Lesson Overview 22.1 What is a Plant?

Characteristics of Plants You are familiar with many examples of plants such as trees, shrubs, and grasses. But did you know that mosses and ferns are plants too? Also, recently biologists have reclassified green algae as plants. What characteristics do all these organisms share?

The Plant Kingdom Plants are classified as members of the kingdom Plantae.   Plants are eukaryotes that have cell walls containing cellulose and carry out photosynthesis using chlorophyll a and b. While most plants are autotrophs, a few are parasites or saprobes (an organism that derives its nourishment from nonliving or decaying organic matter).

What Plants Need All plants have the same basic needs: sunlight, a way to exchange gases with the surrounding air, water, and minerals.

Sunlight Plants use the energy from sunlight to carry out photosynthesis. So every plant displays adaptations shaped by the need to gather sunlight.   Leaves are photosynthetic organs that are typically broad and flat and are arranged on the stem so as to maximize light absorption.

Gas Exchange Plants require oxygen to support cellular respiration, as well as carbon dioxide to carry out photosynthesis. They also need to release excess oxygen made during photosynthesis.   Plants must exchange these gases with the atmosphere and the soil without losing excessive amounts of water through evaporation.

Water and Minerals Plants can lose a lot of water to the air on hot, sunny days. Water is one of the raw materials of photosynthesis, so it is consumed when the sun is shining. Thus, land plants have evolved structures that limit water loss and speed the uptake of water from the ground. As they absorb water, plants also absorb minerals.   Minerals are nutrients in the soil that are needed for plant growth.

Water and Minerals Many plants have specialized tissues that carry water and nutrients upward from the soil and distribute the products of photosynthesis throughout the plant body. Simpler types of plants carry out these functions by diffusion.

The History and Evolution of Plants How did plants adapt to life on land?   For most of Earth’s history, land plants did not exist. Life was concentrated in oceans, lakes, and streams. Although photosynthetic prokaryotes added oxygen to Earth’s atmosphere and provided food for animals and microorganisms, true plants had not yet appeared on the planet.

Origins in the Water The ancestors of today’s land plants were water-dwelling organisms similar to today’s green algae. Most of these photosynthetic eukaryotes were unicellular, although a few were composed of multiple cells.   Although not as large and complex as many plants, green algae have cell walls and photosynthetic pigments that are identical to those of plants. Green algae also have reproductive cycles that are similar to plants. Studies of the genomes of green algae suggest that they are so closely related to other plants that they should be considered part of the plant kingdom.

The First Land Plants Fossil spores of land plants occur in rocks 475 million years old, but the plants themselves from this time period left no fossils. The oldest fossils of land plants themselves are found roughly 50 million years later in the fossil record. Lacking leaves and roots, these plants were only a few centimeters tall.

The First Land Plants The greatest challenge that early land plants faced was obtaining water. They met this challenge by growing close to the ground in damp locations.   Fossils suggest the first true plants were still dependent on water to complete their life cycles. One of the earliest fossil vascular plants was Cooksonia, shown here. Over time, the demands of life on land favored the evolution of plants more resistant to the drying rays of the sun, more capable of conserving water, and more capable of reproducing without water.

The First Land Plants As plants colonized the land, they changed the environment in ways that enabled new species to evolve. New ecosystems emerged, and organic matter began to form soil. Several groups of plants evolved from the first pioneering land plants.   One group developed into mosses. Another lineage gave rise to ferns, cone-bearing plants, and flowering plants.

An Overview of the Plant Kingdom Botanists divide the plant kingdom into five major groups based on four important features: embryo formation, specialized water-conducting tissues, seeds, and flowers. The relationship of plant groups is shown below.

The Plant Life Cycle Land plants have a distinctive sexual life cycle that sets them apart from most other living organisms. The life cycle of land plants has two alternating phases, a diploid (2N) phase and a haploid (N) phase.   The shift between the haploid phase and the diploid phase is known as the alternation of generations, as shown in the figure.

The Plant Life Cycle   The multicellular diploid phase is known as the sporophyte, or spore-producing plant.

The Plant Life Cycle The multicellular haploid phase is known as the gametophyte, or gamete-producing plant.

The Plant Life Cycle A sporophyte produces haploid spores through meiosis. These spores grow into multicellular structures called gametophytes. Each gametophyte produces reproductive cells called gametes—sperm and egg cells.   During fertilization, a sperm and egg fuse with each other, producing a diploid zygote that develops into a new sporophyte.

Trends in Plant Evolution An important trend in plant evolution is the reduction in size of the gametophyte and the increasing size of the sporophyte.