Plant Taxonomy - Ferns & Allies

Plant Taxonomy (BIOL308) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Ferns And Fern Allies

I. Characteristics of the ferns and allies

Vascular plants - possess vascular tissue (xylem and phloem). However, the xylem lacks vessels. Tracheids are the water transport cells.

Gametes are produced in archegonia (eggs) and antheridia (sperm).

Spores are produced in sporangia. These plants do not produce seeds. Spores are the dispersal agents, not seeds. Leptosporangiate ferns (most Polypodiophyta)have a sporangium with a single layer of cells; Eusporangiate ferns & allies (Lycopodiophyta, Psilophyta, Equisetophyta) have sporangia with multiple cell layers.

The sporophyte phase is dominant; the gametophyte is reduced, free living. We will go over the fern life cycle as a "typical" representative of this group. See diagram. Some points to note: (a) there are many variations on this basic theme; (b) sporangia, sorus, indusium, annulus, lip cells; (c) homosporous vs. heterosporous (megaspores/microspores); and (d) leptosporangiate vs. eusporangiate.

Evolutionarily ancient lineage. The ferns arose over 400 mya. Unlike today, during the Carboniferous era (345 – 290 mya) many were large trees. Diverse assemblage – the ferns as a whole are not monophyletic.

There are four major divisions (see below).

II. Lycopodiophyta (which includes the Lycopodiaceae, Selaginellaceae, Isoeteaceae)

Lycopodiaceae - Club Moss Family. Homosporous, terrestrial or epiphytic, moss-like herbs that produce roots directly on stem and leaves, no ligule. Sporangia borne on upper side of sporophylls which are aggregated into strobili that may be terminal or axillary. Lycopodium - club moss, ground pine, running cedar.
Selaginellaceae - Spike Moss Family. Heterosporous, small, prostrate, roots adventitious (produced at the end of small branches called rhizomorphs); moss-like, rhizomorph producing herbs, dichotomously branched, ligulate (flap at base of each leaf). Selaginella - spike moss.
Isoetaceae - Quillwort Family. Heterosporous, aquatic or emergent, quill-like herbs, corm-like stem, sporangia embedded in the base of the leaves, ligulate. Isoetes - quillwort.

III. Psilotophyta. (with a single family)

Psilotaceae - Whisk fern family. Herbaceous, dichotomously branched, stems developing from rhizome; homosporous, sporangia large and bilobed or trilobed, lateral on stem, subtended by a pair of scales. Psilotum - whisk fern.

IV. Equisetophyta (with a single family)

Equisetaceae - Scouring Rush or Horsetail family. Erect, homosporous, herbs with hollow, jointed, fluted silicaceous stems, leaves reduced to whorled scales at the nodes, sporangia borne in terminal strobili, spores containing chlorophyll. Equisetum - horsetails.

V. Polypodiophyta - a rather diverse group, comprised of several orders and families. We’ll feature a few select families.

Ophioglossaceae - Grape Fern Family. One to three-leaved terrestrial homosporous ferns with dimorphic leaf blades; sporangia borne on a branch arising from the base of the sterile part of the blade; vernation not circinnate, annulus absent. Considered to be the most primitive group of temperate North American ferns. Two genera occur in MN: Botrychium - grape fern (leaves pinnately compound or simple and deeply lobed) and Ophioglossum - adder's tongue (simple leaves).

Osmundaceae - Cinnamon Fern Family. Large, terrestrial ferns with dimorphic blades or leaves, differentiated into sterila and fertile segments; sporangia scattered on fertile segments, indusium absent, sporangia maturing simultaneously. Osmunda occurs in MN.

Polypodiaceae - Polypody or Fern family. Diverse collection of primarily terrestrial ferns, sporangia stalked, aggregated in sori, indusium present, annulus vertical and incomplete. Circinnate vernation, crozier formation. There are many species in MN. Common genera include Adiantum, Athyrium, Cheilanthes, Cryptogramma, Cystopteris, Dennstaedtia, Dryopteris, Matteucia, Onoclea, Polypodium, Polystichum, Pteridium, Thelypteris, Woodsia, Woodwardia.

Marsiliaceae - Water clover Family. Clover-like, aquatic, heterosporous ferns developing from rooted rhizomes, produce hard sporocarps. Marsilea.

Salviniaceae - Water fern family. Free floating, heterosporous aquatic ferns which produce soft sporocarps. Salvinia (leaves entire, rootless) and Azolla (leaves bilobed, roots present) are representative members of this family.

VI. Some evolutionary trends in the Ferns (adapted from Scagel et al., Plant Diversity: An Evolutionary Approach).

Character: Primitive Advanced
stem: creeping à upright
fronds: large à small
fronds: much branched à unbranched
venation: open and dichotomous à closed and reticulate
sporangia position: terminal or marginal à superficial on abaxial surface à abaxial surface, protected by indusium
sporangia wall: thick à thin
sporangial stalk: thick, 3+ cell rows à thin (1 cell row)
sporangium size: large à small
Spore number/output: many spores, indefinite number à fewer, constant number
hairs: simple, linear à branched à scales
sequence of maturation: simultaneous à basipetal à random
gametophye size: large à small
gametophytes growth: slow growth à rapid growth
spore differentiation: homosporous à heterosporous
antheridia size: large à small
anteridia wall: more than four cells à one to four cells
sperm production: lots of sperm à few sperm
archegonia: long and straight neck with several to many neck cells à short and recurved with small number of neck cells

References:

Kramer, KV. & PS Green. 1990. Families and Genera of Vascular Plants. Vol 1. Pteridophytes & Gymnosperms. Springer-Verlag, NY.