It Takes Two to Tango

by Fred Rhoades

Figure 1. Old growth conifer forest with rich diversity of bryophytes.
photo: Fred M. Rhoades

Perhaps there is no other group of plants that typifies the Pacific Northwest west-of-the-Cascades forests more than the bryophytes. It is these simple plants that give our temperate rain forests their distinctive, green, sodden look. These spore-producing plants include the mosses, the liverworts and the hornworts. In the Pacific Northwest as a whole (B.C., Wash., Idaho, Ore. and northern Calif.), there are about 900 species of them.

They occupy many spaces on other plants, on their dead remains and on the earth and rock exposures between plants. Some are even adapted to living in the Arctic and alpine regions, where little other plant life occurs. It is a shame that people do not recognize the differences between the species in these groups, but tend to lump them in their minds as a single thing, “green moss.” This term is used indiscriminately and usually refers to all the liverworts, hornworts, and even many of the unrelated lichens.

As a group of organisms offering economic benefit, bryophytes are taken advantage of by humans less than other plant groups. Peat mosses are extracted and used as a source of heat, smoke and water-holding amendment for garden soil; other bryophytes in our woods are occasionally collected for the horticultural trade. If you are a builder of Japanese-style gardens, you will take care to nurture the bryophytes that show up in your garden for they lend a soft beauty to those artificial landscapes. When examined more closely, with care, and particularly with aid of a hand lens (loupe), bryophytes are exceedingly beautiful as individuals, structured in many different adaptive ways.

When you consider how these plants live, their parts and life histories make perfect sense. Unlike vascular plants (flowering plants, conifers, and the spore-producing ferns and their allies), bryophytes do not have roots, but they are photosynthetic and all have leaves or leaf-like parts. Most have stems. They are distantly related to the first plants to move onto land 500 million years ago. Those ancestors gave rise to all modern plant groups, including the present-day bryophytes.

The bryophytes are the amphibians of the plant world. Because those early plants themselves had evolved from algae living in fresh-water habitats, they maintained a kind of duel existence. On the one hand, a phase of growth (called the gametophyte) requires moisture: It is the green phase commonly recognized. Gametophytes have adaptations to hold and use water efficiently and still use environmental water in part of their reproductive cycle. On the other hand, a phase (the sporophyte) takes advantage of the dry conditions of land: This phase is usually much smaller and may start out green, but soon dries to perform its function of spore production and release. All plants have gametophyte and sporophyte phases, but higher plants have greatly reduced the size of their gametophytes and have emphasized the dry land adaptations first seen in bryophyte sporophytes.

In the remainder of this article, I’ll describe a very common moss species which you can find on almost any woodland walk locally. The overall reproductive life of all bryophytes is similar, differing in structural details of the parts. Further descriptions of liverworts and hornworts will have to wait for a future article.

Oregon beaked moss: Eurhynchium oreganum
This moss species gets its common name from its shape of the spore sacks of the sporophyte (more on this below). The green carpet one commonly sees on logs and ground in shady conifer forests is the gametophyte (Figure 2).

Figure 2. The mushroom Mycena epipterygia on needles and a carpet of the moss, Eurhynchium oreganum. Photo: Fred M. Rhoades

A carpet of Eurhynchium may include one or a few genetically different individuals, but it is impossible to know for sure since, over the course of its life, the various bits of one individual may have gotten disconnected. As part of a growth strategy to stay on top of ever-accumulating plant litter, this moss continually grows up and over and may break up into many separate clones.

Figure 4. Eurhynchium oreganum stems and leaves. A leaf is about 1 mm long. Photo: Fred M. Rhoades

A gametophyte is divided into branched stems which look much like little feathers. Arranged around each branch are simple, heart-shaped leaves which are one cell thick, except for a central midrib that helps support the leaf (See Figure 4). Moss gametophytes have no roots and must be saturated with water to function. Unlike higher plants, their leaves are virtually unprotected from drying out. They get the minerals and water they need, not through roots, but from what falls on them in precipitation or what they can wick up from contact with their surrounding environment.

Figure 3. Young sporophytes on green gametophyte of Eurhynchium oreganum
Photo: Fred M. Rhoades

Like most green plants, mosses build their bodies from compounds made mostly of carbon. These compounds ultimately come from photosynthesis, which combines carbon dioxide from the air with water to make sugars. Mosses of the deep forests, like Eurhynchium oreganum, are particularly efficient at photosynthesis in low-light environments.(figure 3) Because mosses like to be dry some of the time (for their sporophytes to function), they thrive in areas of the world that alternate between wet and dry times. Remarkably, the gametophytes can be completely dry but become hydrated and photosynthetically active within just a few minutes. The gametophytes are the main growth phase of mosses, but they grow relatively slowly. A new Eurhynchium frond takes about a year to mature.

Reproductively, gametophytes are the phase that produces gametes (sex cells: eggs and sperm). In some mosses, eggs are produced on one individual and sperm on another. Eurhynchium is such a moss species, but, without close examination, it is impossible to tell the sexes apart. The microscopic details are amazing, but I won’t go into them here. Suffice it to say, sperm from one plant fertilize eggs on another plant. For this process, water is required. Sperm are transferred in splashes of water or perhaps in tiny drops on the legs and feet of wandering critters. Once the sperm are on the other plant, they have to swim to get to the eggs. The fertilized egg slowly grows into the sporophyte phase. For Eurhynchium oreganum, this all takes place in the early fall. The young sporophytes become visible by the end of October (See Figure 3).

Though sporophytes are green at first and produce some food by photosynthesis, they are largely parasites on the parent gametophyte (of the egg) and gain most of their substance up through the connection with the parent. As they develop, they largely lose their green color and develop the structures that will help them in their function, producing and dispersing spores. This development takes place over four to five months, so that by mid-February, fully formed sporophytes are seen on the gametophytes (Figure 5).

Figure 5. Mature sporophytes of Eurhynchium oreganum
Photo: Fred M. Rhoades

The sporophyte of mosses is nothing more than a spore sack (sporangium) on a stick with some protective coverings. You can see several sporangia in the photo in various stages of preparing to release spores. Note that some stages have a covering over the end of the sporangium that is long and pointed. Thus the common name, “beaked,” of this moss. The root “rhynch” in the scientific name refers to a “horn” for the same reason.

The moss sporangium is a wonder of evolution, perfectly structured to disperse spores during the times that mosses are exposed to a drying environment. The spores are produced inside but don’t just drop out of the end in a lump. The end of the sporangium is modified into two rings of teeth that change their shape on wetting and drying (they are hydroscopic). They arch inwards and outwards and drag spores from within, a few at a time, to be released into the passing wind. The spores, just tiny bits of dust (15/1000th of a millimeter in diameter), are easily carried and dispersed relatively long distances away. They land on suitable habitats and grow into gametophytes to repeat the life cycle.

Other moss species differ considerably in the shapes of their gametophytes and the details of the structures, particularly the teeth, of their sporophytes. They are actually taxonomically sorted by features of the sporophytes. However, because this stage is not always available for study, identification can sometimes be tricky. With practice, one can begin to see the subtle differences between the different gametophytes and recognize different species.

Figure 6. Old sporangium of Eurhynchium oreganum showing hygroscopic teeth. About 2 mm long.                 Photo: Fred M. Rhoades

There are fascinating microscopic structures and many other interesting biological details that I have skipped over here, in the interest of keeping your attention. Please investigate these further online, in a biology book, or by asking a bryologist for further details. And please look at the mosses for what they are, interesting individuals adapted to life in some small niches.

Dr. Fred Rhoades did graduate studies in both mycology and lichenology at Oregon State University and the University of Oregon in the 1970s. From 1977 to 2009, Fred was an instructor of biology at Western Washington University until retiring.

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