Dry flats and benches in oak-pine forest, descending (apparently along streams, but not a true mesophyte) to summer-moist flats on the valley floor, about 40007500 feet, fairly frequent in the Sierra Madre Occidental from northern Durango north through Chihuahua to southwestern New Mexico, southeastern Arizona, and adjoining Sonora, and extending east into the region of Bolsón de Mapimí in southern Coahuila.—Map No. 155.—June to October, or sometimes April—June.
Astragalus vaccarum (of cows, from the type-station) Gray, Pl. Wright. 2: 43. 1853.— "Ojo de Vaca, west of the copper mines, New Mexico, Aug. [Wright] 1002."—Holotypus, collected in 1851, GH! isotypi, K, NY, PH, US!—Tragacantha vaccarum (Gray) O. Kze., Rev. Gen. 949. 1891. Hamosa vaccarum (Gray) Rydb. in Bull. Torr. Club 54: 334. 1927.
Astragalus militaris (of Soldier Canyon) Jones, Rev. Astrag. 278, Pl. 70. 1923.—"Soldier Canon near Colonia Juarez Chihuahua Mexico, Sept. 16, 1903, Jones ... "—Holotypus, POM! isotypi, NY, US!—Hamosa militaris (Jones) Rydb. in Bull. Torr. Club 54: 332. 1927.
In the northern Sierra Madre the Cow Spring milk-vetch is quickly identified by its dense racemes of tiny flowers which are succeeded by small, deflexed pods of the hamosoid type; the remaining small-flowered species of the region, other than A. Goldmani, have a fruit stipitate if trigonously compressed and unilocular if sessile. The closely related A. Goldmani, a comparatively local species of southern Chihuahua and Durango, is usually a more densely and loosely pubescent plant; however the vesture is by no means decisive so that the short and plump, few-ovulate pod is required for definite determination. The species most nearly resembling A. vaccarum is A. Hartwegi, but the shorter flower with less oblique calyx and the shorter, fewer-ovulate pod of usually very slender outline are distinctive. Because of the smallness of the flowers in the Micranthi, differences in length of petals are naturally not large, except in proportion to the whole, and are best observed in carefully dissected flowers. Jones (1923, p. 279), Rydberg (1927, p. 334) and others have questioned whether A. vaccarum was more than a form or variety of A. Hartwegi; as the material of the two species has not been disentangled till now, their verdict is not surprising. The Cow Spring milk-vetch is variable in the color of the flowers and of the hairs in the inflorescence, but I have no information as to whether these vary within a colony of plants or only from one colony to another. The type-collection of A. militaris has flowers of a relatively deep purple shade, and Rydberg maintained Hamosa militaris as distinct from supposedly white-flowered H. vaccarum on the basis of this one character; the plant does not differ perceptibly from the majority of collections from Chihuahua and cannot be considered as more than a very minor variant at best. In the Sierra Madre and in Arizona and New Mexico, A. vaccarum flowers during the summer and fall months, depending on the seasonal rains, and I find no record of vernal forms from this region. In southern Coahuila, especially around the edge of Mapimí Basin, there occurs a relatively coarse plant of the vaccarum type which has been collected in flower as early as April and in fruit by June (cf. Gregg, from Rio Nazas, in 1847, NY; Palmer 235, from San Lorenzo de Laguna, GH). The ecology of these populations has not been recorded; their status cannot be determined without further study. They have been mapped provisionally as representing A. vaccarum.
Although common in Chihuahua, A. vaccarum extends only rarely northward into the United States. I have no record from New Mexico later than those of Wright and Thurber, from Ojo de Vaca and Apache Tejú, now over one hundred years old. Kearney & Peebles (1951, p. 468) cited both A. Hartwegi and A. vaccarum (in a form with purple flowers) as found in Arizona, the former collected by Lemmon in Cochise County, the latter by Thornber near Flagstaff. Thornber’s plant (ARIZ) is typical A. vaccarum with rather bright purple flowers. That of Lemmon (No. 2658, from the Babocomori, G, P. or perhaps the Huachuca Mountains, K) is the commoner phase of A. vaccarum with petals of a pallid purplish hue.