Adapting To Land

Although life had flourished in the oceans for more than 3 billion years, no organisms lived on land until about 475 million years ago, when a layer of ozone formed. The ozone protected organisms from the sun's ultraviolet radiation. Eventually, small club-shaped plants began to grow in the mud at the water's edge. Three adaptations allowed plants to thrive on land: the ability to prevent water loss, the ability to reproduce in the absence of water, and the ability to absorb and transport nutrients.

Preventing Water Loss

The move to land offered plants distinct advantages, including more exposure to sunlight for photosynthesis, increased carbon dioxide levels, and a greater supply of inorganic nutrients. However, the land environment also presented challenges. Plants on land are susceptible to drying out through evaporation.

The cuticle (KYOOT-i-kuhl), a waxy protective covering on plant surfaces that prevents water loss, was one early adaptation to life on land. Although it protects a plant by keeping water in the plant, the cuticle also keeps out carbon dioxide. Plants that had small openings in their surfaces, called stomata, were able to survive. Stomata allow the exchange of carbon dioxide and oxygen.

Reproducing by Spores and Seeds

Successful land plants also developed structures that helped protect reproductive cells from drying out. A spore is a haploid reproductive cell surrounded by a hard outer wall. Spores allowed the widespread dispersal of plant species. Eventually, most plants developed seeds. A seed is an embryo surrounded by a protective coat. Some seeds also contain endosperm, a tissue that provides nourishment for the developing plant. Figure 28-1 shows the unusual adaptation of the sugar maple tree for seed dispersal. Seeds are more effective at dispersal than spores are.

objectives

• Name three adaptations plants have made to life on land.

• Summarize the classification of plants.

• Describe alternation of generations.

vocabulary cuticle spore seed vascular tissue xylem phloem nonvascular plant vascular plant seed plant gymnosperm angiosperm sporophyte gametophyte alternation of generations

figure 28-1

The seeds of a sugar maple tree are found inside a winged fruit. Wind can carry the winged fruit away from the tree, thus helping the seeds to disperse away from the parent plant.

figure 28-1

The seeds of a sugar maple tree are found inside a winged fruit. Wind can carry the winged fruit away from the tree, thus helping the seeds to disperse away from the parent plant.

Absorbing and Transporting Materials

Aquatic plants take nutrients from the water around them. On land, most plants absorb nutrients from the soil with their roots. Although the first plants had no roots, fossils show that fungi lived on or within the underground parts of many early plants.

Certain species of plants evolved a type of tissue known as vascular (VAS-kyuh-luhr) tissue, which transports water and dissolved substances from one part of the plant to another. Two types of specialized tissue make up vascular tissue. Xylem (ZIE-luhm) carries absorbed water and inorganic nutrients in one direction, from the roots to the stems and leaves. Phloem (FLOH-em) carries organic compounds, such as carbohydrates, and some inorganic nutrients in any direction, depending on the plant's needs. In addition to transporting absorbed materials, vascular tissue also helps support the plant, which is an important function for land plants.

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