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Learning Goals
By the end of this reading you should be able to:
- Identify the adaptations to life on land that appear in seedless vascular plants
- Describe the classes of seedless tracheophytes
- Compare the lifecycle of seedless nonvascular and seedless vascular plants
- Explain the role of seedless plants in the ecosystem
Introduction
The vascular plants (also called tracheophytes) are the dominant and most conspicuous group of land plants. The 260,000+ species of tracheophytes compose greater than 90 percent of Earth’s vegetation. Several evolutionary innovations explain their success and their ability to spread to all habitats.
Bryophytes may have been successful at the transition from an aquatic habitat to land, but they are still dependent on water for reproduction and absorb moisture and nutrients through the gametophyte surface. The lack of roots for absorbing water and minerals from the soil, as well as a lack of reinforced conducting cells, limits bryophytes to small sizes. Although they may survive in reasonably dry conditions, they cannot reproduce and expand their habitat range in the absence of water. Vascular plants, on the other hand, can achieve enormous heights, thus competing successfully for light. In this group, photosynthetic organs evolved to become leaves, and pipe-like cells evolved into vascular tissues that transport water, minerals, and fixed carbon throughout the organism.
One group of tracheophytes is the seedless vascular plants, in which the diploid sporophyte is the dominant phase of the lifecycle. The gametophyte is this group is an inconspicuous, but still independent, organism. Throughout plant evolution, there is a shift of roles in the dominant phase of the lifecycle; gametophytes in the non-vascular plants to sporophytes in the vascular plants. Like nonvascular plants, the seedless vascular plants still depend on water during fertilization, as the sperm must swim on a layer of moisture to reach the egg. This characteristic limits both of the groups to mainly moist environments.
Adaptations for Life on Land
Xylem
Xylem is the tissue responsible for the storage and long-distance transport of water and nutrients, as well as the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of conducting cells, known as tracheids, and supportive filler tissue called parenchyma (Fig. 1). When xylem cells are mature they are dead, meaning that the internal membrane and structures are lost, leaving just an empty tube. Xylem conductive cells incorporate the compound lignin into their walls and are thus described as lignified. Lignin is a complex polymer that is impermeable to water which “seals’ the dead xylem cells and confers mechanical strength to vascular tissue. With their rigid cell walls, xylem cells provide support to the plant and allow the plant to grow upwards against the pull of gravity. Taller plants often have a selective advantage by being able to