Tomato plants (Solanum lycopersicum) are annual flowering, fruiting plants that do not separate into distinct male and female individuals. Each yellow flower is a “perfect flower,” meaning it contains both the male and female reproductive structures required for fertilization and subsequent fruit development. This self-contained reproductive system allows a single plant to produce fruit without needing a partner plant nearby.
The Reproductive Structure of Tomato Flowers
The anatomy of a tomato flower is designed for self-pollination, as both sets of reproductive organs are enclosed within the same bloom. The female structure, the pistil, is centrally located and consists of the stigma, style, and ovary. The stigma catches the pollen, which then travels down the style to the ovary containing the ovules. These ovules become seeds upon fertilization.
Surrounding the pistil are the male structures, the stamens, composed of the filament and the anther. The anther produces and releases the pollen grains, which carry the male genetic material. In the tomato flower, the five or more stamens are often fused together, forming a hollow, cone-like cylinder that closely encircles the pistil. This arrangement ensures that when pollen is released, it falls directly onto the stigma of the same flower, enabling successful self-fertilization.
Understanding Plant Sexuality in Horticulture
The concept of separate sexes in plants arises from botanical classification. Tomato plants, with their perfect flowers, are distinct from species that exhibit separate male and female parts. For example, some species are dioecious, meaning male flowers (with only stamens) and female flowers (with only pistils) are found on entirely different individual plants, such as kiwi or holly.
Other plants are monoecious, where separate male and female flowers are present on the same individual plant, such as corn or squash. In these cases, pollination must occur between different flowers. Because the tomato flower contains both male and female organs within a single structure, it is considered hermaphroditic and self-fertile.
Pollination and Successful Fruit Set
Although the tomato flower is structurally self-fertile, the pollen is not always shed automatically. The pollen grains are relatively heavy and sticky, requiring physical agitation to successfully transfer from the anthers onto the stigma. This movement is often provided by environmental factors, such as wind or the vibrations caused by insects like bumblebees engaging in “buzz pollination.”
When growing tomatoes in protected environments like greenhouses or indoors, the lack of natural vibration can lead to poor fruit set or “Blossom Drop.” Without the necessary movement, the pollen remains trapped within the anther cone and cannot reach the stigma, causing the flower to wither and fall off. This failure results from the mechanical transfer failing, not a lack of reproductive parts.
Manual Pollination and Environmental Factors
Gardeners can manually assist the process by gently shaking the flower clusters or the entire plant once a day to mimic the effect of wind or insects. Using a battery-powered vibrating device, such as an electric toothbrush, to lightly touch the flower stem can also provide the precise vibration needed to dislodge the pollen. Temperature and humidity also play a significant role. Conditions that are too hot (above 85°F to 90°F) or too humid can cause the pollen to become sterile or too sticky, preventing successful transfer and fertilization.