Plant Diversity

Before plants colonized land from the sea, the continents were barren. Now millions of species of microbes, plants, and animals live on land, and it was the evolution of land plants that made all of this possible. To understand this staggering transformation, we need to understand what exactly plants are and how they evolved to meet the challenges of life on land.

There are certain characteristics that all plants share, from the tallest trees to the smallest moss. First, all plants are multicellular eukaryotes. Second, plants produce the photosynthetic pigment chlorophyll in organelles called chloroplasts, enabling them to produce their own food using energy from the sun. Third, all plants have cells that are surrounded by walls made of cellulose. Lastly, all plants have a life cycle characterized by the alternation of generations, defined as the transition between haploid and diploid multicellular stages over the life cycle. Here, generations refers to two different multicellular phases in the life cycle. One phase is the haploid gametophyte. The gametophyte produces gametes by mitosis, which fuse during fertilization to form a diploid cell, which then undergoes mitosis to develop into a sporophyte. The sporophyte, in turn, produces haploid spores by miosis, completing the cycle by giving rise to new gametophytes.

Although all plants share these characteristics, there are stark differences among the diverse plant lineages in how some of these characteristics are expressed. Let's examine this diversity. Land plants fall into three major groups, the nonvascular plants, the seedless vascular plants, and the seed plants. Each of these groups contains many thousands of species.

The mosses belong to the division Bryophyta. These relatively simple plants have three main characteristics, a lack of vascular tissue, the gametophyte is the dominant free-living stage of their life cycle, and the sporophyte is small, unbranched, and dependent on the gametophyte for nutrition.

The next major group of land plants to evolve was the seedless vascular plants, represented here by the division Monilophyta. This group includes the ferns and horsetails. These plants have true vascular tissue but do not produce seeds like the other vascular plants. The sporophyte is dominant and branched with a waxy cuticle and leaf pores called stomata that regulate gas exchange. The gametophyte is much smaller but is free-living in most members of this group. Fern spores and sperm both require liquid water for dispersal. The extensive root systems and vascular tissues are key innovations in seedless vascular plants that provide structural support and allow for efficient collection of water from the soil.

The seed plants were the next major group to evolve. There are two major lineages in this group, the gymnosperms and the angiosperms. Although they differ in many details, all gymnosperm and angiosperm seeds consist of an embryo and a store of food that is surrounded by a protective shell called the seed coat. The store of food provides the seedling with resources until it is able to feed itself. The gymnosperms include familiar trees like pines, spruces, and ginkgos. The sporophyte is the dominant life cycle stage in the gymnosperms. The gametophytes are very small and grow on male or female cones. Wind carries the male gametophyte, called pollen grains, to the female gametophytes. After fertilization, a female structure called the ovule, develops into a seed. The angiosperms, or flowering plants, were the next group of seed plants to evolve. This group is the most widespread and species-rich group of plants, including many familiar plants and trees like lilies, oaks, and apples. Like the gymnosperms, angiosperm sporophytes are large and free-living and the gametophytes are very small and separated into male and female forms. But in contrast to the gymnosperms, diverse animals can pollinate flowers in addition to the wind, collecting pollen as they visit flowers and distributing it as they leave to visit others. After pollination and fertilization, the ovary develops into a fruit that contains the seeds.

We have now completed our simplified tour of plant diversity that highlighted some key plant traits from vascular tissue to the evolution of fruits. Together, these adaptations enabled plants to dominate most terrestrial biomes. In this lab, you will investigate plant diversity by examining specimens from the different major lineages of plants and observing their structures in both the laboratory and field settings.