The first major transformation was the evolution of alternation of generations (Fig. 33.1; Chapter 30). To understand this change, let’s first consider the presumed ancestral life cycle. The algal relatives of plants produce multicellular bodies made up entirely of haploid cells. These algae form gametes by mitosis, and fertilization results in a zygote, which is the only diploid cell. The algal ancestor of land plants is thought to have released male gametes (sperm) into the surrounding water and relied on water currents to carry the zygote away from the parent plant. The zygote then underwent meiosis, producing haploid cells that developed into new multicellular algae.

Like their green algal relatives, the first land plants would have produced a multicellular body composed entirely of haploid cells and that produces gametes. During periods of rain, the male gametes could swim to female gametes in surface water layers. But zygotes dispersed this way would not be able to travel very far from the parent plant. To enhance dispersal, land plants evolved alternation of generations: The multicellular haploid generation alternates with a multicellular generation composed of diploid cells. Specifically, the diploid zygote develops by mitosis into a multicellular spore-producing plant, called a sporophyte, while still attached to and supported by the haploid gamete-producing plant, called a gametophyte. In the first diverging groups of plants, the multicellular diploid generation produces many haploid spores by meiosis and, because it is typically erect, can release these spores into the air where they can be carried off by a breeze. Furthermore, because the walls of the spores contain sporopollenin, a polymer that is highly resistant to decay, the spores can survive prolonged exposure to air. Thus, the sporophyte generation and the formation of spores represent evolutionary innovations that enhance dispersal on land.

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The first plants lacked roots and thus would have depended entirely on surface water both for fertilization and to maintain the hydration of their cells. As a result, these plants would have been small, with their photosynthetic tissues and gamete-producing structures remaining in close contact with the ground. Photosynthesis would have taken place only when conditions were wet enough to keep cells well supplied with water. Only the sporophyte generation is likely to have extended above the surface, to increase the chances of spores being carried off by a breeze.

This situation radically changed with the second major event in the evolutionary history of plants: the evolution of vascular plants. These plants produce elongate cells for the internal transport of water and other materials. Xylem cells transport water and dissolved nutrients, and phloem cells transport carbohydrates produced by photosynthesis (Chapter 29). The cell walls of the xylem conduits contain lignin, a chemical that greatly strengthens the cellulose wall. Conduits made rigid by lignin allow vascular plants to pull water from the soil and to transport it efficiently through their stems. As a result, vascular plants are taller and able to photosynthesize over a much wider range of conditions than plants dependent solely on surface moisture. The phloem conduits that transport sugars allow roots to grow into the soil, where there is no sunlight to power photosynthesis.

In many vascular plants, xylem and phloem are formed only as stems and roots elongate; they cannot be added to an already formed stem. However, some vascular plants evolved the ability to produce additional or “secondary” xylem and phloem through the formation of a vascular cambium, a layer of actively dividing and differentiating cells that surrounds stems and allows them to increase in diameter (Chapter 31). A second layer of dividing cells, the cork cambium, maintains an intact layer of protective outer bark. Vascular and cork cambia provide the support and water transport capacity needed for plants to grow tall and to support increasing numbers of leaves. Thus, the evolution of secondary growth opened the way to the development of trees and forests.

As plants moved onto land, they retained their ancestral pattern of releasing swimming sperm into the environment. As a result, the gamete-producing generation remained small, constrained by the requirement for surface moisture for fertilization, even as the evolution of vascular tissues allowed the sporophyte to become large. These plants could only reproduce where and when surfaces were sufficiently wet. Eliminating this dependence—through the evolution of pollen and seeds—is the third major evolutionary event in the history of plant diversification.

The life cycle of seed plants differs from that of spore-dispersing vascular plants in a number of important ways (Chapter 30): (1) Spores are not dispersed; instead, they germinate and develop into morphologically distinct male and female gametophytes while still attached to the sporophyte. (2) Male and female gametes are brought together by the transport of the male gametophyte, which is so small that is fits within the spore wall, forming a pollen grain. (3) Following fertilization, the embryo and surrounding tissues develop into a seed, a multicellular structure that replaces unicellular spores as the dispersal unit. In seed plants, alternation of generations persists, but the dominant phase of the life cycle is the sporophyte, and pollen and seeds carry out tasks previously accomplished by swimming sperm and spores. The effect of this transformation is a life cycle that is freed from dependence on surface moisture for fertilization and in which embryos are dispersed along with resources that they can draw upon during germination.

The fourth evolutionary event is the evolution of the flowering plants, also referred to as the angiosperms. Angiosperms are seed plants and thus their life cycle contains all the traits just described. In addition, four new reproductive features are thought to have contributed to angiosperm diversity and success. The first is the flower, a reproductive structure that attracts animal pollinators, increasing the efficiency of pollen transfer compared to wind pollination. The second is the carpel—the closed “vessel” in which seeds develop and that gives angiosperms their name. Because pollen must grow through the carpel to reach the female gametophyte, interactions between pollen and carpel genotypes can affect the probability of fertilization. A third feature is double fertilization, in which one sperm nucleus from the male gametophyte fuses with the egg, and a second sperm nucleus fuses with two haploid nuclei from the female gametophyte. The first union results in the diploid zygote, and the second gives rise to endosperm, which forms the nutritive tissue within the seeds of angiosperms. A consequence of double fertilization is that angiosperms do not expend resources for the next generation until the egg is fertilized. A fourth feature is the development of fruits, structures that surround seeds and attract animals to enhance seed dispersal.

Angiosperms are also distinguished by the presence of wood containing xylem vessels (Chapter 29). Most other seed plants produce only tracheids, which both support the stem and transport water. In contrast, angiosperms have thick-walled, elongate fibers that support the stem and xylem vessels that transport water. Because xylem vessels are not a stem’s sole source of support, they can be wide and long, allowing angiosperms to transport water through their stems at higher rates than plants with tracheids. Their uptake of CO₂ for photosynthesis increases as well because CO₂ uptake is directly linked to water lost to the atmosphere. As a result, angiosperms compete well for light and space and are the dominant plants in most terrestrial ecosystems.

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