In the Botany Photo of the Day entry on Sidalcea malviflora, Taisha noted that two of the difficulties in checkermallow taxonomy are gynodioecy and morphological variation. Today’s photographs of Henderson’s checkermallow illustrate both of these challenges.
Sidalcea hendersonii is native to British Columbia (where it is blue-listed), Washington, and Oregon (extremely rare). Once common in tidal marshes and meadows where it did occur, it is now estimated that fewer than 100 populations remain. All but one of these are in British Columbia and Washington (as of 2005). Today’s photographs were taken a few years ago in the Squamish Estuary, during one of the Whistler BioBlitzes. I remember the coastal breeze waving dozens of these pink wands above the estuarine sedges.
Each individual plant of the species has either female-only flowers or hermaphroditic (bisexual) flowers, hence the use of the term gynodioecy. The left photograph has the hermaphroditic flower with both female and male parts (best seen if you look at the photo at maximum size). The photo on the right shows a plant with pistillate, or female, flowers. In addition to the difference in the reproductive parts, the pistillate flowers are also smaller and a darker shade of pink. While the dimorphism within this species is now well-known, it is easy to imagine how it could be confusing to taxonomists (particularly if one were working with dried plant specimens of this and other species concurrently in a herbarium).
The crème de la crème of information about Sidalcea hendersonii is this article by Melanie Gisler and Rhoda Love in the Native Plant Society of Oregon‘s journal, Kalmiopsis: Henderson’s checkermallow: The natural, botanical, and conservation history of a rare estuarine species (PDF). No one can ever doubt Melanie’s dedication to understanding and conserving rare plants after seeing that photograph on page 7! The Kalmiopsis article also examines why gynodioecy may be maintained in Sidalcea hendersonii, thanks to research from the University of British Columbia:
In Henderson’s checkermallow the persistence of the unusual breeding system, gynodioecy, could not be explained by higher female fitness or genetic control of male sterility. But in the fall of 1994, in a musty, dimly lit hotel room, researchers from the University of British Columbia made a fascinating discovery. While inspecting S. hendersonii fruits collected earlier that day from Vancouver Island, it became evident that many seeds were damaged by weevils, and most of this damage occurred on seeds from bisexual plants. It followed that although production of seeds was equivalent between female and bisexual plants, actual seed survival was much lower in bisexual plants due to selective predation by weevils (Marshall and Ganders 2001). This discovery may explain why female plants are maintained in Henderson’s checkermallow populations.