Shrubs Of The Great Basin: A Natural History

White Burrobush

White Burrobush Hymenoclea salsola ALONG SANDY WASHES in the southern Great Basin there commonly grows a tall shrub which, on occasion, seems to be decorated with pearls. To some it is known as the desert pearl or pearlbush. These pearls are formed by pea,size clusters of papery, translucent bracts fused at the base into a woody affair which surrounds the female flowers. The narrow, often threadlike leaves on light tan stems are another distinguishing feature. On closer ex, amination, some of the leaves produced during the spring may be seen to consist of several or more divisions. Under a hand lens, the narrow leaves can be seen to possess a very unusual feature: a lengthwise groove on the upper surface covered with hairs. The remainder ofthe leafsurface often lacks any pubescence. Like big sagebrush, white burrobush belongs to the aster family, but it differs significantly from big sagebrush in that the stamens and pistils are borne in separate flowers and these, in tum, are in separate clusters. It shares this particular feature with the ragweeds (Ambrosia) and cockleburs (Xanthium) , to which it is closely related. Like the ragweeds it is wind, pollinated and, unfortunately, it is also a significant hay fever plant. In the white burrobush, the bracts surrounding the female flowers are relatively thin and winglike, fused at the base, and make up the characteristic pearl. In the ragweeds and cockleburs, on the other hand, the bracts, while simi, larly fused at the base, develop instead into spines which are frequently hooked. Kathleen Peterson and Willard Payne, who carried out a compre, hensive study of this genus, concluded that it evolved from an ancestor common to the ragweeds and cockleburs, that the primitive pistillate cluster of bracts probably was somewhat like that of some Ambrosias, and that the present arrangement of the bracts is the result of an evolutionary process White Burrobush ASTERACEAE called neoteny. Wings and spines of the pistillate clusters of all Hymenoclea, Ambrosia, and Xanthium species are upright and appressed during the early developmental stages. The burrobushes appear to have retained this juve, nile feature into adulthood. Additionally, Payne proposes that this whole group of ragweeds, cockleburs, and burrobushes first evolved somewhere in the arid Southwest and from there spread elsewhere. Neoteny is said to occur when a juvenile or embryonic structure is re, tained into the adult state. This developmental process is poorly under, stood, but apparently it involves some mechanism which stops differentia, tion before normal maturity has been reached. One example of this among animals is the Mexican axolotl, a type of salamander which, under certain conditions, matures and breeds while still in the larval form, with the char, acteristic external gills of that stage. It is thought by some experts on the evolution of higher plants that neoteny played a very important role in the evolution of flowers from the conelike reproductive structures of their primi, tive precursors over 70 million years ago. Some think that neoteny serves to simplify structures and, in a real sense, free up genes which are then avail, able for other possible roles in evolution or, to be more precise, in individual development as determined by evolution. To get back to the white burrobush, the bracts on the pistillate flower clusters serve to disseminate the seeds by wind or, undoubtedly, by water as well, since this is such a common shrub along washes. In the case of the cockleburs, the hooked fruits disseminate by becoming entangled in the fur of animals and, not infrequently, in human clothing. The clusters of stami, nate flowers are borne on the same branches as the pistillate flowers and adjacent to them; as the seeds develop, the staminate flowers drop from the plant. The aboveground portions of the white burrobush are relatively short,lived. Flowers are borne on two,year,old branches which, following fruit development, die back to the ground. The roots undoubtedly live many more years, though no one has attempted to find out how many. Frank Vasek, H. B. Johnson, and D. H. Eslinger studied the effects of pipeline construction on creosote scrub vegetation in the Mohave Desert of southern California. They found that the most abundant pioneer shrub was the white burrobush, which in some disturbed areas made up as much as 85 percent of the vegetation cover twelve years after the original vegetation had been removed. However, they estimated that, because of the slow...