From Plant Press, Vol. 20, No. 1, January 2017.
By Marcelo R. Pace
The Smithsonian Institution hosts the second largest wood collection in the U.S. and one of the largest in the world, with 5,000 microscopic slides and over 42,500 specimens from 3,000 genera. Such a remarkable wood collection exists thanks to a proud lineage of plant anatomists who have worked at Smithsonian’s National Museum of Natural History, including Richard Eyde, Edward Ayensu, and especially William Stern.
Stern was an accomplished wood anatomist who significantly increased the Smithsonian wood collection before becoming a Professor at the University of Maryland. The collection increased from 14,017 samples when Stern started in 1960, to 35,000 samples by 1967 when he left the Smithsonian. His contribution to advances in wood anatomy continued to extend beyond his tenure at the Smithsonian, being an active member of the International Association of Wood Anatomists (IAWA), and mentoring future generations of plant anatomists. Among his students was Regis Miller who would later go on to become a renowned systematic wood anatomist at the Forest Products Laboratory of Madison, Wisconsin, and who later mentored my own doctoral advisor, Veronica Angyalossy (University of São Paulo, Brazil) when she was a post-doctoral fellow in his lab. Angyalossy has since trained most of the wood anatomists in Brazil. I am fortunate to have the opportunity to return to the Smithsonian to work on the wood collection that Stern had built.
The work that I am carrying out in the anatomy lab of the Department of Botany is inspired by the extraordinary stem anatomy of lianas (woody vines). Unlike stems of trees whose wood is rigid and able to sustain enormous organisms such as the Californian redwoods (Sequoia sempervirens, Cupressaceae), the stems of lianas are flexible and pliable. Indeed, unlike trees they tend to get more flexible as they develop (Rowe et al. 2004 J. Plant Growth Regul. 23: 108-128) and this is at the very core of the lianescent habit.
Twisting and twirling is critical to climbing trees in order to reach the forest canopy to obtain light. This increase in flexibility happens as a result of radical modifications in their internal anatomies; stiff cells of wood get intermingled by soft tissues, such as nonlignified parenchyma or phloem. These special anatomical features are generally derived from unusual activities of the vascular cambium, generating extraordinary evolutionary novelties known as cambial variants. These variants have evolved multiple times in the evolution of woody plants, being present in distantly related groups such as fossil pteridosperms (Medullosa, Dunn et al. 2003 Rev. Palaeobot. Palynol. 124: 307-324), gymnosperms (Gnetum spp. have multiple cambia), and more than 100 families of angiosperm magnoliids, rosids and asterids (Angyalossy et al. 2015 in the book Ecology of Lianas).
It has been known since the 19th century that lianas produce stems of extraordinary anatomies, sometimes with the tissues resulting in unusual shapes when examining the cross section of the stems, such as a cross (tribe Bignonieae, Bignoniaceae with over 300 species), a ladder (as in Schnella, Leguminosae), successive cambia with visible concentric rings of xylem and phloem (as in several Caryophyllales, Menispermaceae, Convolvulaceae), or even a foot (as in Serjania laruotteana, Sapindaceae, which has a compound vascular cylinder). Illustrations as early as the pre-Hispanic Aztec civilization in Mexico seem to have made calendars inspired by the anatomy of lianas, such as the first page of the Codex Fejérváry-Mayer. In this particular calendar codex illustration the Aztecs depicted several plants, including a cross section of a Bignoniaceae liana stem and at least two additional plant species—on the left in the illustration a Ceiba and on the right Theobroma cacao, both members of the Malvaceae.
More interestingly, these unusual shapes in the stem cross-sections tend to be taxon-specific and therefore diagnostic in their taxonomical identification. These features have been used by various taxonomists (e.g., Alwyn Gentry in Bignoniaceae research; Pedro Acevedo-Rodríguez in Sapindaceae research) as key characters for the recognition of genera and even species.
More recently, Pedro Acevedo has been leading a team effort with about 15 contributors to expand and incorporate this knowledge of stem anatomy as a tool for identification of Neotropical lianas and other climbing plants, which include about 11,000 native species from over 660 genera, and 126 families. The first stage of this project is the development of a webpage “Lianas and Climbing Plants of the Neotropics,” which will soon be followed by a publication in book form. Our mutual interest in lianas and their unusual anatomical architectures along with the lavish wood collection at the Smithsonian have stimulated our collaboration.
The focus of my investigation is to explore the diversity and evolution of cambial variants in Malpighiaceae, and how these features may have affected diversification rates within the family. We hypothesize that cambial variants are likely adaptive features that have made lianas a more successful life form. In Malpighiaceae we have identified at least seven different types of cambial variants distributed in different lineages. These include interxylary phloem islands (all Dicella species), phloem wedges (some Niedenzuella, Peixotoa, Heteropterys), and fissured wood (Callaeum, Mezia, Flabellaria) among other stunning types. In contrast, the presence of lianoid lineages within Malpighiaceae that lack cambial variants provide an ideal scenario to test the adaptive value of cambial variants within the family. Through the analysis of a large sample of stems in light of a well-supported phylogeny, we aim to unravel whether the striking wood anatomy of lianas is not only marvelous, but also a key innovation in the evolution of woody vines.