Clammyweed

What is clammyweed?

Clammyweed refers to several plants within the genus Polanisia. Specifically, Polanisia trachysperma is a North American herb belonging to the family Cleomaceae, which is closely related to the mustard family.¹²

This plant typically grows to about 60 cm (2 feet) in height and is characterized by hairy, sticky stems and leaves that emit a foul odor when bruised. It is an annual herb that thrives on sunny waste ground and roadsides in warm-temperate regions. Its flowers feature four sepals, four white petals, and numerous protruding purple stamens. Another variety, P. graveolens, is a strong-scented herb with glandular-pubescent foliage that is common in the western United States.²¹

How does the Cleomaceae family differ from the mustard family?

While both the Cleomaceae and Brassicaceae (mustard) families belong to the order Brassicales and share similarities—such as being primarily herbaceous with flat-topped inflorescences and petals narrowed at the base—they exhibit distinct botanical differences.

Key Distinctions

• Ovary Structure: In Brassicaceae, the ovary is typically positioned above other flower parts and features a two-chambered structure with a partition. In contrast, Cleomaceae ovaries are often borne on a stalk, and the ovules are attached to the walls without internal partitions.
• Stamen Arrangement: Mustard family flowers characteristically feature four long and two short stamens. While six is a common number for stamens in Cleomaceae, they do not follow the specific long-short configuration of the Brassicaceae.
• Physical Features: Cleomaceae species frequently possess prickles on their stems, which may appear in the position of stipules, and their inflorescences often feature leafy bracts. Additionally, some Cleomaceae seeds are arillate (possessing a specialized outgrowth).

Why do some plants evolve to have sticky or hairy leaves?

Plants evolve sticky or hairy surfaces—known as trichomes—as vital adaptations to survive environmental stressors and deter threats. These structures serve several key functions:

• Water Conservation: In arid environments, trichomes increase the reflection of solar radiation, which lowers the internal temperature of the leaf and reduces water loss through transpiration. They can also help trap moisture near the leaf surface, further minimizing desiccation.⁵⁶
• Defense Against Herbivores: Hairy or sticky projections act as physical and chemical barriers. Some trichomes are glandular, secreting resins, oils, or mucilages that trap insects or make the plant unpalatable. Others may be stiff or stinging, providing a direct deterrent against animals that might otherwise consume the plant.⁵⁷
• Resource Acquisition: In some specialized cases, such as with epiphytes like Spanish moss, these hairs are adapted to absorb water and essential minerals directly from the atmosphere.⁵
• Environmental Regulation: By reflecting excess light, these structures protect the plant’s internal tissues from damage in high-light conditions.⁵⁶

What role do stamens play in the reproduction of flowering plants?

The stamen serves as the male reproductive organ of a flower. Collectively known as the androecium, stamens are typically composed of a slender stalk called a filament that supports a two-lobed anther at its tip.¹⁰

The primary role of the stamen is the production and dispersal of pollen. The anther contains four saclike structures called microsporangia, where pollen grains are developed. Once mature, these grains are released—often through a rupture in the anther wall—to be transported by wind, water, or animal pollinators to the stigma of a receptive flower.¹⁰¹¹

Additionally, stamens often facilitate pollination by attracting pollinators. Many flowers feature nectaries at the base of the stamens, which provide food rewards for insects and birds. In some species, the arrangement and timing of stamen maturation are specifically adapted to encourage cross-pollination and prevent self-fertilization.¹⁰¹²