HO-173
YARD AND GARDEN
HORTICULTURE DEPARTMENT COOPERATIVE EXTENSION SERVICE PURDUE UNIVERSITY WEST LAFAYETTE, INDIANA 47907
A GUIDE TO FLOWERING AND WHY PLANTS FAIL TO BLOOM
Juliann Chamberlain, Extension horticulturist, consumer horticulture
Pollination is the initial step in the transformation of flowers into fruits. Pollen is physically transferred from the "male" to the "female" flower structures. Honeybees take most of the credit for transferring pollen for vegetable, tree fruit and berry crops. But the wind also transfers pollen in crops such as nuts and landscape plants. Other natural pollen carriers are moths, butterflies, birds, and water.

Flower Structure
A flower is composed of many parts. It has sepals, which enclose the flower bud; petals, which are frequently attractive and colorful; and stamens, the male flower parts, which hold pollen. A filament and anther make up each stamen. Flowers also have a female part called the pistil. It is composed of a stigma, style, and ovary. Seeds develop inside the ovary. The entire flower rests on the flower’s stem called the pedicel (Figure 1).
Pollination is the transfer of male pollen grains by some means from stamens to pistils. When pollen from the anther sticks to the stigma of the pistil, pollination has occurred (Figure 2). When a grain of pollen sticks to the stigma of the pistil, it moves down the style of the pistil until it reaches an ovule (immature seed) within the ovary. Cells are discharged from the pollen grain inside the ovule where they unite with female sex cells. When this occurs, the process is called fertilization (Figure 3). After fertilization has been completed, the ovule matures into a seed embryo and the ovary enlarges to form a fruit or seed covering. Both pollination and fertilization must occur for a seed to develop.
Some plants develop fleshy pulp around the seeds. We commonly call these fruits. Examples of these are apples, oranges, tomatoes, grapes, etc. Plants which do not develop fleshy pulp around the seeds usually have other types of seed coverings. For example, beans are enclosed within a pod. Corn kernels develop inside a husk and milkweed seeds are found within a dry capsule or follicle.

Flower Types
Some plants bear perfect flowers. These have all parts-petals, sepals, stamens, and pistils in each flower. However, other plants bear imperfect flowers which have one or more parts absent. If stamens are missing, then the pistil must receive pollen from another flower. (Figure 4).
Other plants have evolved so that separate male (bearing stamens) and female (bearing pistils) flowers are on the same plant such as birches, cucumbers, and begonias. These are called monoecious plants. Dioecious plants have male and female flowers but they are borne on separate plants. Holly, willow, and poplar are examples (Figure 5) of dioecious plants. Therefore, for fruits to develop on a holly plant you must have a cultivar bearing female flowers and a nearby source of pollen from any holly cultivar bearing male flowers.
Female flowers may be distinguished from male flowers. Female flowers have an ovary directly behind or within the bloom which usually looks like a tiny, immature fruit (Figure 6).