The Madiinae have achenes of ray flowers enclosed in involucral bracts. A cycle of receptacular bracts separate the ray flowers from the disk flowers. To be sure, some Madiinae lack ray flowers, a fact that confused taxonomists. (Layia platyglossa)
The summit ridge of the Koolau Mountains (Honolulu in the distance). This ridge may look exposed, but it is often cloud-covered and misty, more like a forest understory than a typical mountain ridge. Dubautia laxa can be found along this ridge.
Dubautia knudsenii is a tree of the Kokee region of the Kauai plateau. It has relatively large and thin elliptical leaves, typical of those of a forest tree. The flower heads are pendant, which may aid them in staying dry in the misty cloud forest conditions.
Wilkesia gymnoxiphium is a rosette plant that can reach 5 m or so. Although some plants grow in the open, as here along the edge of Waimea Canyon (Kauai), some reach grow through open dry but humid koa forest to the forest canopy, and flower only when the leaves are at sunny levels of the forest.
Sherwin Carlquist looking at a silversword (Argyroxiphium sandwicense) on a cinder cone in Haleakala caldera, Maui, in 1958. The silversword grows on the cinder cones inside the caldera and also in some places on the outside of the caldera.
The silverswords in Haleakala flower in July; the massive flowering of the plant terminates its life. The exact age of a given silversword plant may vary, but it probably is about 20 years. Argyroxiphium sandwicense has few competitors in the high, dry, sunny alpine areas of Haleakala, which can experience both heat and below-freezing temperatures.
The heads of the Haleakala silversword, Argyroxiphium sandwicense subsp. macrocephalum, are purplish in color, and much larger than those of the tarweeds. However, they are similar in essential respects to those of the continental genus Madia or other tarweeds.
A portion of a section of the leaf of the Haleakala silversword shows massive gel accumulations, which are stained blue here. These gels would fill spaces between the cells in the central part of the living leaf. The gels serve to store water. This feature is otherwise found only in the mainland genus Madia and a few other tarweeds. This is a curious alternative to succulence: succulent plants store water inside large cells rather than outside of the cells. Seen in sectional view are the hairs of the silversword leaf. They form a dense felty covering on the outside of the leaf, and because of their concave surfaces and ultrastructural relief, they probably serve to reflect UV light, high levels of which occur in alpine areas.
A silversword population called Argyroxiphium kauense grows on the rift zone of Mauna Loa, in dry but often cloudy ohia forest. In this locality, frost rarely occurs. This explains why the rosettes of the Haleakala silversword are very close to the ground, where frost exposure is less; the rosettes of the Kau silversword become elevated on short trunks before the plants reach flowering stage. Argyroxiphium sandwicense on Mauna Loa also includes a bog population (at Waiakea). Argyroxiphium sanwicense subsp. sandwicense is rare on Mauna Kea and may once have occurred on Hualalai. The various populations of the silverswords are genetically close, but distinctive in ecology and appearance: they show rapid evolution and rapid adaptive radiation. How many species or subspecies to name in this group is less important than the evolutionary phenomena that they represent.
Although the Haleakala silversword grows in dry lava, the Puu Kukui silversword, Argyroxiphium caliginis, grows in the perpetually wet bog areas of Puu Kukui, on the western part of Maui, which receive rainfall frequently. The capability of the silversword leaf to withstand hot sunshine and high levels of UV light may be of overriding importance in the adaptation of the Puu Kukui silverword to the bog habitat.
A plant of Argyroxiphium caliginis in flower. Notice that the flowering heads are recurved, perhaps an adaptation to prevent water from accumulating in flowers in the frequently misty conditions of the summit bogs of Puu Kukui.
At the margins of the summit bogs of Puu Kukui, one can find the Puu Kukui greensword, Argyroxiphium grayanum. It forms small shrubs that can flower repeatedly before the plant dies. The heads, like those of the Puu Kukui silversword, are recurved.
Greensword plants from East Maui (Haleakala) are placed in the same species as the greensword from West Maui (Puu Kukui), although they have taller inflorescences and narrower leaves, features that may indicate presence of genes in common with the now-extinct East Maui greensword species Argyroxiphium virescens, which had similar features.
Although there is diversity in glandular trichome types in Madiinae, nearly all of them have trichomes somewhere on the plant like these: a stalk two cells wide, with the head subdivided into more than one series of cells. This trichome type, the structure of the heads, and the occurrence of the pectic accumulations among leaf cells were the three major reasons for believing that Argyroxiphium, Dubautia, and Wilkesia should be placed in the same group as the continental tarweeds, and that three alpine continental genera (now known as Raillardella, Anisocarpus, and Carlquistia) formerly assigned to tribe Senecioneae, are tarweeds.
The leaf of Madia sativa. The leaves of Madia and Blepharizonia are filled with pectic compounds. These gels, like those in the silverswords, store water and were an important clue to the relationship of the continental tarweeds with the Hawaiian Madiinae.
The continental tarweeds show a remarkable diversification with respect to glandular (and non-glandular) trichomes, as shown in my 1961 book, “Comparative Plant Anatomy.” There are types other than those shown.
The book Tarweeds and Silverswords represents a collaborative effort to present a synthesis of what is known about the Madiinae. The picture that emerges shows this group to be an amazing example of adaptive radiation, exceeding in dimensions of that phenomenon what is shown by the Darwin’s Finches. The species and genera of continental tarweeds represent an intricate series of phenomena that, paired with the evolution of the Hawaiian Madiinae, make for an unparalleled story of plant diversification and adaptation.
TARWEEDS AND SILVERSWORDS
The tarweeds (Asteraceae, tribe Madieae—or Heliantheae subtribe Madiinae). I knew them from my undergraduate days when I must have collected some of them on my trips between home in southern California and the U.C. Berkeley campus. I certainly knew their wonderful resinous scent. That scent I recognized when in July, 1953, I hiked down into Haleakala crater, Maui, and smelled the silverswords in full bloom there. That scent suggested to me that the California tarweeds might be related to the Hawaiian silverswords. Actually, the plan of the heads, with the disc flowers inside a circle of bracts and the ray flowers each encircled by a bract, is a very distinctive one, and that’s common to the tarweeds and to the silversword group in Hawaii. While I was in Hawaii in 1953, I suspected that that the Hawaiian genera Dubautia and Wilkesia must be closely related to the silversword, Argyroxiphium. I didn’t have much time as a graduate student to deal with the silverswords and their relatives. But soon after I took the job at Claremont, I continued working on them. I was soon convinced by my anatomical work that three Hawaiian genera (Argyroxiphium, Dubautia, and Wilkesia)were relatives of the California tarweeds (which I collected and studied) and must have arrived in Hawaii via long-distance dispersal. I also included what were then the Californian genera Raillardella and Raillardiopsis in the subtribe Madiinae along with the tarweeds and the Hawaiian silversword complex). Raillardella and Raillardiopsis had been put elsewhere in the family (Senecioneae!), but I was sure on the basis of anatomy that the Madiinae included the West Coast tarweeds, the Hawaiian genera (Argyroxiphium, Dubautia, and Wilkesia), and that Raillardella belonged to the tarweeds. People didn’t believe me. David Keck especially—he told me, “I monographed the silverswords to kick them out of the tarweeds.” Ivan Johnston, during a visit to Rancho Santa Ana Botanic Garden, said I was wasting my time studying tarweed anatomy. Everyone thought that the Carnegie group had discovered everything worth knowing about the tarweeds. So I want to be remembered for having studied tarweed anatomy (and chromosomes), for having discovered many interesting things that showed relationships of the tarweeds [ PDF ] and how the group evolved with respect to ecology, and that I correctly delimited the Madiinae (now tribe Madieae), with exactly the genera (generic names now changed in some cases) that it contains today on the basis of DNA evidence. And also, the DNA evidence proved right my claim that Californian tarweeds reached Hawaii by long-distance dispersal and evolved into the silversword complex there.
In 1953, when I was sitting next to a silversword plant in Haleakala, I noticed that its leaves were thick. I thought that they might be succulent, so I broke one open. To my surprise, out came a gel. Immediately, I realized that the silversword leaf was filled with a gel, and that this gel probably stored water, a valuable capability in this dry alpine environment. The gel turned out to occur in intercellular spaces, not in the cells. An interesting alternative to storing of water in big sac-like cells, which is what succulents do. Several points to the leaf-breaking episode. I was interested in the ecology of plants and how their anatomical features were adapted to the climates of the places where they grew. I thought I might study how such a thick leaf originated—the related genera all had relatively thin leaves. And, most importantly, the gels in the intercellular spaces in the leaves of silverswords also occurred in—the West Coast Tarweeds, most conspicuously in Madia. Intercellular gel accumulations like this hadn’t been reported in other plants, so their occurrence in Argyroxiphium, Madia, and and a few other West Coast Madiinae seemed a decisive indicator of relationships between the Hawaiian genera and the genera of western North America [ PDF ]. I would never have thought to study the leaves of the silversword if I hadn’t seen it in the field. The 1953 Hawaiian trip made me realize what a vital role field work could and should play in comparative anatomical studies of plants.
The Madiinae, both Hawaiian and North American, proved to have numerous anatomical features worthy of study. The vessel element length and diameter in Dubautia paralleled leaf size, which in turn was related to ecology (see Ecological Wood Anatomy). Leaf transectional anatomy and leaf venation in Dubautia told similar stories. The differences among the species of Argyroxiphium, Dubautia, and Wilkesia with respect to leaf anatomy and wood anatomy represented key expressions of adaptive radiation, because leaf size, leaf anatomy, and vessel element quantitative features are the tools a plant uses for survival in a particular habitat [ PDF ].
The California floristic province tarweeds didn’t show the range of adaptive radiation that the Hawaiian species did. Not surprisingly, because Californian climates are mostly Mediterranean in character. Most of the West Coast tarweeds grow in habitats that have a relatively brief wet winter and a hot dry summer, so that the annual habit is advantageous. Some of the tarweeds have shifted to vernal flowering, and don’t have adaptations for resisting summer drought. But most of the lowland tarweeds flower at the end of summer, and do have adaptations for that. Leaves of the basal rosette wither early, whereas leaves of the main stem are narrow, often covered with the resinlike coating that gives the group its name. The resins may limit transpiration, but they probably are also distasteful to herbivores (nobody knows!). Narrowness of leaves, and various forms of leaf condensation, such as inrolling of the leaves, characterize the summer-flowering tarweeds. These diversifications are not adaptive radiation, but they are diverse ways the various genera have evolved to cope with surviving summer heat and drought.
The biseriate glandular trichomes of the tarweeds (and silverswords) are very much alike in most of the genera—another key to the phylogenetic unity of Madiinae and why I saw that the Hawaiian genera were closely related to the West Coast genera [ PDF ]. But some tarweeds have evolved elaborate modifications of these glandular hairs. The modifications in Holocarpha [ PDF ] and Calycadenia [ PDF ] are especially interesting. But I suspect that I didn’t cover the entirety of the diversity one might find if one studied those genera more thoroughly.
I didn’t publish much about the tarweeds between 1960 and 2000. But I didn’t forget about tarweed anatomy. I published papers on wood anatomy of the Hawaiian genera, because I had accumulated material of more numerous species and was looking toward a book that was in the planning stages. One of these papers is on Dubautia, a fascinating example of how wood reflects ecology [ PDF ]. Texts of my papers on wood of Argyroxiphium and Wilkesia are not reproduced here (for those references, see Biography). At the national botanical meetings in Seattle meetings, an editor from UC Press asked me if I had any ideas for a book. I said yes, and my idea was to do a book on tarweeds and silverswords with multiple authors. Bruce Baldwin of U.C. Berkeley and Gerald Carr of U. Hawaii were willing to join this project. I submitted the project to U.C. Press when it was finished, in 2002. The Press demanded numerous changes, some silly (ALL scientific terms—even DNA!—would have to be defined) under the impression that it could be readable by high school students. They estimated that the retail price for hardcover copies would be “at least $75” (does that sound like the price of a book likely to become available to a high school student?). So I asked UC Press to have the project back so that I could submit it to another publisher. They agreed. Obviously this book was not going to be a moneymaker for them; otherwise, they might have stayed interested. After recapturing the project, I had Pacific Press in Santa Barbara do the layout and typography, paying for that myself. Bruce and Gerry helped in constructing a final version of the layout. Pacific Press even produced some copies in the form of an actual paperback book. I submitted these to various publishers. One I sent to Peter Raven, Director of Missouri Botanical Garden. Peter, acting for Missouri Botanical Garden Press, accepted it within a week. They published “Tarweeds and Silverswords” in fine form in 2003, and marketed it as a hardcover book for $29.95. Bruce and Gerry were co-editors on the book. None of us received royalties, and my investment in the layout costs was never returned, but I had the satisfaction of having the book well done and published at a reasonable price.