Primitive Plants: Mosses, Ferns, and Allies

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 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 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.

Division Bryophyta, the mosses and liverworts: (bryo = moss; phyto = plant; wort = herb, plant) These are terrestrial, but 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. In addition to the “usual” sexual reproduction, liverworts also reproduce asexually by means of small, cup-like structures called gemma cups containing small pieces of liverwort called gemmae (sing. = gemma). When it rains, the gemmae are splashed out and can grow into new liverworts.

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 Lycophyta = clubmosses: (not a moss) (lyco = wolf) This Division contains two main modern genera: Lycopodium (poda = foot) also known as ground pine or wolf’s claw, 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.

Division Sphenophyta = horsetails: (spheno = a wedge) The main genus in this Division is Equisetum (equis = horse; setum = bristle). Most have an underground rhizome with vertical stems. Many species have whorls of smaller branches at the joints of the stems, thus resembling upside-down horse’s tails. At least one local species has whorls of small, brownish or grayish (dead) leaves at the joints instead of branches, thus sort-of resembling rushes or bamboo. The epidermal cells of this (and many other) species of Equisetum contain silica, thus have been used as potscrubbers, earning the plant the common name of “scouring rush”

Division Pterophyta = ferns: (ptera, ptero = wing, feather) 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.

Note that the ancestors of these primitive plants were present back in “dinosaur times,” and the structures of the modern plants have changed little since then.

For more information on plant taxonomy, check out this site on Systematic Botany at the University of Maryland.