12.2–12.4

The first plants had neither roots nor seeds.

Uluhe ferns (Dicranopteris linearis) growing in Hawaii Volcanoes National Park.

The aquatic ancestors of land plants were green algae, which are classified by many taxonomists as part of the plant kingdom. Like the land plants, green algae are multicellular, photosynthetic eukaryotes, but they live only in water or on very moist land surfaces. As water dwellers, green algae do not require specialized structures to obtain water and nutrients; water simply enters their cells by osmosis, and the nutrients they require are in solution in the water that surrounds them. Some green algae, such as sea lettuce and stoneworts, even look like land plants. But there is enough uncertainty among taxonomists about the best classification of green algae that some classify them within the protist kingdom, and they consider some green algae that look like slime on rocks—organisms called coleochaetes (pronounced koh-lee-oh-keets)—to be the closest relatives of plants (FIGURE 12-5). An individual coleochaete is about the size of a pinhead and is only one cell-layer thick. Coleochaetes can withstand exposure to air, so they survive when the water level in a lake falls and leaves them high and dry. This resistance to drying out was the first evolutionary step that plants took as they moved from water to land.

Figure 12-5Plants’ closest relative: a green alga called a coleochaete.

The first land plants appeared about 472 million years ago. They weren’t impressive-looking, just some patches of low-growing green stems at the water’s edge. They did not have any of the structures we associate with plants today—no roots, leaves, or flowers. But from an evolutionary perspective, those early plants were enormously important because, until terrestrial plants evolved, there was nothing on land for other land organisms to eat. Thus, the first land plants not only set the stage for the tremendous diversity of plant life that we know today but also paved the way for the evolution and diversification of land animals.

As land plants emerged, they faced the same two challenges that were to confront the first terrestrial animals, some 25 million years later: supporting themselves against the pull of gravity and reducing evaporation so they didn’t dry out. The second of these problems was the more urgent. The earliest plants did not have to grow upward—they could creep along the ground—but they did have to avoid drying out (FIGURE 12-6). The material that protects all land plants from drying is a shiny, waxy layer on the stem and leaves called the cuticle.

Figure 12-6Leaving the water.