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The evolution of an efficient vascular system allowed vascular plants to grow larger than their ancestors. There are two types of vascular tissue in vascular plants: xylem and phloem. Xylem is a woody tissue that conducts water and minerals from the soil up through the roots to the shoot of the plant—the part that’s above ground. Xylem also makes the plant more rigid and helps support it against the force of gravity. Phloem conducts dissolved organic nutrients from one part of the plant to another. Leaves are the organs of vascular plants that act as sunlight collectors and are the site of most photosynthesis. Stems support the organs of the shoot and are pathways for fluid flow.

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The earliest fossils of vascular plants date to the Silurian period, over 400 million years ago. These fossils reveal plants with branching stems, but with no roots or leaves. They were anchored in the soil by a horizontal stem called a rhizome. These plants had sporangia at the tips of their stems. Like modern vascular plants, the dominant generation of the first known vascular plants was the sporophyte.

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The sporophyte is the dominant generation in all vascular plants. In some types of vascular plants, the gametophyte is a small free-living organism. In others, the gametophyte remains dependent on the sporophyte for most or all of its lifetime.

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Seedless vascular plants formed dense forests during the Carboniferous period, about 300 million years ago. Their fossilized deposits were converted by the heat and pressure of the Earth into extensive coal beds, important sources of fuel since the Industrial Revolution. Today, the most familiar seedless vascular plants are the ferns.

Many ferns are recognizable by their finely divided leaves, or fronds. Horsetails and lycophytes were important members of the forest community in the Carboniferous period; some grew to the size of giant trees. Today few species of these plants remain. They can be recognized by a cone-shaped organ called a strobilus, which bears spores. Some lycophytes are commonly called club mosses, although they are not true mosses.

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Let’s look at the life cycle of a fern as a representative seedless vascular plant. Like bryophytes, seedless vascular plants require liquid water for sexual reproduction. Their flagellated sperm, produced in a male gametangium, must swim to an egg in a female gametangium in order to fertilize the egg. The young sporophyte that develops from the resulting embryo takes root and grows into the dominant sporophyte. Clusters of sporangia form on some of the fronds of the sporophyte fern. They release spores, which can travel great distances on the wind. In favorable conditions, the spores germinate and develop into new gametophytes, allowing the cycle to continue.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education