Bouteloua rigidiseta var. chihuahuana (Poaceae), New Variety from the Chihuahuan Desert Region

Abstract

The new variety, known only from Sierra de las Pampas, 450 km west of the distri- butional area of the nominate variety, is distinguished especially by its larger stature, the wholly involute leaf-blades, and the simple, flaccid tip of the inflorescence-branch. In August 1972, Tom Wendt, Fernando Chiang, and I were able to make a botanical survey of the Las Pampas Ranch east of Camargo, Chihuahua, Mexico, owing to the kindness of the operators of the ranch, Mr. and Mrs. Lyndon B. Johnson. This property lies on the eastern flank of the north- trending limestone range called Sierra de las Pampas. On 24 August near the northern gate of the property, at the lower northern end of the sierra, I noticed on the roadside a small population, perhaps 75 plants, of a kind of grass that I have seen nowhere else in the Chihuahuan Desert Region, before or since, but which looked rather strikingly like the Texas Grama, Boute- loua rigidiseta (Steud.) Hitchc., that is extremely common in central Texas. Subsequent examination of the specimens confirmed this strong resemb- lance, but also revealed significant differences such as consistently larger plants, about 4 to 5 dm. tall, as opposed to usually 1 to 3 dm. tall in Texas Grama; and wholly involute leaf-blades, versus mostly flat in Texas Grama. But the most important qualitative difference lies in the tip of the axis of the lateral inflorescence-branch. In more than 100 specimens of Bouteloua rigidiseta that I have examined carefully, the inflorescence-branch is a rigid structure with a central awn and at least one and usually two lateral antrorse awns almost as long as the central one. As Gould (1975, p. 342) observes, it is deeply forked or trifurcate. This forked axis is unlike any branch-axis I have found elsewhere in the genus Bouteloua or related gen- era, and is not described for any other species of Bouteloua in the recent monograph (Gould, 1980). In contrast, in the Las Pampas plant, the tip of the inflorescence-branch is simple, flaccid, and not at all awn-like or forked. The decision as to whether to consider the Las Pampas plant as a new var- iety or a new species is arbitrary. I have chosen to emphasize the numerous and convincing similarities of it to the Texas Grama and describe it here at the varietal level: