![]() Accompanying the prominence of the sporophyte and the development of vascular tissue, the appearance of true leaves improved their photosynthetic efficiency. Diagrams of xylem and phloem tissuesĪ third innovation marks the seedless vascular plants. Together, xylem and phloem tissues form the vascular system of plants. Phloem cells are divided into sieve elements (conducting cells) and cells that support the sieve elements. Phloem is the second type of vascular tissue it transports sugars, proteins, and other solutes throughout the plant. By growing higher than other plants, tall trees cast their shadow on shorter plants and limit competition for water and precious nutrients in the soil. Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds further away, thus expanding their range. With their rigid cell walls, the xylem cells provide support to the plant and allow it to achieve impressive heights. Lignin itself is a complex polymer that is impermeable to water and confers mechanical strength to vascular tissue. Xylem conductive cells incorporate the compound lignin into their walls, and are thus described as lignified. ![]() The tissue consists of conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. 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 simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem. The first fossils that show the presence of vascular tissue date to the Silurian period, about 430 million years ago. This step in reproduction explains why ferns and their relatives are more abundant in damp environments. Seedless vascular plants still depend on water during fertilization, as the sperm must swim on a layer of moisture to reach the egg. Throughout plant evolution, there is an evident reversal of roles in the dominant phase of the lifecycle. The gametophyte is now an inconspicuous, but still independent, organism. In seedless vascular plants, the diploid sporophyte is the dominant phase of the lifecycle. Photosynthetic organs become leaves, and pipe-like cells or vascular tissues transport water, minerals, and fixed carbon throughout the organism. Vascular plants, on the other hand, can achieve enormous heights, thus competing successfully for light. Although they may survive in reasonably dry conditions, they cannot reproduce and expand their habitat range in the absence of water. 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. Several evolutionary innovations explain their success and their ability to spread to all habitats.īryophytes 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. More than 260,000 species of tracheophytes represent more than 90 percent of Earth’s vegetation. The vascular plants, or tracheophytes, are the dominant and most conspicuous group of land plants. Identify the main characteristics of seedless vascular plants.Differentiate between vascular and non-vascular plants.The Importance of Seedless Vascular Plants. ![]() Phylum Monilophyta: Class Psilotopsida (Ferns).Phylum Monilophyta: Class Psilotopsida (Whisk Ferns).Phylum Monilophyta: Class Equisetopsida (Horsetails).Ferns and Other Seedless Vascular Plants.
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