Vascular plants are often called Tracheophytes and are thought to represent a natural group, that is, they are each others closest relatives and are all descendents from a common ancestor.
There are early free-sporing tracheophytes - pteridophytes or cryptogams - and seed plants; we are just beginning to fully understand how all these vascular plants are related to each other.
World-wide pteridophytes number about 12,000 species so they are a big group. Cryptogams means "secret marriage" as Linnaeus, who used the term, didn't know how they reproduced. Pteridophytes refers to "the ferns and the fern allies."
Four traditional groupings - phyla - are traditionally recognized, but not all of these are natural.
- Lycopodiophyta : lycopods
- Psilotophyta: whisk ferns; unlike the rest of the groups, lack roots or leaves
- Equisetophyta: horsetails
- Polypodiophyta: ferns
Why are these four phyla grouped together?
The production of free spores, the principal dispersal units. Spore: a reproductive cell, capable of developing into an adult without fusion with another cell. Spores have the advantage of being cheap to produce, but the disadvantage of being fragile and small.
The germination of spores and the susequent development to produce gametophytes that exist independently of the spore-producing plants. These two independent alternation of generations. Typically the more conspicous and dominant plant is the sporophyte (diploid, 2n) which is usually perennial and lives for an indefinite period. The gametophytes (haploid, n) tend to be inconspicuous (usually less than 1 cm in largest dimension) and short-lived; when the fertiliztion occurs and the new embryonic sporophyte forms, the gametophyte forms.
The fertilization of eggs by flagellate, swimming sperms need water to reproduce.
The can also be viewed by what is absent: no seeds, no flowers, and no fruits
General description of cryptogams:
They are often thought as retaining many primitive features of the earliest land plants. These were the first plants to turn to land.
There is a great diversity in form, life cycle, and habitat preference. They occur world-wide from icy tundras above the artic circle to tropical forests. More diverse in the tropics: arborescent, epiphytic, and climbing. In North America the optimal habitat is rich, moist forests.
Basically herbs, only a few get to the height to 2-3 m tall (tree ferns). But wide range exists - Azolla a floating aquatic herb is one millimeter long and some tree ferns Cyatheaceae and Dicksoniaceae - are up to 18 meters tall and bear leaves that are more than 4 meters long. The longest fern leaf is a fern leaf whose rachises never stop elongating, Lygodium and Salpichlaena whose leaves can reach over 30 m long!!
With two exceptions, Botrychium and Isoetes, no modern pteridophyte displays true secondary growth (there is evidence of secondary growth in fossil pteridophytes); there is no cambium and they lack secondary vascular tissues.
Compared to seed plants, there are of little economic importance with few exceptions - unless you would like to consider that most of our electricity comes from burning the fossil remains of pteridophytes that grew in the extensive coal-forming swamps of the Carboniferous periods! Azolla harbors cyanobacteria (formally and eronaeous thought of as blue-green algae); the cyanobacteria fix nitrogen and is a rich source of nitrogen and used for centuries in SE Asia as an organic fertilizer in flooded rice paddies. There are also some cultivated ferns and a few invasive ferns.
II. Lycopodiophyta [also called Lycophyta]
This group arose in the Later Silurian about 420 million years ago and was dominant in the Carboniferous and used to be much more diverse.
Leaves are microphylls: generally small, simple, one-veined leaves. Microphylls evolved by the process of enation and vascularization. An enation is a veinless, lateral protrubence on the stem. When the enation becomes vascularized, it is termed a microphyll.
Sporangia: the spores produced by the sporophytes are located singly on the upper surfaces of an axis either of bracts of a cone or of green leaves.
Lycopodiaceae: Club moss family
15 genera about 375 species; cosmopolitan distribution most diverse in tropics
Stems elongate and dichotomously branching. Leaves often densely covering the stem. Homosporous plants - produce only one kind of spore and gametophyte; cones terete (rounded); the oily compounds in the cell walls ignite rapidly into a flash of light and were used by magiciancs and sorcerers in the Middle Ages, was also used as a flash early in photpgraphy, and in experimental photocopying machines. Some member of the family have chromosomes numbers up to 275. The genus Huperzia is the only pteridophyte that has alkaloids and is currently being tested for its supposed action of slowing the onset of Alzheimer's.
The genus Lycopodium club mosses - now usually broken up into other genera.
Examples seen in lecture:
Lycopodium obscurum, groundpine
Lycopodium lucidulum, shining clubmoss [Huperzia lucidula]
Lycopodium digitatum, crowfoot clubmoss, southern running-pine, southern ground-cedar [Diphasiastrum digitatum]
Lycopodium inundatum, bog clubmoss [Lycopodiella inundata]
Selaginellaceae: Spike moss family
1 genus, Selaginella, 750 species with a mainly tropical distribution with a few species extending into arctic regions of both hemispheres
Leaves spirally arranged and often 4-ranked on the secondary and ultimate branches. Can look a lot like Lycopodiaceae, but the leaves are ligulate: tongue shaped or strap-shaped outgrowth near the base of the upper surface of each microphyll. Spores borne in or near the axils of well-differentiated sprophylls, usually on 4 sided stroboli. Unlike Lycopodiaceae, Selaginellaceae are hetersporous with different types of spores: microspores and megaspore (not related to size) - the microspores give rise to male gametophytes and the megaspores give rise to the female gametophytes·
Examples seen in lecture:
Selaginella rupestris - rock spikemoss
Selaginella selaginoides - northern spikemoss
Isoetaceae - quillwort family
2 genera, 50-70 species. Only two species of Isoetes in Wisconsin.
Aquatic or semi-aquatic plants with corm-like stems with secondary growth. Leaves with ligules and quite long (15+ cm) for microphylls. Heterosporous plants with sporangia borne at the base of sporophylls which are similar to vegetative leaves.
III. Equisetophyta [also called Sphenophyta]
One extant family Equisetaceae and 1 genus, Equisetum (horsetails and scouring rushes), of 15 species with a cosmopolitan distribution except for Australia or New Zealand [9 species in Wisconsin]. Fossils have been known since the Devonion 408-360 mya; also more abundant and important during the Carboniferous where they are abundant understory plants.
Plants are primarily colonizers of unforested areas, lake margins and wetlands.
Leaves are in whorls; leaves one veined, verticillate, united to form a sheath around the stem; these leaves are probably reduced megaphylls: Megaphylls are larger than microphylls have a blade that has a complex system of veins. Megaphylls are believed to have evolved from entire branch systems in which there was (1) unequal branching that resulted in stronger branches overtopping weaker ones, this was followed by (2) planation or the subordinal lateral branches, and the final step (3) was fusion or webbing of the separate lateral branches to form a primitive lamina.
Internodes with conspicuous vertical ridges; jointed stems; stems hollow. Silica in the stems made them useful to early European settler in North America for scouring cookware.
Sporangia clustered terminally in cones composed of polygonal, umbrella-like structures with sporangia beneath. Spores have elators.
Examples seen in lecture:
Equisetum arvense, common horsetail, field horsetail
Equisetum hyemale, common scouring rush, pipes, scouring rush horsetail
Equisetum laevigatum - smooth horsetail or scouring rush
Equisetum scirpoides, dwarf scouring rush, sedge horsetail
Equisetum sylvaticum, wood horsetail, woodland horsetail
IV. Polypodiophyta [also called Pterophyta]
Much of the diversity talked about the cryptogams in general can be applied to the fern group in terms of the diversity of size and growth form. The ferns range from aquatic, to epiphytes to trees. They also represent the bulk of the species diversity of cryptogams: about 11,000 species worldwide and are the largest group of plants other than the angiosperms.
Leaves of megaphyllous origins. Leaves are sometimes referred to as fronds and the petioles as stipes. Fronds show circinate vernation: vernation is the arrangement folded leaves in a bud, forming a crozier or fiddlehead, i.e. coiled or rolled up at the tip and unfolding lengthwise when emerging - due to auxin and differential growth of tissue.
Sporangia borne on the margin or the lower surface of the leaf; often grouped in sori (pl.) sorus (sing.); often associated with an annulus a cluster or row of cells with thick walls that open the sporangium and catapult the spores into the air; indusium is a protective flap of tissue over the sorus, and may or may not be present. Members are both homosporous and heterosporous.
There is much disagreement on the circumscription of families. For our purposes, there are around 30 fern families worldwide. 9 fern families are recognized in the Wisconsin checklist.
Characters used to circumscribe families often related to cryptic feature such as the presence or absence of annulus and it orientation, length and diameter (number of cells) of the sporangial stalk, spore shape, stem and petiole cross-section anatomy.
Of some importance is that most families are leptosporangiate (based on 1 cell layer in sporangium, presence of an annulus), whereas a few (Ophioglossaceae for example) are eusporangiate (based on 2+ cell layers in sporangium, no annulus). All other cryptogams are also eusporangiate.
Descriptions of the selected families can be obtained by clicking on the links which will take you to the Plant Systematics Collection.
Ophioglossaceae (2 genera / 12 species)
Botrychium virginianum - rattlesnake fern
Botrychium lunaria - moonwort
Botrychium mormo - goblin fern
Osmundaceae (1 genus / 3 species)
Osmunda cinnamomea - cinnamon fern
Osmunda claytoniana - interrupted fern
Osmunda regalis var. spectabilis - American royal fern
Adiantaceae (4 genera / 6 species)
Adiantum pedatum - northern maidenhair fern
Cryptogramma stelleri - slender cliff-brake fern
Polypodiaceae (1 genus / 1 species)
Polypodium virginianum - common polypody, rock-cap fern
Dennstaedtiaceae (2 genera / 2 species)
Pteridium aquilinum var. latiusculum - bracken fern
Thelypteridaceae (2 genera / 4 species)
Thelypteris palustris var. pubescens (Dryopteris thelypteris var. pubescens) - marsh fern
Aspleniaceae (9 genera / 27 species)
Asplenium platyneuron - ebony spleenwort
Asplenium viride - green spleenwort
Athyrium angustum (A. filix-femina var. michauxii) - northeastern lady fern
Dryopteris intermedia (D. spinulosa var. i., D. austriaca var. i.) - glandular wood fern
Polystichum acrostichoides - Christmas fern
Onocleaceae (2 genera / 2 species)
Onoclea sensibilis - sensitive fern
Salviniaceae (1 genus / 1 species)
Azolla mexicana (A. caroliniana) - mosquito fern