Evolution in Plants
Five hundred million years ago, the invasion of land was under way. Why then? Astronomical numbers of photosynthetic cells had come and gone, and oxygen-producing types had changed the atmosphere. High above Earth, the sun’s energy had converted much of the oxygen into a dense ozone layer, a shield against lethal doses of ultraviolet radiation. Until then, life had not ventured above the surface of water and mud.
Algae were evolving at the water’s edge, and one group – probably the charophytes – gave rise to plants. Cooksonia, a simple branching plant a few centimeters tall, evolved by 430 million years ago. It took another 160 million years for the taller Psilophyton to evolve. Then the evolutionary pace picked up. It took only 60 million years for plants radiate from the swampy lowlands to high mountains and nearly all places in between. They did so through modifications in their structure, function, and reproductive modes.
Roots, Stems, and Leaves
Underground absorptive structures evolved as plants colonized the land, and in some lineages they became systems of roots. Most roots had a large surface area that helped plants absorb more water and dissolved mineral ions. As they do today, young roots probably associated with fungal mycorrhizal symbionts, which help plants obtain water and dissolve nutrients. In many lineages, older roots started anchoring the plant.
Above ground, shoot systems evolved. Their stems and leaves captured energy form the sun and carbon dioxide from the air. Stems became erect, taller, and branched when plants developed a capacity to make and incorporate lignin, a glue-like polymer, in their cell walls. Lignin-strengthened walls supported the stems as they grew upward and outward, in patterns that increased the light-intercepting surface of leaves.
Vascular tissues, xylem and phloem, first appeared in plants called rhyniophytes. As happens now, xylem distributed water and mineral ions through the plant;
Algae were evolving at the water’s edge, and one group – probably the charophytes – gave rise to plants. Cooksonia, a simple branching plant a few centimeters tall, evolved by 430 million years ago. It took another 160 million years for the taller Psilophyton to evolve. Then the evolutionary pace picked up. It took only 60 million years for plants radiate from the swampy lowlands to high mountains and nearly all places in between. They did so through modifications in their structure, function, and reproductive modes.
Roots, Stems, and Leaves
Underground absorptive structures evolved as plants colonized the land, and in some lineages they became systems of roots. Most roots had a large surface area that helped plants absorb more water and dissolved mineral ions. As they do today, young roots probably associated with fungal mycorrhizal symbionts, which help plants obtain water and dissolve nutrients. In many lineages, older roots started anchoring the plant.
Above ground, shoot systems evolved. Their stems and leaves captured energy form the sun and carbon dioxide from the air. Stems became erect, taller, and branched when plants developed a capacity to make and incorporate lignin, a glue-like polymer, in their cell walls. Lignin-strengthened walls supported the stems as they grew upward and outward, in patterns that increased the light-intercepting surface of leaves.
Vascular tissues, xylem and phloem, first appeared in plants called rhyniophytes. As happens now, xylem distributed water and mineral ions through the plant;