The Evolution and Diversity of Land Plants

Aristotle pioneered the classification of the living world into two broad categories: animals, which moved, and plants, which did not. This division still prevails today. Land plants, known as embryophytes, comprise a vast group encompassing approximately 300,000 to 315,000 species. This category includes flowering plants, conifers, ferns, and mosses, but not algae and fungi.

Remarkably, all land plants trace their lineage back to a single green algae species, the charophytes, which ventured onto land around 1.2 billion years ago. Many green algae thrived at the fringes of ponds and lakes, places prone to drying. Darwin's theory of natural selection suggests that those algae capable of adapting to life above the waterline survived when water levels receded. These early land plants, emerging some 450 million years ago, thrived under conditions offering increased access to bright sunlight, ample carbon dioxide, and nutrient-rich soil—ideal circumstances for generating organic molecules through photosynthesis. Seed plants, representing 85–90 percent of all land plants, appeared 360 million years ago. Flowering plants followed 140 million years later, and the most recent major group, the grasses, evolved around 40 million years ago.

Green plants exhibit a vast range in size, from diminutive weeds to towering redwood trees. All possess eukaryotic cells containing nuclei and cell walls composed of cellulose. The majority of green plants harness energy through photosynthesis. The initial seed plants were the now-extinct seed ferns. Seeds consist of an embryo, a fertilized egg, and its nourishing supply, enclosed within a protective shell. Such seeds can remain dormant for extended periods after detachment from the parent plant.

Around 12,000 years ago, humans across various regions began cultivating wild seed plants, transitioning from hunter-gatherer lifestyles to agriculture. Seed plants are primary sources of sustenance, alongside providing fuel, wood products (absent in seedless plants), and medicinal resources. In contrast, seedless plants, which encompass ferns, mosses, liverworts, and horsetails, lack flowers and do not reproduce through seeds.

Land plants exhibit a wide range of sizes, from unassuming weeds to towering giants like redwood trees. The depicted moss lacks a water transport system, necessitating a moist habitat for growth and the presence of liquid water for reproduction. Roof tiles, capable of retaining moisture in rainy climates, serve as an ideal environment for these mosses.