Threads from the Web of Life: Stories in Natural History

Silversword: Flowers of the Sun

12 Silversword Flowers of the Sun JOHN Muir came to the Sierra Nevada in the 1870s to document the natural heritage of the region and to preach conservation. He was alone with his thoughts and his visions for preservation of the wilderness for weeks at a time while he broke trails through the mountains, cataloging the flora and fauna. Muir was brought up in Victorian Scotland in the strict Calvinist tradition. He worked to understand the natural order he discovered as evidence of God’s Great Plan. But his catalogues presented him with evidence to support a contravening notion—a theory sweeping the ranks of natural historians at the time—propounded by his contemporary Charles Darwin. In the organization of his collections of Sierra plants, Muir recognized relationships connecting one species with the next. It seemed that the species had originated from one another in succession, not that they had been created with a single grand stroke. Particular plants were well adapted to their ideal habitat, but where one habitat gave way to the next, the plants on the boundary were adapting to the changes. These adapted varieties eventually evolved into new species, in a process that appeared to be ongoing. This “adaptive radiation” was heresy, troubling to him, but it seemed that even the overarching plant families themselves might have come into being by such a process of evolution, as if their creation occurred not in an instant but over an inconceivably long continuum of time. The controversy no doubt animated many a conversation between Muir and his friend and mentor Asa Gray, curator of the Harvard Herbarium. Gray preserved Muir’s collections of pressed and dried specimens, cross-checked the identifications, and assigned Latin names to those that proved new to science. 130 Threads from the Web of Life In recognition of Muir’s prodigious efforts, Gray honored him by assigning his name as the species designation for many of his novel discoveries. In one example, Muir’s name was conferred upon a new tarweed in the sunflower family from the southern Sierra: Raillardiopsis muirii. Muir’s notebook records were copied, with description of the plant as a low perennial that occurred on stony granite outcrops, sending its roots deep into the sandy soil to tap the last reservoirs of seasonal snow melt. The two naturalists could not know the significance of that one discovery. They filed the unprepossessing specimen of tarweed in the museum archives and moved on. But by its own example, this particular plant would eventually come to epitomize the paradigm of adaptive radiation, substantiating Darwin’s novel theory. It was a story that unfolded long before any man walked the mountains. But at the time of Muir and Gray, it was a story that had yet to be told. FIVE million years before Muir arrived in the Sierra, one seed head from one tarweed plant departed the region. A dead bough from an overhanging tree fell and crushed a tarweed plant. As the bough broke through the flowering crown, a tarweed stem carrying a seed head snagged in a cleft in the deadwood and snapped off, wedged in place. The falling branch did not stop on impact but bounced down slope, careened over the granite precipice and spun off into space, finally crashing into the river far below. The log righted itself in the water, raising its dripping cargo of broken plant material topmost like a pennant. Then the little vessel set off bobbing through the steep mountain canyons, across the coastal plain on an ambling delta, finally to be delivered into the ocean for a journey longer still. A persistent offshore breeze from the southwestern desert drove the broken branch out to sea and into the current flowing parallel to the coast. After many days, the current bent southward , then southwest, carrying the little vessel far from shore toward the equator at a steady five miles per hour. Where the current reached beyond the influence of the southeast trade winds it slowed, its course coming around to due west. For fifty days, the drowning log flew its wildflower ensign Silversword 131 halfway across the tropical Pacific. A crop of beetle parasites matured within the seed head, and as the males hatched, each flew off upwind, testing the air in search of mates. A few of the outer seeds succumbed to the constant salt-spray imbibition and germinated, quickly dying in the...