By the end of this reading you should be able to

• Discuss the challenges to plant life on land
• Describe common characteristics of all land plants
• Relate specific adaptive features to the unique challenges of life on land

Current evolutionary thought holds that all plants—green algae as well as land dwellers—are monophyletic; that is, they are descendants of a single common ancestor. At some point in the evolutionary history of plants, there was a transition from an aquatic environment to the terrestrial one. This transition from water to land imposed severe constraints on plants. Plants had to develop strategies to avoid drying out, to disperse reproductive cells in air, to provide structural support, to exchange gases without water, and as they grew larger to move molecules by bulk flow.  While some groups of plants developed adaptations that allowed them to populate even the aridest habitats on Earth, full independence from the water did not happen in all plants. As a group, most seedless plants still require a moist environment for at least some of their physiological functions. Land plants can be separated by the degree of development of their structures, both vascular as well as reproductive, and the variation in their alternation of generations lifecycle.

Early land plants, like the early land animals, did not live very far from an abundant source of water and developed survival strategies to combat dryness. One of these strategies is called tolerance. Many mosses, for example, can dry out to a brown and brittle mat, but as soon as rain or a flood makes water available, mosses will absorb it and are restored to their healthy green appearance. Another strategy is to colonize environments with high humidity, where droughts are uncommon. Ferns, which are considered an early lineage of plants, thrive in damp and cool places such as the understory of temperate forests. Later, plants moved away from moist or aquatic environments using resistance to desiccation, rather than tolerance. These plants, like cacti, minimize the loss of water to such an extent they can survive in extremely dry environments.

Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages (Fig 3).

Haplontic refers to a lifecycle in which there is a dominant haploid stage (like most fungi), and diplontic refers to a lifecycle in which the diploid is the dominant life stage (like animals). Most plants exhibit alternation of generations, which is described as haplodiplodontic: the haploid multicellular form, known as a gametophyte, is followed in the development sequence by a multicellular diploid organism: the sporophyte. The gametophyte gives rise to the gametes (reproductive cells) by mitosis. This can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain, in the higher plants (a common collective term for the vascular plants). The sporophyte stage is barely noticeable in lower plants, like moss, or massively huge, like diplontic sequoia and pine trees. The sporophyte stage produces spores by meiosis and each spore is capable of giving rise to a multicellular gametophyte.

The sporophyte of seedless plants is diploid and results from syngamy (fusion) of two gametes. The sporophyte bears the sporangia (singular, sporangium) (Fig. 4): organs that first appeared in the land plants. The term “sporangia” literally means “spore in a vessel,” as it is a reproductive sac that contains spores.

Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis. The spores are later released by the sporangia and disperse in the environment. Seedless non-vascular plants produce only one kind of spore and are called homosporous. The gametophyte phase is dominant in these plants. After germinating from a spore, the resulting gametophyte produces both male and female gametangia, usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores (Fig. 1). The male spores are called microspores, because of their smaller size, and develop into the male gametophyte; the comparatively larger megaspores develop into the female gametophyte. Heterospory is observed in a few seedless vascular plants and in all seed plants.

Gametangia (singular, gametangium) are structures observed on multicellular haploid gametophytes. In the gametangia, germline cells give rise to gametes by mitosis. The male gametangium (antheridium) releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia: the female gametangium. The embryo develops inside the archegonium as the sporophyte. Gametangia are prominent in seedless plants but are very rarely found in seed plants.

• As plants adapted to dry land and became independent from the constant presence of water in damp habitats, new organs and structures made their appearance.
• Early land plants did not grow more than a few inches off the ground, competing for light on these low mats. By developing a shoot and growing taller, individual plants captured more light.
• Because air offers substantially less support than water, land plants incorporated more rigid molecules in their stems (and later, tree trunks).
• In small plants such as single-celled algae, simple diffusion suffices to distribute water and nutrients throughout the organism. However, for plants to evolve larger forms, the evolution of vascular tissue for the distribution of water and solutes was a prerequisite. The vascular system contains xylem and phloem tissues. Xylem conducts water and minerals absorbed from the soil up to the shoot, while phloem transports food derived from photosynthesis throughout the entire plant.
• A root system evolved to take up water and minerals from the soil and to anchor the increasingly taller shoot in the soil.
• In land plants, a waxy, waterproof cover called a cuticle protects the leaves and stems from desiccation.
• However, the cuticle also prevents the intake of carbon dioxide needed for the synthesis of carbohydrates through photosynthesis. To overcome this, stomata or pores that open and close to regulate the traffic of gases and water vapor appeared in plants as they moved away from moist environments into drier habitats.
• Water filters ultraviolet-B (UVB) light, which is harmful to all organisms, especially those that must absorb light to survive. This filtering does not occur for land plants. This presented an additional challenge to land colonization, which was met by the evolution of biosynthetic pathways for the synthesis of protective flavonoids and other compounds: pigments that absorb UV wavelengths of light and protect the aerial parts of plants from photodynamic damage.
• Plants cannot avoid being eaten by animals. Instead, they synthesize a large range of poisonous secondary metabolites: complex organic molecules such as alkaloids, whose noxious smells and unpleasant taste deter animals. These toxic compounds can also cause severe diseases and even death, thus discouraging predation.  In contrast, as plants co-evolved with animals, the development of sweet and nutritious metabolites lured animals into providing valuable assistance in dispersing pollen grains, fruit, or seeds. Plants have been enlisting animals to be their helpers in this way for hundreds of millions of years.

Land plants acquired traits that made it possible to colonize land and survive out of the water. All land plants share the following characteristics: alternation of generations, with the haploid plant called a gametophyte, and the diploid plant called a sporophyte; protection of the embryo, formation of haploid spores in a sporangium, the formation of gametes in a gametangium, and an apical meristem. Vascular tissues, roots, leaves, cuticle cover, and a tough outer layer that protects the spores contributed to the adaptation of plants to dry land.

Figure 1. Image courtesy of James St. John [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]

Figure 2. Image courtesy of Avenue [GFDL (http://www.gnu.org/copyleft/fdl.html)]

Figure 3. Image created and provided by D. Jennings

Figure 4.   Top Image courtesy of Guido Gerding – Own photograph, URL: Ex :: Natura – Freies Portal für Umweltbildung (Environmental Education), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=499406.   Bottom Image courtesy of Vaelta at English Wikipedia. [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]