Chapter 29 Introduction

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CHAPTER 29

Plant Structure and Function

Moving Photosynthesis onto Land

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Frank Krahmer/Getty Images.

Core Concepts

  1. The evolution of land plants from aquatic ancestors introduced a major challenge for photosynthesis: acquiring carbon dioxide without drying out.
  2. Leaves have a waxy cuticle and stomata, both of which are important in regulating carbon dioxide gain and water loss.
  3. Xylem allows vascular plants to replace water evaporated from leaves with water pulled from the soil.
  4. Phloem transports carbohydrates to the non-photosynthetic portions of the plant for use in growth and respiration.
  5. Roots expend energy to obtain nutrients from the soil.

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In the ocean, where life first evolved, organisms are bathed in seawater that often matches the osmotic concentration of their cells. In contrast, organisms on land are surrounded by air and thus at risk of drying out. Excessive water loss, or desiccation, is a constant challenge for life on land, especially for photosynthetic organisms, which expose large surface areas to the air in order to obtain sunlight and carbon dioxide (CO2). In Chapter 27, we saw that land plants evolved from aquatic green algae. What structural and functional innovations enabled plants to colonize the land so successfully?

The story begins about 470 million years ago, when a group of green algae began the transition from water to land. There are few fossils to show us what the first land plants looked like. We know they were small, at most only a few centimeters tall. These early colonists resembled their algal ancestors; they had no means of obtaining water from the soil and, at best, only a limited capacity to restrict water loss from cells. Therefore, their water content would have fluctuated wildly. Water is the medium for life, and metabolism is possible only when the water content of cells is high. Thus, these first land plants would have been able to carry out photosynthesis only when the surface of the land was wet.

Today, descendants of those first land plants dominate terrestrial habitats, having evolved the ability to draw water from the soil and limit water loss from their leaves. These are the vascular plants, and their evolution transformed the physical and biological environment on land. Vascular plants can carry out photosynthesis even when the soil surface is dry and can sustain the water content of photosynthetic leaves elevated as much as 100 m into the air. In many ways, the capacity to control the uptake and loss of water made possible the extraordinary evolutionary success of vascular plants.