Fossil evidence indicates that the first vascular plants (tracheophytes) were simple, dichotomously branched organisms about 5–10 cm in height. In the earliest vascular plants, the gametophyte and sporophyte are believed to have been free-living and approximately equal in size. These early land plants had specialized vascular tissue for conducting water, sugar and minerals (see Sections 1.8.2–1.8.4). Morphological and molecular evidence supports the division of living vascular plants into three clades: lycophytes, ferns and allied taxa (monilophytes) and seed plants (lignophytes) (see Figure 1.3). Within the vascular plant clade, we shall see a progressive reduction in the size and life span of the gametophyte generation and an increase in the size and importance of the sporophyte generation. Plants that are recognized as ferns, pine trees and flowering plants are diploid sporophytes. We will first examine the vascular spore plants, the lycophytes and the monilophytes. In these two groups, although sporophytes are more prominent, both sporophytes and gametophytes are free-living, independent plants. In both groups, sporophytes produce spores that are wind dispersed.
Extant lycophytes form a distinct clade that split from other tracheophytes about 400 mybp. Today they are few in number, about 1200 species, but during the Carboniferous period, diverse lycophytes dominated the landscape and gave rise to coal deposits. Extinct members of this clade include large trees that produced woody trunks by processes similar to those found in woody seed plants. Living lycophytes are represented by the club mosses (e.g. Lycopodium), the spike mosses, commonly found in the tropics (e.g. Selaginella), and the quillworts (e.g. Isoetes) (Figure 1.9). Unlike bryophytes, lycophyte sporophytes have true roots, stems and leaves. Their leaves are small and have a single vein. Sporangia, borne on specialized leaves, produce wind-dispersed spores. Gametophytes of lycophytes are independent plants that have a simple morphology and lack vascular tissue. They produce flagellate sperm that swim through a film of water to reach eggs.
The monilophytes, which arose more than 360 mybp, include the ferns (Ophioglossaceae, Marattiales and Polypodiales), horsetails (Equisetales) and whisk ferns (Psilotaceae) (Figure 1.10). Along with the lycophytes, the ferns and allied groups contributed to the formation of coal during the Carboniferous period. Today, although they have limited economic value, they play important ecological roles. Ferns are numerous (exceeding 11 000 species) and abundant, especially in the tropics.
The phylogenetic relationships among monilophytes have been inferred from gene sequences. When only morphological characters were available for comparison, systematists did not consider these plants monophyletic. For example, the leafless, rootless whisk ferns (e.g. Psilotum) were thought to be more primitive than the lycophytes, but molecular phylogeny places them firmly in the monilophyte clade.
Although some fern orders (e.g. Salviniales) are aquatic, most ferns are terrestrial. As in lycophytes, the fern life cycle includes a large sporophyte generation and an inconspicuous, though independent, gametophyte (Figure 1.11). Fern sporophytes are well adapted to land; their above-ground parts are covered by a cuticle and an epidermis in which stomata are embedded. Leaves are generally large, multiveined and may be highly divided. They are often the only above-ground part of the fern plant. Most ferns have a horizontal underground stem (a rhizome) and a complex root system. Fern sporophytes produce wind-dispersed, haploid spores in sporangia borne on the undersides of leaves. Spores germinate to form haploid gametophytes that may have male and female gametangia on the same plant or on separate gametophytes. In a few species, sporophytes produce two types of sporangia: megasporangia that make large spores (megaspores) and microsporangia that make small spores (microspores). Megaspores develop into female gametophytes and microspores develop into male gametophytes.
Fern gametophytes are photosynthetic. They are small, rarely more than 1 cm in diameter, and usually only one to two cells thick. Gametophytes lack cuticle, vascular tissue and true organs; simple unicellular rhizoids anchor them to the substrate. They can only survive in damp areas. Gametophytes produce eggs in archegonia and flagellated sperm in antheridia. The sperm swim through a layer of water to reach the egg. After fertilization, the diploid zygote develops into an embryonic sporophyte within an archegonium of the gametophyte. As the young sporophyte produces leaves, stems and roots, it overgrows the parent gametophyte.
Sporophytes of vascular spore-producing plants (lycophytes, ferns and allied groups) are well adapted to life on dry land. However, their gametophytes remain tied to wet areas by their anatomy and the fact that fertilization requires water. Because sporophytes begin life attached to gametophytes, they must start life in moist areas. The problem of a vulnerable gametophyte was overcome when the seed plants evolved.
Living seed plants are divided two monophyletic clades, the gymnosperms, including five major lineages—the cycads, the pine family, other conifers, gnetophytes and Ginkgo—and the angiosperms (the flowering plants) (see Figure 1.3). The gymnosperm lineages contain more than 800 extant species. Flower-bearing angiosperms are by far the largest seed plant lineage, including more than 254 000 species. This number is probably an under- estimate because it is likely that many members of this clade await discovery.
Seed plants provide many key resources, including food, lumber, fiber and fuel. Given their significance in our lives and their centrality to this book, we will describe in detail their reproduction, structure and development. First, we will examine the phylogeny and reproductive biology of gymnosperms and angiosperms separately. Later, in Sections 1.7–1.10, we discuss the anatomy and development of angiosperms noting similarities and differences with gymnosperms.
Seed plant sporophytes produce two kinds of sporangia: ovules (megasporangia) and pollen sacs (microsporangia). Spores produced in these sporangia are not released, but divide in situ to produce gametophytes. Ovules enclose female gametophytes; pollen sacs contain male gametophytes, called pollen.
Three major reproductive advances in adapting to life on land are found in seed plants. First, haploid gametophytes are reduced in size and are protected within sporangia on the parent sporophyte. Second, seed plants no longer require a film of water for fertilization. The male gametophyte, a pollen grain, develops a protective coat and is delivered to the vicinity of the female gametophyte by wind or by animal pollinators. Finally, a new dispersal stage, the seed has evolved. A seed, with its protective seed coat, contains a new sporophyte with a source of food. We shall see below how each of these features is incorporated into the life cycles of first a gymnosperm and then an angiosperm.
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