Introduction to Plant Diversity

All plants are multicellular eukaryotes that produce chlorophyll in chloroplasts for photosynthesis. They have cells surrounded by cellulose walls and undergo alternation of generations, cycling between haploid gametophyte and diploid sporophyte stages. These shared traits distinguish plants from other organisms despite their remarkable diversity.

Early land plants developed a waxy cuticle to prevent water loss and pores for gas exchange. They evolved vascular tissue to transport water and nutrients while providing structural support. Plants also developed gametangia to protect reproductive structures and seeds to disperse offspring in dry environments, enabling colonization of terrestrial habitats.

Alternation of generations is the transition between haploid and diploid multicellular stages in a plant's life cycle. The haploid gametophyte produces gametes by mitosis that fuse during fertilization to form a diploid sporophyte. The sporophyte then produces haploid spores by meiosis, which develop into new gametophytes, completing the cycle.

Nonvascular plants like mosses lack vascular tissue and are the most ancestral group. Seedless vascular plants such as ferns evolved vascular tissue for transport and support. Seed plants, the most diverse group, include gymnosperms with bare seeds and angiosperms with flowering and fruiting structures, occupying the broadest range of terrestrial habitats.

Seedless plants require moist environments for sperm to swim between male and female structures. Seed plants evolved pollen grains, small mobile male spores that travel via wind or pollinators directly to female gametophytes, depositing sperm at the egg. This adaptation allows seed plants to reproduce successfully in dry terrestrial environments.

Seeds contain a plant embryo with a nutrient supply, protecting and nourishing the developing sporophyte. This allows seed plants to reproduce without requiring moist conditions for gamete transport. Seeds enable efficient dispersal and establishment in diverse terrestrial environments, contributing to the dominance of seed plants across most terrestrial biomes.

Before plants colonized land from the sea, continents were barren. Plant evolution enabled the establishment of millions of species of microbes, plants, and animals on land, fundamentally transforming terrestrial environments. This staggering transformation made modern terrestrial ecosystems possible and established the foundation for complex life on land.