12.0.2 12.4: The evolution of vascular tissue made large plants possible.

Figure 12.11: Snapshot of the vascular seedless plants: ferns and horsetails.

What a difference some tubes make. That’s all vascular tissue is—an infrastructure of tubes that begins in a plant’s roots and extends up its stem and out to the tips of its leaves. The evolution of vascular tissue allowed early land plants to transport water and nutrients faster and more effectively than the cell-to-cell diffusion that non-vascular plants must rely on. Vascular plants’ roots penetrate far enough into the soil to reach moisture even when the soil surface is dry. Roots that reach deep into the soil also provide the support that a plant needs to grow upward without falling over. As a consequence, vascular plants can grow taller than non-vascular plants and are more successful in areas where the surface of the ground dries out between rains (FIGURE 12-11).

Ferns are the most familiar of the primitive vascular plants. During the Carboniferous period, which extended from 360 to 300 million years ago, ferns were a major component of the huge swamp forests that eventually formed the coal deposits we now mine. A related group of plants, the horsetails, like their Carboniferous ancestors, grow in wet habitats where the oxygen concentration around the roots is low because the spaces between soil particles are filled with water (rather than air). Horsetails developed hollow stems that allow oxygen from the air to diffuse down to the roots. The thin leaves of horsetails have a single nutrient- and water-carrying vessel extending from the base to the tip.

Simply having vessels, as horsetails do, is an important evolutionary innovation, but ferns have a further one. In addition to the central vessel in each leaflet, the leaves of ferns have vessels branching from the central vessel to the edges of the leaflet. This arrangement places a channel for the movement of water and nutrients close to each cell in the leaf.

Like non-vascular plants, horsetails and most ferns reproduce with spores. In horsetails, the central hollow stem is topped by a conical structure where the haploid spores are produced and released. Many ferns have sporangia (sing. sporangium) on the undersides of the leaves where spores are produced (FIGURE 12-12). Because the haploid gametophyte is much smaller and simpler than the diploid sporophyte, ferns are described as having a dominant sporophyte.

Figure 12.12: Haploid and diploid life stages in ferns. The photograph shows the sporangia, the spore-producing bodies, located on the underside of most fern leaves, on the diploid plant. The diploid and haploid plants in the fern life cycle are very different in appearance.

Horsetails and ferns are taller than non-vascular plants, so their spores can be blown by the wind when they are released, and they may settle some distance from the parent plant. This increased dispersal ability was an important adaptation—the non-vascular plants are so low-growing that wind can’t play much of a role in moving their spores.

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A spore that lands on moist soil grows into a tiny heart-shaped structure called a prothallus, which is the free-living haploid stage of a fern (see Figure 12-12). The prothallus produces the haploid gametes: some cells produce eggs and others produce sperm. A sperm “swims” through drops of rainwater to fertilize an egg, and the fertilized egg (a diploid zygote with two sets of chromosomes) grows into an adult fern.

TAKE-HOME MESSAGE MESSAGE 12.4

Vessels are an effective “circulatory system” to carry water and nutrients up from the soil to a plant’s leaves. The first vascular plants—including the earliest ferns and horsetails—were able to grow much taller than their non-vascular predecessors.

In what ways was the evolution of vascular tissue beneficial to plants?

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