Spurge family (Euphorbiaceae)
Description: This native annual plant branches frequently at the base, forming a spreading mat against barren ground that is about 6-18" across, but usually less than 1" tall. However, where other vegetation is present, the stems may ascend to 6" in height. The slender round stems are more or less covered with hairs, and often turn pale red in the presence of bright sunlight. The opposite leaves are up to ¾" long and ¼" across, but they are usually smaller in size. They are oblong in shape, smooth or slightly toothed along the margins, and have short petioles. Each leaf is more or less oblique at the base (i.e., asymmetrical where the petiole joins the leaf blade), and it often has a red blotch near the middle of the upper surface. Both the stems and the major veins of the leaves contain a milky juice.
From the axils of the leaves, there often appears either a single flower or, more commonly, a small leafy cluster of flowers. Like other Chamaesyce spp. (Ground Spurges), Prostrate Spurge is usually monoecious, each plant having separate male and female flowers. Less commonly, a plant may be unisexual, producing either all male flower or all female flowers, but not both. Each male or female flower is less than 1/8" across, and consists of a small floral bract in the shape of a cup, which contains 4 glands with petal-like extensions along its upper edge. These petal-like structures are either white or light red and quite small. Each cup-shaped bract has a slit on one side that is 1/4–1/3 as long as the bract. Each female flower has a 3-valved seed capsule that is hairy and hangs along the side of the bract from a stout pedicel. Each valve of the capsule contains a single 4-angled seed that is minutely pitted and has faint transverse ridges across its surface. The seeds are hydrophilic and can adhere to surfaces when they are wet. Each male flower has 4 stamens with yellow anthers that extend beyond the rim of the cup-shaped bract. Neither female nor male flowers have any petals or sepals. The blooming period occurs from mid-summer through the fall and lasts about 2 months for individual plants. There is no noticeable floral scent. The root system consists of a slender taproot. This plant spreads by reseeding itself.
Cultivation: The preference is full sun, dry conditions, and open barren ground that is sandy, gravelly, or rocky (including cracks in pavement). The Ground Spurges develop slowly, and don't become conspicuous until late summer.
Range & Habitat: Prostrate Spurge is a common plant that occurs in every county of Illinois. Habitats include glades, dry sand prairies, cropland, gravelly areas along railroads and roadsides, lawns and gardens, cracks in sidewalks and pavement, borders along buildings, and sterile waste areas containing sand, gravel, or compacted soil. This plant prefers disturbed areas, and it is quite common in urban areas where there is a decaying infrastructure.
Faunal Associations: The nectar of the flowers attracts small bees, flower flies, and wasps. Some upland gamebirds eat the seeds, including the Mourning Dove and Greater Prairie Chicken. The milky juice of the foliage is somewhat toxic to mammalian herbivores, and so this plant is rarely eaten by them. Because the seeds become sticky when wet, they can cling to the fur of animals and to the bottoms of shoes, and so they are distributed by animals and humans to some extent.
Photographic Location: The plant was sprawling over the edge of a sidewalk at the webmaster's apartment complex in Urbana, Illinois.
Comments: The taxonomy of the Ground Spurges is rather confused. Some authorities assign them to the Euphorbia genus, rather than the Chamaesyce genus. Over the years, reassignment of the names of species has been rather common. Other scientific names for Prostrate Spurge include Euphorbia supina and Chamaesyce supina. Sometimes Chamaesyce nutans (Nodding Spurge) is incorrectly referred to as Chamaesyce maculata. Another problem is that the Ground Spurges are similar in appearance and difficult to identify. The use of a magnifying glass is sometimes required. Prostrate Spurge can be distinguished from many Ground Spurges on the basis of its hairy stems and hairy seed capsules. This is still insufficient to distinguish it from the very similar Chamaesyce humistrata (Spreading Spurge), if indeed they are separate species. According to the Field Guide to Indiana Wildflowers (2000) by Kay Yatskievych, the slit in the cup-like bract of Prostrate Spurge is 1/4–1/3 the length of the bract, while the corresponding slit in Spreading Spurge is 1/2 the length of the bract. On top of each seed capsule, there are 3 tiny styles that are each divided into 2 parts toward the apex. According to the Vascular Flora of Illinois (2002) by Robert Mohlenbrock, the styles of Prostrate Spurge are 0.3–0.5 mm. in length and divided less than halfway to the base, while the styles of Spreading Spurge are about 0.7 mm. in length and divided halfway to the base.