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.
Kingdom Plantae first appeared about 410 million years ago as green algae transitioned from water to land. Though challenging, this transition benefited early colonizers in several ways. Initially, most living organisms (including plants and animals) were ocean dwelling, making aquatic environments crowded and highly competitive. In contrast, land was a relatively uncolonized environment with ample resources and little to no predators or competitors. Terrestrial environments also offered more light and carbon dioxide, required by plants to grow and survive. Accordingly, new terrestrial niches were ripe with possibility for the first semi-aquatic algae that transitioned to dry environments.
However, the stark differences between land and sea posed a formidable challenge to early colonizing species. These challenges prompted many new adaptations that have resulted in the wide variety of plant forms observed today. Adapting to life on land required fundamental changes to the structure, reproductive strategies, feeding and defense mechanisms of plant species. For instance, aquatic plants generally rely on a liquid environment for the direct absorption of water and nutrients, buoyancy for physical support, and the transport of gametes through water for fertilization. For land plants, these strategies became impossible. Such obstacles to life on land played a critical role in the early evolution of terrestrial plants and continue to shape their evolution today.
One early adaptation was the development of an outer waxy coating, called a cuticle1. Cuticles serve to protect plants from desiccation, or extreme drying, by trapping moisture inside. However, this adaptation prevented the direct exchange of gases across the surface of plants. As a result, pores developed on the outer surfaces of plants that allowed the absorption of carbon dioxide and release of oxygen. These pores, called stomata, can be opened or closed depending on environmental conditions. By contracting guard cells surrounding the stomata, plants close these openings during dry periods to prevent excess moisture loss. These adaptations helped to retain water for land-dwelling plants. However, additional structures were necessary to facilitate the transport of water and nutrients from soil to the superior portions of the plant. As a result, vascular tissue developed that not only serves to transport water and nutrients to all areas of the plant, but can also provide structural support as stems grow taller and stronger.
To accommodate reproduction on land, several changes occurred to the structures and mechanisms of plant fertilization and development. First, terrestrial plants developed gametangia, which are reproductive structures that protect gametes and embryos from the harsh environment outside the plant. In males, this structure is called the antheridia while in females it is called the archegonia. To facilitate the transport of sperm from the antheridia to eggs within the archegonia, different strategies evolved. These include sperm swimming from one structure to the next, being carried by the wind, or being transported by pollinators like bees and birds. The specific mode used is unique to each classification of plants. Following fertilization, eggs are retained within the archegonia to protect and nourish the developing embryo, or sporophyte.
Another important reproductive adaptation was the generation of seeds. Though not all terrestrial plants are seeded, the use of seeds is advantageous for many reasons. Without these structures, plants require moist environments to transport gametes from one place to another. Often in such plants, male and female spores are approximately the same size and both travel. However, seeded plants generally contain small male spores adapted to be highly mobile, called pollen grains. Pollen travels to female gametophytes to deposit sperm directly to the egg. Once fertilization occurs, a seed is formed that contains the plant embryo and a supply of nutrients. Many seeds also have a protective coat and are able to survive in dry environments and disperse over long distances. Some can even exist in a dormant state for prolonged intervals of time, “waiting” for the appropriate environmental conditions to trigger germination. These adaptations have created plant species well adapted to life in terrestrial environments.
Though countless varieties of plants now exist, all can be divided into one of three groups: non-vascular, vascular seedless, and vascular seeded. Non-vascular plants are the most ancestral and least complex, including mosses, liverworts, and hornworts. Because these plants lack vascular structures and seeds and posses only a thin cuticle or none at all, they are reliant on water to survive and reproduce. Certain species may enter dormancy during dry periods until additional rainfall facilitates growth or reproduction. A lack of supporting structures in these plants results in forms that are generally low, seeming to hug the surface on which they are growing. To reproduce, non-vascular plants release bare sperm that must swim through surrounding water to the archegonia. Though these plants possesses very few of the adaptations that other terrestrial groups have, non-vascular plants are specialized to live in the moist environment in which they are found.
Next, the vascular seedless plants include ferns and horsetails. These can be found in wet habitats, commonly in the understory of temperate rain forests. Unlike non-vascular species, these plants have a thicker cuticle, functioning stomata, and vascular tissue that allow them to grow taller and actively transport water and nutrients. Ferns do not have seeds, but instead use spores to transport gametes though moisture from antheridia to archegonia. As a result, these species represent an intermediate evolutionary lineage that can live in dry environments, but require moist conditions to reproduce.
The last group, vascular seeded plants, includes all remaining species. This group is the most diverse and occupies the widest range of habitats2. However, all species are characterized by several common adaptations, including vascular tissue, highly mobile pollen, and seeds. This large group is split into two major sub groups, angiosperms and gymnosperms. Angiosperms include all flowering and fruiting plants, with pollen carried by the wind or transported by pollinators3-4. The development of flowers and fruits are adaptive for the distribution of pollen and seeds. Many animals, including bees and hummingbirds, assist in the transport of pollen from one flower to the next. Fruit produced by this group is extremely important to the diet of many animals, including humans. By its biological definition, fruit includes any structure that bears seeds and is formed from the ovary, encompassing commonly known varieties including apples and oranges in addition to products like tomatoes, avocados, and cucumbers. Consumption or transport of fruits by humans and other animals can help spread seeds over large distances. In contrast, gymnosperms are non-flowering plants including conifers, cycads, and ginkgo trees. These species produce bare seeds not protected by fruit and pollen carried by wind. Both angiosperms and gymnosperms make up the vast majority of the plants observed today.
In addition to species that evolved naturally over large spans of time, humans have participated in the artificial selection and breeding of many species of plants for human use or consumption. For example, the wild mustard plant within the Brassica group has undergone extensive artificial selection to produce kale, broccoli, Brussel sprouts, cabbage, turnips, kohlrabi, and cauliflower. Humans have also changed the landscape of plant life by introducing invasive species to non-native areas. Such species often outcompete native organisms, as they often lack natural competitors or predators in the new environment. One example is kudzu, a vined pea plant that grows quickly and spreads efficiently. Kudzu was intentionally introduced to the eastern United States from Southeast Asia in an attempt to stabilize soil and prevent erosion near roads and farms. However, once introduced, kudzu quickly outgrew native species, blocking light and over-consuming resources. It is now estimated that kudzu causes over 500 million dollars in forestry and agricultural damages each year. The ecological impacts of this and other invasive species are a major concern to biologists and economists alike.
