JARS v62n2 - The Word: Truss

The Word: Truss
Bruce Palmer
Cutten, California

The word is truss . It's from the French trousser , to pack into a bundle. We're discussing a flowering structure here, not something that supports a bridge or a hernia. What could be a more appropriate word at this season as our rhododendrons are reaching toward their peak bloom? Not all of the members of the genus ( genus is Latin for kind or sort; in biology the word denotes a classificatory rank between family and species) bear their flowers in trusses. In some members of the genus the flower stalk, the rachis (Greek: spine, backbone or ridge) is very short, and the inflorescence has the general appearance of an umbel (Latin: a sunshade), as in onions and some members of the parsley family. The inflorescence may even be reduced to a single flower. A clear majority of the hybrid rhododendrons and many of the species in our gardens, though, are in the subgenus Hymenanthes (Greek: hymen , a membrane, and anthos , a flower), a subgenus whose members bear their flowers in clusters we call trusses. In Hymenanthes the trusses are at the ends of branches. What is a truss? It is a horticultural or florists' term, not a technical botanical one. The RHS Rhododendron Handbook defines it as the flower cluster. For Rhododendron subgenera that have multiple flowers the handbook uses the term inflorescence, a term more widely employed among botanists. Technically, the inflorescences in the genus Rhododendron are racemes (Latin: racemus , a bunch of grapes). The raceme is a single rachis with flowers on the sides, typically with flower stems the same length, and usually without a flower at the end. In some members of the genus, the rachis is very short, and the inflorescence has the general appearance of an umbel or may even be reduced to a single flower.

Rhododendron truss drawing
Drawing by Marilee Mannen

The rachis in rhododendrons is not obvious as long as the truss is in bloom. Look at what's left when you deadhead, though, and it is clear that it resembles the leftover grape stem after you have eaten the grapes. Despite the derivation of the term raceme, grapes do not bear their flowers and fruits in racemes. Grape inflorescences are cymes (Latin: cyma , a cabbage sprout). Cymes typically have multiple stalks, not just one, and they bloom entirely differently. The difference is major. Inflorescence types in flowering plants that bear multiple flowers are separated primarily by whether they are racemose or cymose. In racemose clusters, such as trusses, the flowering begins at the bottom or outside of the inflorescence and progresses toward the top or inside. Cymose inflorescences begin blooming at the top or center and progress outward or downward. Until the last couple decades the difference was considered a major evolutionary distinction, but that isn't so widely agreed upon lately. Nevertheless, trusses aren't at all like grapes. Trusses don't bear edible fruit, but for rhodo enthusiasts they don't need to. Out in our gardens about now, the rhododendrons are demonstrating why we keep so many of them around and continue to buy or propagate them. Let's get out there and enjoy them at their best from now until late spring or early summer.

Bruce Palmer is a member of the Eureka Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii System for twenty-five years.