Plant of the Week
Arrowhead (Sagittaria cuneata)
By Walter Fertig
After crossing half a continent and reaching the Pacific Ocean, the Lewis and Clark expedition spent the cold, dreary winter of 1805-06 cooped up at Fort Clatsop, near modern Astoria, Oregon. Among the foods that kept the explorers nourished through the winter was the starch-rich tuber of the Indian potato or Wapato (Sagittaria cuneata). Also known as Arrowhead for the characteristic shape of its leaves, Indian potato was one of the primary vegetable foods of the indigenous cultures of the lower Columbia River. The tubers have a potato-like texture but more of the flavor of water chestnuts when boiled or roasted to remove their slightly bitter taste when raw. Arrowhead tubers grow in muddy soil underwater and were harvested by Indians using sticks or with their bare feet (once freed, the tubers float to the surface to be gathered).
Sagittaria cuneata is one of the most common and widespread of the 24 North American species in the genus, ranging from eastern Alaska to Newfoundland and south to Arizona, Texas, Illinois, and Pennsylvania. Despite the superficial resemblance of its corms to the Common or Irish potato (Solanum tuberosum), the two species are wholly unrelated. Arrowhead is a monocot in the Water plantain family (Alismataceae) and is characterized by flower parts in multiples of three and parallel-veined leaves. Common potatoes by contrast are dicots, with five-parted flowers and pinnately veined leaves and are in the same family as nightshade and tomato (Solanaceae).
Arrowheads are wetland plants that can grow on muddy soils adjacent to receding shorelines of small ponds or creeks, or submerged in shallow, perennial water bodies. When growing underwater, the leaves often fail to develop the typical arrowhead-shaped or hastate leaf blades and consist instead of a grass-like leafstalk.
The flowers of Arrowhead occur in whorls of three on an elongated inflorescence borne above the water surface. Each flower has three white petals and either a cluster of yellow pollen-bearing stamens or numerous flattened pistils that will ripen into achenes. Typically, the staminate flowers are found at the top of the inflorescence and pistillate flowers occur at the base. The existence of flowers with separate sexes on the same individual plant is called “monoecy” and is an adaptation to promote cross-fertilization. Some populations of Arrowhead, however, consist of plants that are either staminate (male) or pistillate (female), a condition termed “dioecy”.
The presence of monoecious and dioecious populations in the same species is very uncommon in the botanical world, and thus of great interest to scientists researching the evolution of plant breeding systems. Researchers in Ontario have found that monoecious populations tend to occur in ephemeral wetlands like shorelines, creeks, and ditches, while dioecious populations are more prevalent in permanent water bodies, such as ponds, lakes, and rivers. Apparently, both types of plants are fully capable of interbreeding and the two forms are not diverging into separate species.