Non-vascular land plants The non-vascular land plants, namely the mosses (Bryophyta),
hornworts (Anthocerotophyta), and
liverworts (Marchantiophyta), are relatively small plants, often confined to environments that are humid or at least seasonally moist. They are limited by their reliance on water needed to disperse their
gametes; a few are truly aquatic. Most are tropical, but there are many arctic species. They may locally dominate the ground cover in
tundra and Arctic–alpine habitats or the epiphyte flora in rain forest habitats. They are usually studied together because of their many similarities. All three groups share a
haploid-dominant (
gametophyte) life cycle and unbranched
sporophytes (the plant's
diploid generation). These traits appear to be common to all early diverging lineages of non-vascular plants on the land. Their life-cycle is strongly dominated by the haploid gametophyte generation. The sporophyte remains small and dependent on the parent gametophyte for its entire brief life. All other living groups of land plants have a life cycle dominated by the diploid sporophyte generation. It is in the diploid sporophyte that vascular tissue develops. In some ways, the term "non-vascular" is a misnomer. Some mosses and liverworts do produce a special type of vascular tissue composed of complex water-conducting cells. However, this tissue differs from that of "vascular" plants in that these water-conducting cells are not lignified. It is unlikely that the water-conducting cells in mosses are homologous with the vascular tissue in "vascular" plants. It was assumed that the gametophyte dominant phase seen in bryophytes used to be the ancestral condition in terrestrial plants, and that the sporophyte dominant stage in vascular plants was a derived trait. However, the gametophyte and sporophyte stages were probably equally independent from each other, and that the mosses and vascular plants in that case are both derived, and have evolved in opposite directions. During the Devonian period, vascular plants diversified and spread to many different land environments. In addition to vascular tissues which transport water throughout the body, tracheophytes have an outer layer or cuticle that resists drying out. The sporophyte is the dominant generation, and in modern species develops leaves, stems and roots, while the gametophyte remains very small.
Lycophytes and euphyllophytes All the vascular plants which disperse through spores were once thought to be related (and were often grouped as 'ferns and allies'). However, recent research suggests that leaves evolved quite separately in two different lineages. The lycophytes or lycopodiophytes – modern clubmosses, spikemosses and quillworts – make up less than 1% of living vascular plants. They have small leaves, often called 'microphylls' or 'lycophylls', which are borne all along the stems in the clubmosses and spikemosses, and which effectively grow from the base, via an intercalary
meristem. It is believed that microphylls evolved from outgrowths on stems, such as spines, which later acquired veins (vascular traces). Although the living lycophytes are all relatively small and inconspicuous plants, more common in the moist tropics than in temperate regions, during the
Carboniferous period tree-like lycophytes (such as
Lepidodendron) formed huge forests that dominated the landscape. The euphyllophytes, making up more than 99% of living vascular plant species, have large 'true' leaves (megaphylls), which effectively grow from the sides or the apex, via marginal or apical meristems. Others have questioned whether megaphylls evolved in the same way in different groups.
Ferns and horsetails The ferns and horsetails (the Polypodiophyta) form a clade; they use spores as their main method of dispersal. Traditionally, whisk ferns and horsetails were historically treated as distinct from 'true' ferns. Living whisk ferns and horsetails do not have the large leaves (megaphylls) which would be expected of euphyllophytes. This has probably resulted from reduction, as evidenced by early fossil horsetails, in which the leaves are broad with branching veins. Ferns are a large and diverse group, with some 12,000
species. A stereotypical fern has broad, much divided leaves, which grow by unrolling.
Seed plants ,
Aesculus hippocastanum Seed plants, which first appeared in the fossil record towards the end of the
Paleozoic era, reproduce using
desiccation-resistant capsules called seeds. Starting from a plant which disperses by spores, highly complex changes are needed to produce seeds. The sporophyte has two kinds of spore-forming organs or sporangia. One kind, the megasporangium, produces only a single large spore, a megaspore. This sporangium is surrounded by sheathing layers or integuments which form the seed coat. Within the seed coat, the megaspore develops into a tiny gametophyte, which in turn produces one or more egg cells. Before fertilization, the sporangium and its contents plus its coat is called an ovule; after fertilization a seed. In parallel to these developments, the other kind of sporangium, the microsporangium, produces microspores. A tiny gametophyte develops inside the wall of a microspore, producing a pollen grain. Pollen grains can be physically transferred between plants by the wind or animals, most commonly insects. Pollen grains can also transfer to an ovule of the same plant, either with the same flower or between two flowers of the same plant (
self-fertilization). When a pollen grain reaches an ovule, it enters via a microscopic gap in the coat, the micropyle. The tiny gametophyte inside the pollen grain then produces sperm cells which move to the egg cell and fertilize it. Seed plants include two clades with living members, the
gymnosperms and the
angiosperms or flowering plants. In gymnosperms, the ovules or seeds are not further enclosed. In angiosperms, they are enclosed within the carpel. Angiosperms typically also have other, secondary structures, such as
petals, which together form a flower.
Meiosis in sexual land plants provides a direct mechanism for
repairing DNA in reproductive tissues. Sexual reproduction appears to be needed for maintaining long-term
genomic integrity and only infrequent combinations of extrinsic and intrinsic factors allow for shifts to asexuality. ==References==