directly in front of the three sepals, but in front of the spaces between the sepals. In the same way the three outer stamens alternate with the petals; the inner stamens alternate with the outer ones; and the three carpels alternate with the inner set of stamens. It is very uncommon to find one set standing directly in front of the next outer set, and this position opposite the other set always needs some special explanation. As a rule, therefore, the flower sets _alternate_ with one another, but in some cases a set may be _opposite_.
The history of a flower does not end with the opening of the blossom. If the stigma has succeeded in receiving some pollen, and the pollen has succeeded in doing its work, the ovules within the ovary become gradually transformed into seeds, and the ovary becomes transformed into the fruit, the outer sets of the flower usually disappearing. In the lily these fruits take the form of dry pods, some of which may be seen in the illustration. Such pods have various ways of opening to discharge their ripened seeds.