Primitive Plants: Mosses, Ferns, and Allies

Plants and Alternation of Generations:

Plants are multicellular eukaryotes. Nearly all are photosynthetic, and most are terrestrial (some are secondarily aquatic). Plants contain a variety of cell types that are specialized to perform different functions: the aerial parts generally have a waxy cuticle to prevent water loss, but the roots don’t so they can absorb water. The photosynthetic pigments include chlorophylls A and B plus various carotenoids. Plant cells have cell walls which are made of cellulose. Plants grow by mitosis, and reproduce by meiosis (which produces spores) and alternation of generations with special adaptations of the gametes and embryos to survive in a non-aquatic environment. The gametes are produced in gametangia. The male gametangium is called an antheridium (anthe = flower), while the female gametangium is an archegonium (arche = first, beginning; goni = seed). The eggs are fertilized within the female archegonium to reduce desiccation, and often stay within the archegonium to continue their development. In all plant groups except Division Bryophyta, the 2n sporophyte is the dominant generation.

The mosses, ferns and their allies are among the most primitive of plants, having relatives from the coal age. Like other plants, they are adapted to an environment where they are not totally surrounded by water, but most must live in very humid environments which are especially necessary for transfer of sperm in these groups.

The various Divisions (remember botanists use Division = Phylum) of plants are classified based on presence/absence of a vascular system and/or seeds. If seeds are present, the location of the seeds (whether on the surface of a reproductive structure or within some type of ovary) becomes significant.

The mosses and their allies lack true roots, stems, and leaves as such because they do not contain vascular bundles (veins). Many of them, however, do have rhizoids (rhizo = root; -oid = like, form) — root-like non-roots, as well as stemlike and leaflike structures. The ferns, like other more familiar plants, have true vascular bundles of xylem (xylo = wood; phloeo = the bark of a tree) and phloem for transport of water and nutrients. The fern allies have stems, but rootlike and leaflike structures vary with the group.

In the vascular plants, soil minerals and water are absorbed by the roots (which also anchor the plant and have no cuticle so water can be absorbed). Air, light, and CO₂ are absorbed by the leaves, while the O₂ “waste” from photosynthesis is released from the leaves. The stem functions in the support of the plant, and its vascular system transports water and nutrients. The diploid sporophyte is the dominant generation. Some gametophytes are monoecious and some are dioecious. “Monoecious” means that the male and female reproductive organs are in/on the same individual plant, while “dioecious” refers to having separate male and female plants. The vascular system (transportation system) is composed of xylem (xylo = wood) which transports water and nutrients up from the roots and must be dead to function (this consists of hollow tubes where cells have died) and phloem (phloeo = the bark of a tree), which distribute sugars around the plant and consists of live cells. In general, in the vascular bundles in the stems, xylem is found “inside” the bundle, surrounded by phloem, and in leaves, xylem is found on “top” of the bundles with phloem underneath.

Division Bryophyta, the Mosses and Liverworts:

Class Musci (Mosses):

(bryo = moss; phyto = plant; wort = herb, plant; musc = moss)
These are terrestrial plants, but must live in very damp, shady places. They have no vascular system so must directly absorb water from the soil. Mosses have root-like rhizoids (rhizo = root; -oid = like, form) and leaf-like structures, but these are not true roots/leaves because they have no vascular system. The dominant generation in the life cycle is the 1n gametophyte.

A germinating moss spore gives rise to an algalike protonema (proto = first, original; nema = a thread) which later develops rhizoids and “leafy” shoots. These moss gametophytes produce antheridia (male sex organs — anthe = flower; -idium = small) or archegonia (female sex organs — arche/archeg = ancient, first, beginning; goni = seed) on the tips of the “stems.” Sperm produced in the antheridia fertilize the eggs in the archegonia giving rise to the sporophyte generation which forms a stalk and capsule on top of the “stem” of the gametophyte. Within the capsule, certain cells undergo meiosis to form spores, the start of the new gametophyte generation, and when the spores are mature and the weather is right, the calyptra (the lid — calypt = covered, a cover) opens and the spores are released. These spores grow via mitosis to turn into the new, 1n gametophyte generation.

Below are a couple photos of clusters of moss plants. The low, “fuzzy” growth is all the 1n gametophyte moss plants. The stalks sticking up with capsules on top are all there is of the 2n sporophyte generation.

In wet weather, the sperm are released, swim to an archegonium, swim down the opening in the archegonium, and fertilize the egg. The resulting zygote remains within the archegonium for protection from dessication, and grows by mitosis to form the new, 2n sporophyte generation. The sporophyte never leaves the archegonium, but grows upward from there. In this photo, there are two 1n female moss gametophytes, each with a 2n sporophyte growing out of the top of it.

Class Hepatica (Liverworts):

(hepat = liver)
While closely related to the mosses, liverworts look quite different. Again, the dominant generation is the 1n gametophyte generation. The body (thallus — thall = twig, young shoot) of a liverwort is flat and ribbon-like (approximately 1 cm wide) with a “midrib”. Because they also have no vascular system to distribute water, they must remain small and be in almost direct contact with moist soil. One commonly-occurring genus of liverwort is Marchantia.

The male gametophytes produce antheridial heads that look sort-of like umbrellas. Each antheridial head contains several antheridia full of sperm. The female gametophytes produce archegonial heads that look like clusters of “fingers.” Several archegonia, each containing one egg, are located on the underside of each finger. When it rains, the antheridia release sperm. Raindrops then splash the sperm onto the archegonial head, where they swim to and into an archegonium, then fertilize that egg. Again, the zygote remains within the archegonium, and grows into an inconspicuous, small, round, 2n sporophyte. Meiosis occurs within the sporophyte, resulting in 1n spores which are then released and grow into the next gametophyte generation. This photo shows several male and female reproductive structures. Note that one version is labeled so you know which is which, and the other version is “plain” so you can see those structures more easily.

In addition to the “usual” sexual reproduction, liverworts also reproduce asexually by means of small, cup-like structures called gemma cups (gemma = a bud) containing small pieces of liverwort called gemmae (sing. = gemma). Gemma cups are located periodically on the thallus. When it rains, the gemmae are splashed out and can grow into new liverworts.

TRACHEOPHYTA (VASCULAR PLANTS)

(tracheo = the windpipe)
These plants all have a vascular system, comprised of xylem and phloem, to transport water and nutrients.

Division Lycophyta, the Clubmosses:

Unlike what the Division common name might suggest, these are not mosses. This Division contains two main modern genera: Lycopodium (lyco = wolf, poda = foot) also known as ground pine or wolf’s claw (hence the genus name), which is the larger of the two, and Selaginella, which is smaller in size. Many tropical species in this Division are epiphytes (epi = upon, over), plants that use another plant as a substrate upon which to live but are not parasitic. Most of our local species have a rhizome, an underground, horizontal STEM from which the roots and branches arise. Their leaves, while tiny, are true leaves with a vascular system.

The Lycophyta bear single sporangia (sporo = seed; angio = vessel, box, case) in the axils (axil = armpit) of special leaves called sporophylls, usually near the tips of the branches. The sporophyte generation has a horizontal rhizome, branching aerial stem, and numerous small leaves. These leaves do have a vein in the center. In each sporangium, special cells undergo meiosis to form spores. These spores produce underground tuberous gametophytes, some of which are dependent on a certain fungus for their nutrition. For Lycopodium, Eastern Redcedar growing nearby also appears to be important to survival. Gametophytes produce eggs and sperm which unite to form the new sporophyte.

Wolf’s Claw:

Here is a photo of Running Ground Pine, Lycopodium flabelliforme.

Division Sphenophyta, the Horsetails:

(spheno = a wedge)
The main genus in this Division is Equisetum (equis = horse; setum = bristle). The sporophyte consists of an underground rhizome from which roots and aerial branches arise. Fertile “stems” do not branch but bear a cone at their tips and a whorl of scalelike leaves at each joint on the stem. The non-fertile “stems” of many species of horsetail have smaller branches at the joints, resembling a horse’s tail while those of Scouring Rush resemble the fertile branches. This is the sporophyte generation from which spores form in the cone. When these spores germinate, they form small gametophytes (similar to those of the clubmosses) which eventually bear antheridia and archegonia. When the sperm fertilizes the egg, the zygote develops into a sporophyte again.

Field Horsetail:

A local species called Field Horsetail (Equisetum arvense — arvens = a field) shows the branching pattern from which the group gets its name.

Scouring Rush:

Another local species called Scouring Rush (Equisetum hyemale — hyemal = winter) has whorls of small, brownish or grayish (dead) leaves at the joints instead of branches, thus sort-of resembling rushes or bamboo. Scouring Rush gets its species name (hyemale) because it is evergreen, while Field Horsetail dies back to the ground in winter. The epidermal cells of this (and many other) species of Equisetum contain silica, thus have been used as potscrubbers, earning the plant its common name of “scouring rush.” Its strength is due to the presence of significant amounts of silica in its stems. If the top of the main stalk is damaged, Scouring Rush may send out side branches similar to those normally produced by Field Horsetail. Scouring Rush may occur in large stands in moist areas such as along stream banks.

Division Pterophyta, the Ferns:

Class Filicineae (Ferns)

(ptera, ptero = wing, feather; filic = a fern)
Ferns also exhibit alternation of generations, but for them, the 2n sporophyte generation is the dominant generation, and the 1n gametophytes are inconspicuous. The sporophyte plant consists of an underground, horizontal rhizome from which arise the roots and leaves.

A frond is a mature fern leaf, and these are often compound (divided into leaflets). A fiddlehead or crozier is a developing “baby” fern leaf. New fern leaves are coiled and uncoil as they grow, thus initially resemble the top of a violin above the pegs (fiddlehead) or a shepherd’s or bishop’s staff (crozier). Most ferns have horizontal stems or rhizomes. Some ferns having vertical stems are called tree ferns.

Mature Fern Frond
Young Fern Fiddleheads
Another Species of Fern with Fiddleheads
A Tree Fern from California

Christmas Fern Sterile and Fertile Leaves
Rattlesnake Fern Sterile and Fertile Leaves Fern sporophytes have two kinds of leaves: sterile and fertile. The sterile leaves are “regular” leaves, primarily for photosynthesis. The fertile leaves bear the spore-producing areas. In some types of ferns, the sterile and fertile leaves look almost alike, except for the presence/absence of the spore-producing areas, while in other types of ferns, the sterile and fertile leaves look very different from each other.

On the underside of the fertile leaves are numerous brown spots called sori (singular = sorus — sorus = a heap).

Each sorus contains numerous sporangia in which the spores are produced via meiosis. A single sporangium consists of a stalk made of several elongated cells, and a capsule. The “backbone” of the capsule is a ring of cells called an annulus (annul = a ring), while the capsule itself is made of thin-walled cells. Note that each of the cells in the annulus has a thin outer wall and thick inner walls. In drier weather, evaporation of water from the cells of the annulus causes them to dehydrate somewhat. Thus, the annulus becomes straighter, causing the sides of the capsule to tear open and spores to be released. Because glycerol also will pull water out of cells, it can serve to dehydrate them, thus causing the capsule to tear and spores to be released for examination.

Each spore grows into a small, heart-shaped gametophyte called a prothallium (pro = before, in front of; thall = a young shoot, twig) which produce antheridia and sperm or archegonia and eggs. Sperm swim to and fertilize the eggs. Typically one zygote per female gametophyte will develop into a new sporophyte fern. The new 2n sporophyte forms within the archegonium until it sends up its first leaf and roots. Once the sporophyte is established, the gametophyte dies and decays.