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The earliest
vascular plants developed from green algal ancestors. It is hypothesized that
they were derived from an algal ancestor in which the gametophytic stage
became adapted to temporarily dry habitats before a vegetative
(=multicellular) sporophyte developed (interpolation theory), OR they were
derived from ancestors in which both gametophyte and sporophyte initially
were free-living and adapted to temporarily dry habitats and that, over time,
the sporophyte secondarily began growing on the gametophyte (transformation
theory). Recent phylogenetic analyses support the progressive development of
the sporophyte growing on the gametophyte to explain alternation of
generations in embryo-forming plants. The conquest of the land must have
entailed the development of adaptations that permitted water retention, such
as a waxy cuticle that covered the plant body, high volume to surface ratio,
and specialized pores called stomata through which gas exchange occurs.
Vascular tissues for support and conduction of water and nutrients were
developed subsequently.
The lycopods, selaginellas, horsetails, and ferns are vascular plants that
reproduce by spores, not by seeds. Both diploid and haploid phases of the
life cycle form vegetative structures in these plants. The haploid stage
(gametophyte; 1n) is reduced, while the diploid stage (sporophyte; 2n) is
large and visible. Meiosis occurs in sporangia located on the adult
sporophyte, and results in the formation of haploid spores. If the haploid
spores are dispersed to an appropriate habitat, each will germinate to
produce a small, vegetative structure called a gametophyte (1n) on which male
and/or female sex organs are formed. The gametes or sex cells are produced in
the sex organs. Fertilization occurs in the female sex organ of the
gametophyte and the zygote that is formed will grow eventually to form a
mature sporophyte. Selfing or outcrossing may occur, depending on whether or
not the male gamete originates from the same or from a different gametophyte
as the female gamete.
The majority of ferns and their allies produce one type of haploid spore in
one type of sporangium (homospory). The spores, when dispersed to a suitable
habitat, will germinate to form a bisexual (=hermaphroditic) gametophyte. A
few other taxa, such as Selaginella and water ferns (e.g., Marsilea,
Salvinia), produce two types of spores in two differentiated sporangia
(heterospory); in these taxa, the spores germinate to form individual male
and female gametophytes. Homosporous plants have slow-germinating spores
20-40 microns in diameter and gametophytes that are either photosynthetic or
mycorrhizal (with fungal association). They typically occur in a mesophytic
(moist) habitat, which facilitates fertilization since the motile sperm have
to swim to the egg on the gametophyte. The formation of two types of spores
(heterospory) on one plant enabled the differentiation and specialization of
male and female gametophytes. Heterosporous plants occur in dry or wet
habitats and form female gametophytes that use stored food from the spores.
The spores from heterosporous plants rapidly germinate, with the male spores
15-35 microns in diameter and the female spores 100-500 microns in diameter.
Heterospory was a necessary precursor for the development of the seed habit,
which required a separation of the male and female gametophytes, a means of
storing food reserves in the female gametophyte, and adaptations that would
prevent or slow down water loss from the reproductive structures.
Spore-producing plants may be distinguished from seed plants by the
fertilization process. In spore- producers, sperm swim freely through dew or
rainwater to gain contact with an egg nucleus. In seed plants, sperm do not
swim freely; instead, they are found inside the pollen (=male gametophyte)
and usually are transported aerially to the structure that houses the female
gametophyte. In general, the reproductive structures of spore- producers are
located on or near the ground, while in seed producers, the reproductive
structures are borne aerially on above-ground stems.
The origin of leaves in vascular plants is believed to have occurred in two
different phylogenetic groups. In one group, small leaflike appendages called
enations eventually became larger and vascularized with a single vein to form
microphylls (micro=small; phylls=leaves). Enations and microphylls occur in
extinct groups as well as the living lycopods. In the second group,
megaphylls (mega=large; phylls=leaves) are believed to have been derived from
the overlapping of flattened and expanded stems (telomes). Megaphylls
typically possess branching veins, and are found in the horsetails, ferns,
and modern seed plants.
In general, the earliest land plants likely exhibited dichotomously branching
stems, simple sporangia (called eusporangia), homospory, photosynthetic or
mycorrhizal gametophytes, and fertilization on ground-dwelling gametophytes.
The main groups of seedless vascular plants that are living today (extant)
are the lycopods, selaginellas, horsetails, and ferns. Before conifers and
flowering plants developed, members of these spore-producing groups formed
the dominant terrestrial vegetation on the earth. In fact, some of these
early vascular plant groups, now extinct, had relatives that formed seedlike
structures. Members of the seedless vascular plant families usually may be
readily identified by habit and by arrangement of sporangia.
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1. Order
Lycopodiales a. Family Lycopodiaceae (Lycopodium, commonly called lycopod,
clubmoss, or ground pine).
These plants are mostly evergreen herbs that usually occur in forest
habitats. The plants have dichotomously branching or single stems, spirally
arranged leaves, and sporangia aggregated into terminal cones (strobili) or
distributed along the stem. The leaves that subtend sporangia (sporophylls)
are often similar to the vegetative leaves. Lycopods are homosporous with
mycorrhizal gametophytes.
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Lycopodium dendroideum (photo credit:
W. Judd)
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2. Order
Equisetales, Family Equisetaceae; Equisetum, commonly called horsetail or
scouring rush.
Horsetails are rhizomatous perennials with annual or perennial aerial stems.
They often are weedy and can form large colonies at the margins of wetlands
or even in dry sites. They have jointed stems that fragment easily, whorled
leaves, and cavities (or canals: central, carinal, and vallecular) that
extend throughout the stems. The sporangia are borne on specialized stalks
called sporangiophores (may be interpreted as modified sporophylls) and
aggregated into terminal cones (=strobili). The homosporous plants produce
only one type of sporangium. The spores of various species may germinate to
form hermaphroditic gametophytes or they may form both hermaphroditic and
male gametophytes. The spores are equipped with long slender structures
called elaters that respond to changes in humidity. Several spores often are
distributed together because their elaters become entangled.
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Equisetum sylvaticum (photo credit: C.
Campbell)
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Equisetum arvense (photo credit: W.
Judd)
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3.
Pteridophytes (several orders, many families, commonly called ferns).
Ferns are mostly herbaceous perennials and homosporous, and exhibit a wide
variety of habits, from the true aquatic condition to terrestrial to epiphytic
(epi=on, phyte=plant; growing on another plant but not deriving any nutrition
from it). They usually have rhizomes that give rise to simple or compound
leaves (=fronds) that expand from a circinate or coiled position. Although
most species produce one type of leaf that is both photosynthetic and
fertile, a few species produce two leaf types--sterile photosynthetic leaves
and fertile leaves. On the lower side of fertile leaves, there are clusters
of sporangia called sori (singular, sorus). Some ferns have a special flap of
tissue called an indusium that surrounds and presumably protects a sorus.
There are two types of sporangia found in ferns, eusporangia and
leptosporangia. Eusporangia are large and several cells thick, produce many
spores, and open (or dehisce) by a simple slit. Leptosporangia are small and
one cell thick, produce few spores, and open or dehisce usually by
specialized cells comprising an annulus. Evaporation puts an annulus under
tension, the cells then collapse, and the annulus opens and often forcibly
releases the spores. Fern spores are either trilete (tetrahedral) or monolete
(bean- shaped).
The morphological trends observed in ferns, from the ancestral or
plesiomorphic condition to the derived or apomorphic condition are:
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