Zosteraceae

Growth Form and Habit

Perennial, submerged marine herbs (seagrasses). Extensive vegetative growth occurs via creeping, branched rhizomes that anchor the plant in sediment or attach to rocks (Phyllospadix). Erect shoots bearing leaves arise from the rhizomes, often forming dense meadows.

Leaves

Leaves are alternate and arranged in two ranks (distichous) on upright stems. They are typically long, linear, and ribbon-like, with a distinct sheathing base that encloses the stem. Leaf tips can be rounded, notched, or pointed, which can be useful for identification. Venation is parallel. Leaves contain air channels (aerenchyma) for buoyancy and gas transport, and lack stomata as gas exchange occurs directly with the water.

Inflorescence

Flowers are borne on a distinctive flattened, spike-like axis called a spadix (though structurally different from the spadix in Araceae). This spadix, bearing rows of flowers, is enclosed within a modified leaf sheath known as a spathe. The entire structure (spadix + spathe) is often referred to as a spatheate spike or rhipidium and is typically borne on a specialized reproductive shoot.

Flowers

Flowers are highly reduced, unisexual, and lack a typical perianth (sepals/petals). Plants can be monoecious (both sexes on the same plant, e.g., Zostera) or dioecious (separate male and female plants, e.g., Phyllospadix).

• Male Flowers: Consist essentially of a single stamen, which has two pollen sacs (thecae). They release distinctive thread-like (filiform) pollen grains, adapted to drift in water currents.
• Female Flowers: Consist of a single carpel forming a superior ovary with one locule containing a single, pendulous ovule. The style is usually divided into two long, thread-like stigmas adapted to capture drifting pollen from the water.
• Retinacula: Small, flap-like or scale-like bracts (retinacula) are often associated with the flowers on the spadix, possibly aiding in protection or pollen release/capture.

Pollination and Fruits

Pollination is specialized for underwater conditions (hydrophily). Thread-like pollen is released into the water column and drifts with currents until it contacts the long, receptive stigmas of female flowers. This process often occurs near the water surface or entirely submerged.

The fruit is small, developing within the spathe. It is typically an achene (a dry, one-seeded fruit where the seed coat is not fused to the fruit wall) or a utricle (similar, but with a thin, bladder-like wall). Some descriptions refer to it as drupe-like due to fleshy outer layers in some species. Seeds lack endosperm; the embryo is surrounded by nutritive tissue from the nucellus.

Chemical Characteristics

Possess physiological adaptations for survival in saline marine environments, including mechanisms for osmoregulation. Contain phenolic compounds, such as caffeic acid derivatives, which may play roles in defense or allelopathy.