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Natural Perspective

The Plant Kingdom: Mosses and Allies

(Last modified: 4 May 2015)

Mosses and their allies are small green plants that are simlutaneously overlooked and deeply appreciated by the typical nature lover. On the one hand, very few people pay attention to individual moss plants and species. On the other hand, it is the mosses that imbues our forests with that wonderful lush "Rainforest" quality which soothes the soul and softens the contours of the earth.

These wonderfully soft carpets of green are, in fact, Nature's second line of attack in its war against rocks. After lichens have created a foothold in rocks the mosses move in, ultimately becoming a layer of topsoil for higher plants to take root. The mosses also hold loose dirt in place, thus preventing landslides.

Ecologically and structurally, mosses are closer to lichens than they are to other members of the plant kingdom. Both mosses and lichens depend upon external moisture to transport nutrients. Because of this they prefer damp places and have evolved special methods of dealing with long dry periods. Higher plants, on the other hand, have specialized organs for transporting fluid, allowing them to adapt to a wider variety of habitats.

Bryophytes used to be classified as three classes of a single phylum, Bryophyta. Modern texts, however, now assign each class to its own phylum: Mosses (Bryophyta), Liverworts (Hepatophyta), and Hornworts (Anthoceraphyta). This reflects the current taxonomic wisdom that the Liverworts and Hornworts are more primitive and only distantly related to Mosses and other plants.

Mosses (Phylum: Bryophyta)

All plants reproduce through alternating generations. Nowhere is this more apparent than in the mosses. The first generation, the gametophyte, forms the green leafy structure we ordinarily associate with moss. It produces a sperm and an egg (the gametes) which unite, when conditions are right, to grow into the next generation: the sporophyte or spore-bearing structure.

The moss sporophyte is typically a capsule growing on the end of a stalk called the seta. The sporophyte contains no clorophyl of its own: it grows parasitically on its gametophyte mother. As the sporophyte dries out, the capsule release spores which will grow into a new generation of gametophytes, if they germinate.

Mosses, the most common, diverse and advanced brypophytes, are categorized into three classes: Peat Mosses (Sphagnopsida), Granite Mosses (Andreaopsida), and "True" Mosses (Bryopsida or Musci).

Shown: Class: Bryopsida; Order: Hypnales; Family: Brachythecia; Homolathecium nutalli (probably)

Leafy Liverworts (Phylum: Hepatophyta, Class: Jungermanniidae)

While people typically know what a moss is, few have even heard of liverworts and hornworts.

These primitive plants function much like mosses and grow in the same places, often intertwined with each other. The liverworts take on one of two general forms, comprising the two classes of liverworts: Jungermanniidea are leafy, like moss; Marchantiopsida are leaf-like (thalloid) similar to foliose lichens.

The leafy liverworts look very much like mosses and, in fact, are difficult to tell apart when only gametophytes are present. The "leaves," however, are simpler than moss and dont have a midrib (costa). The stalk of the sporophyte is translucent to white; its capsule is typically black and egg-shaped. When it matures, the capsule splits open into four equal quarters, releasing the spores to the air.

The liverwort sporophyte shrivels up and disappears shortly after releasing its spores. Because of this one hardly ever sees liverwort sporophytes out of season. Moss sporophtyes, on the other hand, may persist much longer.

Shown: Class: Jungermanniidea; Order: Jungermanniales; Family: Scapaniaceae; Scapania spp. (probably)

Leaf-like Liverworts (Phylum: Hepatophyta; Class: Marchantiopsida)

The leaf-like (thalloid) liverworts are, on the whole, more substantial and easier to find than their leafy counterparts. The gametophyte is flat, green and more-or-less strap-shaped. The body may, however, branch out several times to round out the form.

When the gametophyte has become fertilized and is ready to produce its sporophyte generation it may grow a tall green umbrella-shaped structure called the carpocephalum. The sporophyte grows on the underside of this structure, often completely hidden from view.

During the dry season, leaf-like liverworts may shrivel up and completely disappear from view until the rains arrive again.

Thalloid liverworts are much easier to identify than their leafy counterparts due to the wider variety of gametophyte shapes.

Shown: Class: Marchnatiopsida; Order: Marchantiales; Family: Aytoniaceae; Asterella californica

Hornworts (Phylum: Anthoceraphyta)

Hornworts are very similar to liverworts but differ in the shape of the sporophyte generation. Instead of generating spores in a capsule atop a stalk, the hornwort generates spores inside a green horn-like stalk. When the spores mature the stalk splits, releasing the spores.

Under the microscope, hornwort cells look quite distinct as well: they have a single, large chloroplast in each cell. Other plants typically have many small chloroplasts per cell. This structure imparts a particular quality of color and translucency to the body (thallus) of the plant.

Hornworts are all grouped into a single class, Anthocerotae, containing a single order, Anthocerotales.

Shown: Class: Anthocerotae; Order: Anthocerotales; Family: Anthocertaceae; Phaeoceros spp.

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Collaboration and inspiration thanks to Susan Kornfeld

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