SEEDLESS VASCULAR PLANTS

The fossil record indicates that vascular plants developed during the Silurian period, approximately 400 million years ago. The earliest vascular plants were basically dichotomously branching stems (forking regularly into two branches) that reproduced by spores formed in sporangia. With subsequent development, the stems gave rise to both roots and leaves. Roots anchor the plant and absorb water and minerals. Leaves typically function as the major photosynthetic organ.

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.

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.


Lycopodium dendroideum (photo credit: W. Judd)

b. Family Selaginellaceae (Selaginella, commonly called selaginella or lesser clubmoss).
The selaginellas are creeping, evergreen herbs often occurring on rock outcrops. They are distinguished by the spirally arranged leaves with a small flap of tissue (ligule) on the upper surfaces and by heterospory with the male sporangia located above the female sporangia in the axils of the upper leaves (sporophylls). The female gametophyte is developed within the spore and derives nutrition from stored nutrients.


Selaginella plana (photo credit: W. Judd)

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.


Equisetum sylvaticum (photo credit: C. Campbell)


Equisetum arvense (photo credit: W. Judd)

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:

 

 

 

ANCESTRAL OR

 

DERIVED OR

PLESIOMORPHIC

 

APOMORPHIC

homospory

-->

heterospory

trilete spores

-->

monolete spores

many spores per sporangium

-->

few spores per sporangium

few sporangia

-->

--> many sporangia and clustered

weak line for dehiscence of

of sporangium

-->

--> specialized cells for

dehiscence (= annulus)

no indusium

-->

indusium

 


Osmunda cinnamonum
(photo credit: W. Judd)


Polypodium virginianum
(photo credit: W. Judd)