Bryophytes: Mosses

As you recall we are studying bryophytes. These have always been members of Kingdom Plantae. While some bryophytes are arguably vascular, they have been excluded from the "vascular plants" by virtue of not being capable of lignin biosynthesis. Thus the conducting tissues are not xylem and phloem...rather they are hydroid and leptoid. Should this make them non-vascular? Well, not in my opinion.

The mosses belong to class Muscopsida in phylum Bryophyta in most classifications. As is true of all bryophytes, mosses are eukaryotic, have chlorophyll a and b as well as xanthophylls and carotenoid pigments for photosynthesis.

Mosses store starch, have pectin-cellulose walls, mitosis is open (nuclear membrane disappears in mitosis), and cytokinesis is by formation of a cell plate along a phragmoplast. These characteristics are common for ALL members of Kingdom Plantae. All mosses have a sporic (diplohaplontic) life cycle that is oogamous. In some ways, then, mosses are fundamentally related to flowering plants.

The Gametophyte Thallus

The dominant phase of the moss life cycle is the gametophyte (haploid). The small green plant you find as a moss in the woods is the haploid gametophyte. Many people think of mosses as living only in moist woods. But this idea misses a lot of mosses. Mosses can be found growing on bare rocks and sand dunes...even glaciers!

With a sporic life cycle, however, the moss gametophyte will host the sporophyte as shown in these pictures.

The moss sporophyte is completely dependent upon the gametophyte for its nutrition. The sporophyte however is responsible for producing spores. Here you can see the fancy decoration on the spore surface that is waterproofed by sporopollenin. The SEM provides us with excellent surface views. The TEM shows that the spore inside the wall is a typical cell with nucleus, vacuoles, chloroplasts, mitochondria, etc.

The gametophyte plant is produced by the germination of a haploid moss spore. This is typically a light-triggered event when a moss spore lands in a moist environment.

Sprouting from the spore wall is a developing filament of haploid cells, called a protonema. This branching filament in nature looks very much like a green alga. In fact it would be difficult to distinguish the protonema from an alga for many biology students. If you remember the phrase: "ontogeny recapitulates phylogeny" you see a prime example here in the mosses!

The prepared slide has artificially stained blue walls. From this tangle of branching filaments, the protonema forms a clump of cells that can become asexual propagules called bulbils that can withstand cold, heat, or drought. But under good conditions, the protonema ultimately grows into a young gametophyte plant.

The gametophyte plant body is called a thallus, which means that it lacks xylem and phloem and therefore does not have true stem, true leaf or true root. The plant body may have conducting tissue and some of this has a familiar look.

The xylem-like water-and-mineral-conducting tissue is called hydroid. The phloem-like sugar-and-amino-acid-conducting tissue is called leptoid. Because the details are not precisely xylem and phloem, mosses are referred to as non-vascular. Obviously this is a contradiction in terms between name and function. I suspect in a few years, we will lose the "non-vascular" name thanks to efforts on the part of bryologists to educate other botanists on the functions of hydroids and leptoids.

Mosses have a stem-like axis with leaf-like appendages (sometimes called phyllids). Most moss "stems" are erect but some are prostrate. As mosses lack lignin biosynthesis capacity, they are never tall. The phyllid arrangment is usually spiral:

The phyllids of mosses such as Mnium may be a single cell thick, reminiscent of the algal sheets in Monostroma but with a midrib with hydroids and leptoids!

The phyllids of other mosses, such as Polytrichum have a pad of cells and filamentous strands of photosynthetic cells. Thinking back to the liverwort thallus and to photosynthetic filaments of algae, we can see some recapitulation of phylogeny here!

The thallus is anchored to the substrate by rhizoids. A rhizoid is a hair-like structure that may be only one cell wide, though it can be more than one cell in length. There is no root in any bryophyte! This level of anchorage will of course also never support a tall "stem" whether it had lignin or not!

The epidermal layers of the moss gametophyte may possess cutin to prevent desiccation in the terrestrial environment, if so, the thallus then needs pores to allow for gas exchange. In some mosses the pores are surrounded by a single "doughnut shaped" guard cell. In others we may find truly functional stomata.

For sexual reproduction, the moss gametophyte produces gametangia. The gametangia of both genders may be on the same thallus (homothallic or monoecious) or on separate gametophytes (heterothallic or dioecious) depending upon the species being observed. Both the antheridium and archegonium have a sterile jacket of cells, which better protects the gametes against desiccation in the terrestrial environment.

Here you can see the splashcup of the tip of male Polytrichum gametophytes. The splashcup contains many antheridia as revealed best in the longitudinal section through the splashcup.

The antheridium consists of a stalk, a sterile jacket, and spermatogenic tissue as shown in the closer view below. The antheridium sterile jacket has a cap cell which disintegrates when turgor pressure rises, for example when the gametophyte is wetted with hypotonic rain water in the splashcup, allowing for sperm release into the surrounding water. The spermatogenic tissue inside the sterile jacket is a haploid tissue; the sperm differentiate from haploid products of mitosis of haploid spermatogenic cells. The flagellated sperm are curved cells that are chemotactic and swim through free-water up a concentration gradient of the chemotactic agent to find the open archegonium. The second raindrop to splash into the splashcup will assist in dispersing sperm onto adjacent plants.

The archegonial head of female Polytrichum gametophytes lacks a splashcup. The longisection reveals that the archegonium consists of a stalk, a venter surround the egg, and a long neck. The neck is filled with canal cells. The sterile jacket has a cap cell which disintegrates when turgor pressure rises (when surrounded by rain water for example).

All cells of the archegonium, including the egg cell, are produced by mitosis of haploid gametophyte cells. Likewise the neck canal and ventral canal cells disintegrate and ooze out the end opened by the breakdown of the cap cell. The diffusion of the breakdown products of these cells allows for chemotaxis by the sperm to swim to the egg. In the archegonia below, the sperm have arrived and the archegonia have been visited. The venter on the left has a archegonium on the right has a very young sporophyte developing inside the venter. The twisted appearance of the necks and open end are the telltale signs that these archegonia are no longer "virgin". The twisted sperm distort the necks.

The Sporophyte Thallus

The sperm and egg join in syngamy. The resulting diploid zygote and subsequently-developing diploid sporophyte is typically not photosynthetic and so is parasitic (dependent!) on the gametophyte for its nutrition. Here you can see the early development of the sporophyte within the archegonium neck (which becomes the calyptra). Also you can see a progression of the elongation of the seta.

The sporophyte thallus is also determinate; having a specific final body size. Moss sporophytes consist of the haustorial foot (nutrition exchange area), a supporting (seta) stalk (with hydroids and leptoids), and (sporangium) capsule.

The sporangium has a sterile jacket of cells surrounding the sporocytes. The sporocytes divide meiotically to produce haploid spores. The spores develop a wall impregnated with sporopollenin (a water-proofing agent) and are distributed in the terrestrial environment by wind. Spores landing in a suitable environment germinate into new gametophytes.

The sterile jacket can be quite complex in structure.

It may have an operculum (cap) that must be removed before the spores can be released. Under the operculum there may be a diaphragm which keeps the open end of the capsule dry after the operculum is shed.

The opening may have peristome teeth that raise and lower the diaphragm, or otherwise open and close the open end of the capsule as needed. Under dry conditions the peristome opens the capsule for shedding spores into the wind; under moist conditions, the peristome keeps the capsule closed and spores are not shed.

In other species, the diaphragm is supported by a column of cells running up the middle of the sterile jacket; this column of cells is called the columella.

Spore Dispersal by Insects?!

In certain dung-mosses, for example Splachnum, the spores are sticky and the peristome teeth are large and colorful and fold back to expose the columella, covered with the spores. The capsule wall shrinks around the base of the columella so that the columella looks like a pollen-covered stigma and the peristome appears to be a perianth. Flies are attracted to the dung on which this moss grows, and seeing the "flowers" here, the flies hope to get a bit of free lunch as well as a place to lay their eggs. They get spores on their hairy bodies and then fly off to another dung lay their eggs, but end up also depositing some moss spores on the next dung pile. So dung mosses mimic flowers to trick flies as a "magic bullet" to get their spores from one dung-pile to another.

In Sphagnum moss, the sporophtyes are nearly spherical but have a definite cap. They are a dark color which also assists the spore dispersal.

The Sphagnum capsule contains spores but also some gas and fluid space. The dark capsules are heated in the sunlight and the liquid and gas in the capsule is heated. The pressure in the capsule builds. The heat dries the surface of the capsule and it shrinks, adding to the internal pressure.

The pressure eventually exceeds the holding force of the cap on the capsule, and it blows into the air. The spores are spewed into the air.

In more passively opened sporangia in other species the peristome shape is important. It causes the air passing over the open end of the capsule to swirl and lift spores via the Bernouilli effect out of the sporangium and into the air.

Asexual Reproduction

The moss sporophyte typically has no method for asexual reproduction. The gametophyte of most mosses may reproduce asexually via gemmae in gemmae cups, bulbils budding from gametophyte surfaces, or fragmentation of the branching protonema.

When is a "moss" not a moss?

As our parting shot, I offer this photo of "Spanish moss" hanging on trees in the southern part of the US. This plant is neither of Spanish origin nor is it a moss. This is a New World species that is related to pineapples! It is a flowering plant with some very fine violet flowers in summer. Definitely NOT a moss.