THE LIVING WORLD
Unit Seven. Plant Life
32. Evolution of Plants
Once plants became established on land, they gradually developed many other features that aided their evolutionary success in this new, demanding habitat. For example, among the first plants there was no fundamental difference between the aboveground and the belowground parts. Later, roots and shoots with specialized structures evolved, each suited to its particular below- or aboveground environment. The evolution of specialized vascular tissue allowed plants to grow larger. For example, compare the size of a moss, a more primitive type of plant, to the more recently evolved tree on which it grows.
As we explore plant diversity, we will examine several key evolutionary innovations that have given rise to the wide variety of plants we see today. Table 32.1 provides you with an overview of the plant phyla and their characteristics. While other interesting and important changes also arose, four key innovations discussed in this chapter serve to highlight the evolutionary trends exhibited by the plant kingdom. These innovations lead to the evolution of the major plant groups, as shown in figure 32.4.
Figure 32.4. The evolution of plants.
TABLE 32.1. PLANT PHYLA
Four key evolutionary innovations serve to trace the evolution of the plant kingdom:
1. Alternation of generations. Although algae exhibit a haploid and a diploid phase, the diploid phase is not a significant portion of their life cycle. By contrast, even in early plants (the nonvascular plants indicated by the first vertical bar in figure 32.4), the diploid sporophyte is a larger structure and offers protection for the egg and developing embryo. The dominance of the sporophyte, both in size and the proportion of time devoted to it in the life cycle, becomes greater throughout the evolutionary history of plants.
2. Vascular tissue. A second key innovation was the emergence of vascular tissue. Vascular tissue transports water and nutrients throughout the plant body and provides structural support. With the evolution of vascular tissue, plants were able to supply the upper portions of their bodies with water absorbed from the soil and had some rigidity, allowing the plants to grow larger and in drier conditions. The first vascular plants were the seedless vascular plants, the second vertical bar in figure 32.4.
3. Seeds. The evolution of seeds (see section 32.6) was a key innovation that allowed plants to dominate their terrestrial environments. Seeds provide nutrients and a tough, durable cover that protects the embryo until it encounters favorable growing conditions. The first plants with seeds were the gymnosperms, the third vertical bar in figure 32.4.
4. Flowers and fruits. The evolution of flowers and fruits were key innovations that improved the chances of successful mating in sedentary organisms and facilitated the dispersal of their seeds. Flowers both protected the egg and improved the odds of its fertilization, allowing plants that were located at considerable distances to mate successfully. Fruit, which surrounds the seed and aids in its dispersal, allows plant species to better invade new and possibly more favorable environments. The angiosperms, the fourth vertical bar in figure 32.4, are the only plants to produce flowers and fruits.
Key Learning Outcome 32.2. Plants evolved from freshwater green algae and eventually developed more dominant diploid phases of the life cycle, conducting systems of vascular tissue, seeds that protected the embryo, and flowers and fruits that aided in fertilization and distribution of the seeds.