Plants I: The Evolution of Land Plants

This page is part of the lab Plants I, which includes these lab pages:

This is the first of several labs on plants; each lab will build on the previous one. In fact, it makes more sense to view this as one plant lab spread over several sessions. By the time you complete these labs, you should have a good start in understanding plant evolution, structure, and function.

This lab involves a lot of microscope use; you should also take a look at the microscopes page.

There will be several labs on plants. This one is designed to acquaint you with the major evolutionary groups of plants and their distinguishing characteristics. In later labs, you’ll take a closer look at some specific plant structures and how they work.

Objectives

Explain how terrestrial life imposes specific physiological and reproductive challenges for plants.
Compare and contrast algae, nonvascular plants, vascular non-seed plants, and vascular seed plants in terms of their anatomy and life histories and their various adaptations to life on land.

The other pages within this lab have additional objectives.

What to do in lab

You don’t need to turn anything in for this lab. Before lab, you should go through the pages listed above and get a sense of what this lab is about. Also, do the reading in Campbell (see the reading section below). During lab, look at the specimens listed in these pages and set out for you in lab. Your goal should be to become familiar with the specimens so you can recognize them and understand their significance. After you have looked at the specimens, go to the Plant Lab Review, where you’ll find some questions and vocabulary lists. Use these to test your knowledge. As you go over the review, you should probably go back and look at the specimens to make sure you see the connections. You don’t need to turn in answers to the review questions, but you will see questions like them on a lab exam and quiz.

Introduction

Not all photosynthetic organisms are plants. Photosynthesis first appeared in unicellular prokaryotic organisms, perhaps similar to green algae such as Chlamydomonas or Spirogyra. Photosynthesis first evolved in the water, but modern plants are highly adapted to life on land. In this lab you’ll see that the major groups of plants are defined by characteristics that make them increasingly suited to surviving and reproducing out of the water.

Challenges of life on land

Terrestrial plants must solve some problems that aquatic algae don't encounter. Algae only grow surrounded by water; therefore, each cell can absorb water, nutrients, and gases directly from the environment. Since most of the cells are more or less the same, each cell also absorbs its own sunlight to carry out photosynthesis. Even in large multicellular algae, each cell is capable of leading a somewhat independent existence.

This isn't true for terrestrial plants. Plants living on land must solve a particular series of problems imposed by their environment and their way of life:

Avoiding desiccation (water loss): This is the first big problem that must be solved for life out of the water. Vascular plants living in the air have a cuticle, which is a waxy layer on the outside that reduces evaporation. This reduces water loss but creates another problem: it blocks gas exchange.
Gas exchange: Algae or plants living in the water can simply absorb dissolved CO₂ from the water. However, the waxy cuticle blocks gas exchange for vascular land plants. This requires these plants to have additional adaptations for gas exchange, which you’ll encounter both in lecture and in the vascular plant structure lab.
Fertilization: Water provides a medium for getting reproductive cells together. For example, some algae have sperm that swim. For land plants, this is much more problematic: sperm cells can’t swim through the air, and they would dry out quickly if exposed to the air. Some land plants (such as mosses and ferns) rely on water to carry their sperm, and can reproduce only in wet conditions; vascular seed plants have adaptations that allow them to achieve fertilization even in dry conditions.
Obtaining nutrients: Land plants generally need specialized root structures for absorbing water and nutrients from the soil.
Transport: For large land plants, in which photosynthesis occurs in leaves while nutrient absorption happens in the roots, vascular tissue transports essential substances between different parts of the plant.
Support: Land plants typically must hold up their own weight, while aquatic algae are supported by the surrounding water.
Growth: Because vascular land plants have complex tissue and organ structures, growth must occur in an organized way to produce all the needed structures in an appropriate way. The growth process is far more complex in vascular plants than in algae, which are essentially masses of undifferentiated cells.
Protecting vulnerable phases of the life cycle: Single-celled stages are vulnerable; some land plants keep these protected, as you'll see in seed plant life cycles.

The plant labs of Bio 6A are about the specializations that make growth and reproduction possible for land plants. I’m dividing these specializations into two parts: Vascular plant structure and function: This theme addresses the ways that land plants use vascular tissue to transport materials from the roots and the leaves to the rest of the plant. In order to understand how this works, you also need to think about the problems of growth that vascular plants face, and how these problems are solved. Read Chapter xx of campbell and the Stems, Roots, and Leaves pages of this site. Plant life cycles. This theme is important because it shows how reproduction became possible on land, starting from ancestors that were purely aquatic. Start with Chapter 29 of Campbell and the Life Cycles page on this site.

One basic problem of life on land is that the water and nutrients are generally on or under the ground, while the sunlight comes from above. For this basic reason, plants have specialized tissues and organs to do specific jobs. At the most basic level, a land plant is divided into a root, for absorbing water and nutrients, and a shoot, for absorbing sunlight, performing photosynthesis, and doing reproduction. Since these functions are specialized in different regions of the plant, it is also necessary to have a mechanism for transporting substances from one part of the plant to another. You'll look closely at these structural specializations in Plants II, Vascular Plant Structure and Function.

Plant Cladogram

Plant cladogram

This cladogram provides a framework for understanding all the adaptations we will discuss in the plant labs. The ancestors of all plants, though long extinct, were similar to modern green algae. By the time you finish these labs, you should be able to place the major adaptations of terrestrial plants in their proper places on the branches of this cladogram.

Life cycles

Many of the defining differences among the major groups of photosynthetic organisms are related to the life cycle -- the series of events the organism must pass through in order to grow and reproduce. In order to understand many of the specimens you'll see in lab, you'll need to connect them with the organism's life cycle. To understand plant life cycles, you'll find it very helpful to read Chapter 29 (Plant Diversity I) in Campbell, paying particular to the life cycle diagrams and the explanation of alternation of generations.

Review

For review, go to the Plant Lab Review, which covers all three plant labs.

References & further reading

The essential reference:

Campbell Biology -- your textbook! It's not just for lecture. For this lab you should read (or at least skim) Chapter 29 -- Plant Diversity I: How Plants Colonized Land. There's a lot of information in that chapter; if you want to know what will be on the lab exam, you should rely on this website.