Although I had heard of liverworts, one of the more primitive plant forms, I didn’t become truly aware of these little wonders until a chapter field trip that Jane Strong led to the West Fork of the San Gabriel River in early 2009. Much of that area is damp, with ample ferns, mosses, liverworts, lichens, and the like.
Then a year ago, winter 2012-2013, I noticed liverworts closer to home, on the hillsides above our house, on the edge of the San Rafael Hills in western Pasadena. They were smaller than what we’d seen along the West Fork, but they had that strap-like leaf, or thallus, that extends flat along the ground, a characteristic of certain types of liverwort. At first I saw just a few. Then as the weather became rainy, there were more. In fact, after good rain, they were a multitude, sometimes in mats! They gradually faded away, and by early summer were gone completely. These liverworts are always on bare ground, often mixed with mosses, low down on the north-facing slopes of canyons with plenty of shade and little or no direct sunlight.
It looked as if I was seeing just the one species of liverwort, but the big question was how to identify it. The on-line calflora.org and The Jepson Manual don’t include these “lower” plant forms. So where does one start? Jane Strong pointed me to some articles on Cal State University web sites concerning bryophytes, — Wikipedia defines bryophytes as plants “that do not have true vascular tissue”, i.e., they have no veins, which includes mosses, liverworts, and hornworts. Bryophytes have no flowers or seeds, but have reproductive structures that produce spores.
Most helpful among these references were Bryophytes of the Santa Monica Mountains by Sagar and Wilson (those mountains are suitably close in order to be potentially similar), and Images of California Bryophytes (csun.edu web site). I also found the extensive article (109 pages) by Doyle and Stotler in a 2006 special issue of Modroño that constitutes a complete catalog, although it’s highly technical reading. And I discovered that CalPhotos (calphotos.berkeley.edu) has a selection of liverwort photographs. All of these helped in determining that the liverworts on our hillside are Asterella californica, the most common Asterella in California This is the only species of the genus in California that is dioicous, having separate female and male plants.
A small group of Asterella californica seen on three occasions, February 4, 7, and 17 respectively, illustrating before and after rain, which fell on February 6. Note the old spore receptacles on stalks.There being no pollen for insects or wind to disperse, bryophytes depend on the water from raindrops, so that sperm from the male part of a plant can swim to the eggs in the female part of a plant. To describe it in simplistic terms, fertilized eggs then go through a process that ends up producing a stalk supporting a capsule, or receptacle, from which spores are dispersed, typically by wind. There is also a vegetative method of reproduction. It’s all a complicated and risky arrangement!
In the current season, the 2013-2014 winter, I’ve watched the liverworts more closely, especially to see if I can witness their reproductive cycle. Alas, up to mid February, the time of writing, this winter has seen only a couple of moderate-sized wet periods. Each storm immediately resulted in masses of liverwort leaves, on ground that appeared bare on the previous day. But there was no ongoing follow-up rain, and most of that liverwort mass disappeared again within a few weeks.
In several places I found liverworts with old stalks and spore receptacles, which presumably date from the rainy season a year ago. And even the old stalks seemed to freshen up after rain! (See photos at right.)
The phenomenon of plants responding so strikingly to rain and then disappearing again is fascinating, and the adventure to find liverworts on our hillside undergoing the full cycle of producing spore receptacles is ongoing. Maybe that won’t be seen again until we have a more prolonged, continuous wet season in some future year when the rainfall pattern reverts to the so-called normal.