TRIDENS X OKLAHOMENSIS (T. FLAVUS X T. STRICTUS), AN INTERSPECIFIC STERILE HYBRID IN THE ERAGROSTEAE (GRAMINEAE)

The incidence of Tridens oklahomensis Feath. with T. flavus (L.) Hitchc. and T. strictus (Nutt.) Nash together with its morphological intermediacy between the latter two species suggests a hybrid origin. Field and laboratory studies conducted over several years provide substantial evidence of an interspecific cross. Tridens oklahomensis appears in generally disturbed habitats, such as roadsides and cleared areas, where the adventive (for this region) T. strictus occurs as overlapping populations with the ubiquitous T. flavus . At no time has the supposed hybrid been recorded solely with one or the other of the two species. On a moisture gradient, T. strictus is found mainly in swales and ditches while T. flavus occurs on the drier and more general upland. The hybrid occurs toward the moist end of gradient, near T. strictus . On this basis it is reasoned that the latter is the pistillate parent. The hybrid is sterile and has no vegetative means of spreading. Under these conditions its presence is limited to chance fertilization in those situations where T. strictus has invaded the range of the more widespread T. flavus .