Sexual Reproduction in Angiosperms
Angiosperms are seed-producing plants that carry out sexual reproduction and form their seeds inside
flowers. Over half of all plants are angiosperms. Some, such as lilies or roses, produce large showy
flowers while others, such as grasses, form tiny flowers that are easy to overlook. The flower is the
organ of sexual reproduction for an angiosperm. The flower contains both the female parts (called the
pistil) and the male parts (called the stamen). The pistil consists of a sticky tip called a stigma, a stalk
that supports the stigma called a style and a swollen base called an ovary. The ovary contains the
female sex cells (female gametes) called the ovules (eggs).
Before seeds can form, the male sex cells (male gametes) must reach the ovules and fertilize them,
forming the zygote (the first cell in a new plant). This process is called pollination. The male gametes
are found in the pollen, which is located on the tip (or anther) of the stamen. The stamen is supported
by a stalk called the filament. In self-pollination, the male and female gametes come from the same
flower while in cross-pollination the gametes are from different flowers. The advantage of crosspollination is that it creates a greater diversity of seeds than existed before sexual reproduction. Plants
therefore use various ingenious strategies to ensure their pollen is transferred to the flower of another
plant. The petals serve to attract pollinators (such as insects, birds or rodents) while the smaller, green
parts called sepals serve to protect and nourish the flower.
Pollination occurs when pollen lands on the stigma of a flower and sperm form. A structure called a
pollen tube grows down the style and enters an ovule. Sperm then move down the pollen tube and
fertilize one egg in the ovule, forming a zygote or fertilized egg. Other cells in the ovule develop into the
cotyledon, which provides stored food to eventually nourish a developing seed. The zygote also
undergoes mitosis to form the embryo and a hard coat develops around the seed. In angiosperms, the
wall of the ovary develops into a fruit, such as the flesh of an apple or a pea pod.
Figure 1: Male Parts of the
Flower
Figure 2: Female Parts of
the Flower