Introduction to Plant Diversity

On early Earth, the continents were barren. Over time, early land plants evolved from green algal ancestors in water and gradually colonized land.

Today, continents support microorganisms, insects, plants, and animals, many of which depend on land plants for survival.

All land plants, from mosses to seed-bearing plants, share several key characteristics.

First, these plants are multicellular eukaryotes.

Second, plant cells contain chloroplasts. These are membrane-bound organelles that contain the photosynthetic pigment chlorophyll in structures called thylakoids.

Chlorophyll absorbs light energy from the sun. Plants use this energy, along with carbon dioxide and water, to carry out photosynthesis and make carbohydrates as food.

Third, plant cells have a cellulose-rich cell wall outside the plasma membrane.

All plants have a life cycle called alternation of generations. In this cycle, the plant alternates between two multicellular stages: the gametophyte, a haploid stage, and the sporophyte, a diploid stage.

In the gametophyte stage, the male sex organs, called antheridia, and the female sex organs, called archegonia, produce haploid gametes through mitosis. Antheridia produce sperm, while archegonia produce eggs. During fertilization, these gametes fuse to form a diploid zygote.

The zygote then divides by mitosis and develops into the diploid sporophyte.

The sporophyte produces haploid spores by meiosis. These spores then grow into haploid gametophytes, completing the life cycle.

Land plants are classified into two major groups: nonvascular plants and vascular plants.

Nonvascular plants include liverworts, mosses, and hornworts. Vascular plants are further classified into seedless vascular plants and seed plants. Each of these groups contains many thousands of species.

Over time, these groups evolved different traits, from vascular tissue to fruits. Together, these adaptations helped plants spread across most terrestrial biomes.