Shrubs Of The Great Basin: A Natural History

Blackbrush

Blackbrush Coleogyne ramosissima THE BLACKBRUSH or, as it is sometimes called, the burrobrush gets its common name from the older stems, which have a rough, black bark. On younger stems the bark is ashy gray. A characteristic feature of the blackbrush is its pattern of very intricate branches, with the branchlets usually two at a node, opposite each other. Each successive pair of branchlets tend to be at right angles to the stems from which they arise. The smaller branchlets end in spines. This combination ofcharacters makes the blackbrush easy to recognize, even from a distance. Ordinarily, the blackbrush varies from a third of a meter to a meter in height, though in rare cases it may reach a height of 2 meters. The deciduous leaves are small, gray, and narrow, between 5 and 12 millimeters in length, and characteristically clustered at the ends of smaller branchlets. Actually, like the branchlets, they are borne two at each node. They are finely pubescent and have two to four grooves beneath. Under a hand lens the fine hairs on the leafwill be seen to be attached at the middle. The individual flowers-yellow to brownish and about a centimeter wideare produced from early spring until midsummer. The yellow color is due to the sepals, of which there are four. These are fused at the base and subtended by one or two pairs of greenish, three-lobed bracts. There are no true petals. The numerous stamens are attached to a sheath which surrounds the pistil. This is a unique structure, sometimes called a torus, and in this case it is densely white-pubescent inside. The pistil which the torus encloses contains only one seed at maturity. The style, which appears to arise about twothirds of the way up the side of the ovary, is densely pubescent at its base. At maturity, the ovary with its single enclosed seed becomes hard and forms a small, brown nutlet or achene. 157 ROSACEAE Desert bighorn sheep and deer apparently make some use of this plant for browse, while ground squirrels and quail collect the achenes. For cattle and horses, however, it is considered to be poor to useless. Under the auspices of the U.S. National Herbarium at the Smithsonian Institution, Ivar Tidestrom published the Flora of Utah and Nevada in I925. Tidestrom pointed out that the blackbrush within the Great Basin occupied a broad belt between the northern and southern deserts. In the north it is commonly associated with hopsage and may extend into the pinyon,juniper community, while to the south the creosote bush may be a companion. In many instances, blackbrush may form relatively pure stands with only ami, nority of other shrubs present. It is particularly abundant on gravelly slopes and foothills as well as desert mesas below 6,000 feet. Some authorities con' sider that blackbrush marks the upper limit of Mohave Desert vegetation. To the west, the blackbrush extends into the Colorado Desert of southern California, while in Utah it is found in the area of the Green and San Juan rivers in Emery and Grand counties. It just barely gets into southwestern Colorado and is common in northern Arizona. The genus name Coleogyne is derived from two Greek words-kaleos, sheath, and gune, ovary-in reference to the torus which surrounds the pistil. The species name ramosissima means with many branches. There are no other species in the genus. As a member of the rose family, Rosaceae, blackbrush is a rather atypical representative, since relatively few roses have evolved into desert shrubs. It is also unusual in having four sepals rather than five, as do most other members of the family. Even more unique, how, ever, is the torus or sheath surrounding the pistil. The individual genera in the rose family have flowers in which the sepals, petals, and stamens are variously fused to form a disk or a cup. The apple fruit is considered by some analysts to be such a cup, which has fused with the pistil making up the core of the apple. However, this is not the explanation for the torus of the black, brush. The answer to this minor but interesting problem remains for some avid student of our desert botany to formulate. ...