Plant Diversity

From Water to Land

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.

Major Lineages of Plants

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 pollinators^3-4. 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.

References

1. Ziv, C., et al. (2018). ‘Multifunctional Roles of Plant Cuticle During Plant-Pathogen Interactions.’ Front Plant Sci 9: 1088.
2. Gupta, R. and R. Deswal (2014). ‘Antifreeze proteins enable plants to survive in freezing conditions.’ J Biosci 39(5): 931-944.
3. Jurgens, A., et al. (2012). ‘Pollinator-prey conflict in carnivorous plants.’ Biol Rev Camb Philos Soc 87(3): 602-615.
4. Thomann, M., et al. (2013). ‘Flowering plants under global pollinator decline.’ Trends Plant Sci 18(7): 353-359.