Chapter 27: Seedless Plants

Adaptations of Plants to Life on Land

• The colonization of land by plants required the evolution of many anatomical, physiological, and reproductive adaptations.
  • Plants have a waxy cuticle to protect against water loss and stomata for gas exchange needed for photosynthesis.
• Plant life cycles have an alternation of generations in which they spend part of their life cycle in a multicellular haploid gametophyte generation and part in a multicellular diploid sporophyte generation.
  • The gametophyte plant produces gametes by mitosis.
  • During fertilization these gametes fuse to form a zygote, the first stage of the sporophyte generation.
  • The zygote develops into a multicellular embryo that the gametophyte protects and nourishes.
  • The mature sporophyte plant develops from the embryo and produces sporogenous cells (spore mother cells).
  • These cells undergo meiosis to form spores, the first stage in the gametophyte generation.
• Most plants have multicellular gametangia with a protective jacket of sterile cells surrounding the gametes.
  • Antheridia are gametangia that produce sperm cells, and archegonia are gametangia that produce eggs.
• Ferns and other vascular plants have xylem to conduct water and dissolved minerals and phloem to conduct dissolved sugar.
• Plants probably arose from a group of green algae called charophytes.
  • This conclusion is based in part on molecular comparisons of DNA and RNA sequences, which show the closest match between charophytes and plants.

Bryophytes

• Unlike other land plants, bryophytes are nonvascular and lack xylem and phloem.
  • Bryophytes are the only plants with a dominant gametophyte generation.
  • Their sporophytes remain permanently attached and nutritionally dependent on the gametophytes.
• Mosses (phylum Bryophyta) have gametophytes that are green plants that grow from a filamentous protonema.
• Many liverworts (phylum hepatophyta) have gametophytes that are flattened, lobelike thalli; others are leafy.
• Hornworts (phylum Anthocerophyta) have thalloid gametophytes.
• The green moss gametophyte bears archegonia, antheridia, or both at the top of the plant.
  • During fertilization, a sperm cell fuses with an egg cell in the archegonium.
  • The zygote grows into an embryo that develops into a moss sporophyte, which is attached to the gametophyte.
  • Meiosis occurs within the capsule of the sporophyte to produce spores.
  • When a spore germinates, it grows into a protonema that forms buds that develop into gametophytes.

Seedless Vascular Plants

• Seedless vascular plants have several adaptations that algae and bryophytes lack, including vascular tissues and a dominant sporophyte generation.
  • As in bryophytes, reproduction in seedless vascular plants depends on water as a transport medium for motile sperm cells.
• Sporophytes of club mosses (phylum Lycopodiophyta) consist of roots, rhizomes, erect branches, and leaves that are microphylls.
• Ferns (phylum Pteridophyta) are the largest and most diverse group of seedless vascular plants.
  • The fern sporophyte consists of a rhizome that bears fronds and true roots.
  • Phylum Pteridophyta also includes whisk ferns and horsetails.
  • Sporophytes of whisk ferns have dichotomously branching rhizomes and erect stems; they lack true roots and leaves.
  • Horsetail sporophytes have roots, rhizomes, aerial stems that are hollow and jointed, and leaves that are reduced megaphylls.
• Fern sporophytes have roots, rhizomes, and leaves that are megaphylls.
  • Their leaves, or fronds, bear sporangia in clusters called sori.
  • Meiosis in sporangia produces haploid spores.
  • The fern gametophyte, called a prothallus, develops from a haploid spore and bears both archegonia and antheridia.
• Homospory, the production of one kind of spore, is characteristic of bryophytes, most club mosses, and most ferns, including whisk ferns and horsetails.
  • In homospory spores give rise to gametophyte plants that produce both egg cells and sperm cells.
• Heterospory, the production of two kinds of spores (microspores and megaspores), occurs in certain club mosses, certain ferns, and all seed plants.
  • Microspores give rise to male gametophytes that produce sperm cells.
  • Megaspores give rise to female gametophytes that produce eggs.
  • The evolution of heterospory was an essential step in the evolution of seeds.