Diagnostic Characteristics
Flowers yellow, buds erect, petals 2-4 cm, anthers 8-12 mm, ovary terete or nearly so; fruit terete, obscurely and roundly 4 angled or sulcate; stamens equal; seeds horizontal, very angular; sepal appendages separate from the base, subterminal.
Habitat
Oenothera argillicola is an endemic of the Appalachian shale barrens, occurring in the states of Virginia, West Virginia, Maryland and southern Pennsylvania (Stinson 1953).
The term "shale barren" is a general reference to certain mid-Appalachian slopes that possess the following features: 1) southern exposures, 2) slopes of 20-70 degrees and 3) a covering of lithologically hard and weather-resistant shale or siltstone fragments (Dix 1990). These barrens support a sparse, scrubby growth of Quercus ilicifolia, Q. prinus, Q. rubra, Pinus virginiana, Juniperus virginiana, Prunus alleghaniensis,, Rhus aromatica, Celtis tenuifolia, Kalmia latifolia, Bouteloua curtipendula, Andropogon scoparius, Phlox subulata var. brittonii, Silene caroliniana ssp. pensylvanica, Sedum telephoides, Antennaria spp., Aster spp., and species of Solidago (Dix 1990). Considerable variations in associated flora may occur locally.
Although adequate moisture is available for most plants within the substrata of the shale layers, adverse surface conditions act to restrict germination and establishment success of plants (Platt 1951). It is primarily the effect of high surface temperatures that limits reproductive success in these habitats. Surface soil temperatures are often well above the physiological tolerance of most plant species, reaching maximum temperatures of 63 degrees Celsius (Dix 1990). Such temperatures are high enough to cause direct damage to seedlings. For additional detailed information pertaining to the shale-barren community, see Dix (1990).
Artz (1948) stated that O. argillicola is able to exist in habitats other than shale barrens, but always requires areas with little or no competition. This inability to withstand competition may be one of the chief reasons why O. argillicola is largely restricted to shale barrens (Artz 1948). Typically, shale barren endemics are apparently obligates of high light intensity, a soil adequate for extensive root development, and a low level of competition (Platt 1951). The sparsity of plant life in the shale barrens has been explained as the result of a restriction in seedling establishment imposed by low soil moisture and high soil temperatures (Stinson 1953). Oenothera argillicola is apparently able to withstand and thrive in such conditions.
Ecology
Oenothera argillicola flowers in late summer, typically July through October (Gleason and Cronquist 1963).
Oenothera argillicola is unusual among eastern euoenotherids in its tendency to display only minor cytogenic aberrations typical of the subgenus, as well as its restriction to a specific habitat (Stinson 1954).
According to Stinson (1954), O. argillicola consists of: "ecologically restricted, essentially lethal-free, open-pollinated individuals with all-pairing chromosomes or small circles and several pairs. In natural populations, open-pollination should lead to considerable crossing among individuals occupying the same area. At the same time, as a result of the absence of lethals, one would expect that whenever self-pollination in the circle-bearing forms did take place segregation for chromosomes configuration would occur."
Stinson (1954) further stated that based on segmental end arrangements of chromosomes, O. argillicola is most closely related to O. hookeri of the southwestern United States. The two taxa share a number of similar features, including the presence of mostly-paired chromosomes, alethal complexes, open-pollination and ancestral end arrangements.
Morphologically, however, O. argillicola most closely resembles O. parviflora. Both show similar narrow, thick glabrous leaves, subterminal sepal tips and bent stem tips (Stinson 1954). In addition, O. parviflora is able to occupy the same shale barren habitat as O. argillicola. Based on primitive characters present in O. argillicola, Stinson (1954) surmised that it is a relict of a once widely-distributed taxon that invaded the East from its ancestral home in the Southwest. This belief was also shared by Keener (1970).
It has been suggested that O. argillicola likely played a role in the origin of O. parviflora, through crossing with ancient O. strigosa or O. biennis populations in the East (Stinson 1954).
Reproduction
Easily cultivated and self seeding, this species shows exceptional growth in sandy or heavy soils. Germination percentage high.