…and another entry by Tamara Bonnemaison today:
Thank you to to Robert Klips (aka aka Orthotrichum@Flickr) for posting a lovely shot of broom moss in the snow (with a little fern moss, Thuidium delicatulum, also making in appearance). Robert took this photo last February in Delaware County, Ohio, USA. For a close-up image of Dicranum scoparium with sporophytes at different stages of maturity, see this image by Phil Pullen.
The widely-distributed Dicranum scoparium is found on moist and sunny rocks, cliff edges and logs across North America, Europe, Asia, Australia and New Zealand. The 2-8cm tall stems form soft turfs. The hair-like leaves have a “swept” appearance, giving this species its common name.
Robin Wall Kimmerer inspired me to write about this species. Kimmerer has written a very entertaining book, Gathering Moss, A Natural and Cultural History of Mosses. Moss reproduction is a fascinating subject in general (well, at least if you are a plant geek like me – but as you are reading this, I suspect you are!) and I found the way that Kimmerer describes Dicranum reproduction particularly colourful. Moss reproduction is often hit and miss, and in the case of Dicranum scoparium, the female of the genus has taken control of the process in order to ensure a higher chance of success. Broom moss spores have no gender, and the ultimate sex depends on where the spore lands in relation to other female members of its species. If the spore lands away from the nearest turf of broom moss, it will develop into a female. Those spores that land on an established female develop into something called a “dwarf male”.
Dwarf males grow to only a few millimeters, and to the visible eye appear as nothing more than a miniscule cluster of leaves, growing as an epiphyte on a full-sized female. There are many advantages to this arrangement. By commanding the least amount of space and resources possible, dwarf males do not compete with the females, who need moisture, sunlight, and nutrition in order to support the growth of their sporophytes. Even more importantly, dwarf males are placed as close to the female gametophyte as possible, greatly reducing the amount of ground that a swimming sperm needs to cover in order to be successful. This reproductive tactic is quite effective, with higher rates of fertilization occurring on the females hosting the largest numbers of dwarf males.
Kimmerer wrote that a hormone emitted by female Dicranum scoparium causes the males of the species to grow as dwarves, but a literature review by Pichonet and Gradstein (2012) (9 years after Kimmerer’s book was published) did not find conclusive evidence that dwarf males were caused by chemical factors. Instead, Pichonet and Gradstein conclude that “genetic factors, environmental factors, and unfavourable nutrient conditions” can all result in dwarf males.