Chapter 33 Introduction

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CHAPTER 33

Plant Diversity

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Gaby Wojciech/age fotostock.

Core Concepts

  1. Angiosperms make up approximately 90% of all plant species found today.
  2. Bryophytes form persistent, photosynthetic gametophytes and small, unbranched sporophytes; today they grow in environments where the ability to pull water from the soil does not provide an advantage.
  3. Spore-dispersing vascular plants today are primarily small plants that grow in moist environments, but in the past included tall trees.
  4. Gymnosperms produce seeds and woody stems and are most common in seasonally cool or dry regions.
  5. Angiosperms are distinguished by flowers, fruits, double fertilization, and xylem vessels; their diversity is the result of traits that increase the efficiency of completing their life cycle and building their bodies.

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Five hundred million years ago, the land surface supported little more than a green crust made up of a mixture of photosynthetic and non-photosynthetic microbes. Then, through the process of natural selection, a group of freshwater algae began the transition to survival on land. Fast-forward 465 million years—to the present time—and plants are everywhere. Much of what makes up a plant, including roots, leaves, tree trunks, and seeds, evolved quickly. Within the first quarter of the time since these algal ancestors began to colonize the land, all of these features were present. But one thing was missing: flowers. Flowering plants, also known as angiosperms, do not appear on the scene until the last quarter of this history, and yet today they make up a remarkable 90% of all plant species. Many of the plant groups that diverged before angiosperms appeared are still present, but where they live and grow has been shaped by the evolution of flowering plants.