Introduction and Learning Objectives
Plants are a dominant component of terrestrial ecosystems and are the source of energy for the majority of other terrestrial organisms. Modern plants descended from an ancestral plant that lived in an aquatic environment. We will study the evolutionary history of the plant kingdom to better understand the selective forces that have shaped plants' development and led to the diversity of forms in existence today.
First we'll examine the similarities and differences between members of the Kingdom Plantae and the red, brown, and green algae covered in Tutorial 9. Of the three groups of algae, green algae are the closest living relatives to modern land plants. Adaptations for the transition from an aquatic to a terrestrial habitat distinguish members of the plant kingdom, so these features will be discussed in detail. Plants have been evolving for at least 450 million years, and based on their major adaptive features, four major plant lineages (taxonomic groups) are currently recognized. This tutorial will introduce each of these groups. You will also examine the nonvascular plants, the most primitive of the living plants lineages, Throughout the tutorials discussing plant evolution and diversity, a good strategy is to understand the major characteristics of each group, which characteristics are unique to a group, or are found across groups, and how these characteristics reflect adaptations to different environmental conditions. By the end of this tutorial you should have a working understanding of:
- The origins of plants and the major factors shaping their evolution
- Plant features that are adaptations to the terrestrial environment
- The major plant lineages and features that are characteristic of each group
- The evolutionary relationships among the nonvascular plants, seedless vascular plants, nonflowering seed plants, and flowering seed plants
- The distinguishing characteristics of nonvascular plants and their life cycles
- The three groups (mosses, liverworts, hornworts) of nonvascular plants
- Identify the characteristics of plants
- Describe the benefits and challenges of living in a terrestrial environment
- Summarize the adaptations to terrestrial environments seen in plants
- Explain alternation of generations and the change in the dominant generation as plants evolved
- For each major group, identify the major characteristic(s) of that group and be able to discuss representatives that demonstrate the diversity of the group (this objective also applies to Tutorials 23 and 24).
- Identify the characteristics of non-vascular plants
- Describe the adaptations in members of this group that allow them to be successful in terrestrial environments
- Summarize the life cycle of a moss, a heterosporous plant, and identify the dominant stage in the life cycle
Plants Share a Common Ancestor With Green Algae
Modern land plants share some features with the group of green algae called charophytes, and charophytes are the closest relatives of the plant kingdom.
This phylogenetc tree (Fig. 1) depicts the evolutionary relationships between charophytes and plants. It is thought that the first true plants were derived from a charophyte. This means that the ancestor of all modern plants was a green algae living in an aquatic environment. There are many lines of evidence (e.g., chemical, structural, and genetic data) for the close evolutionary relationship between charophytes and plants.
Chlorophytes (green algae) have the same photosynthetic pigments found in plants. Chlorophyll a is common to other photosynthetic organisms, but chlorophyll b is shared only by green algae and plants. Characters such as a cell wall composed primarily of cellulose, storage of carbon in the form of starch, and formation of a cell plate at cytokinesis are not limited to green algae and plants; however, these shared characters provide further evidence of their relatedness. Molecular evolutionary analyses of RNA and DNA sequence data from green algae and plants also clearly place these two groups together.
Figure. 1. Overview of plant evolution. (Click image to enlarge)
Despite the similarities between charophytes and plants, plants are classified in a separate kingdom (Plantae). Charophytes are adapted to an aquatic environment (Fig. 2), and the features that distinguish members of the plant kingdom from charophytes are their adaptations to a terrestrial environment.
Figure 2. Chara, a genus of Charophyte (Click to enlarge) ([http://src.sfasu.edu/~jvk/PineywoodsPlants/Bryophytes_Charophytes/Charophytes/Characeae/Characeae.html]
Alternation of Generations
Life cycles were discussed in Tutorial 11. All eukaryotes that reproduce sexually have both a haploid and diploid stage in their life cycle. Although there is great variety in modes of reproduction among algae, some algaes (Brown, Red and Green) have a life cycle that includes alternation of generations; this type of life cycle is also found in plants.
Alternation of generations (Fig. 3c) refers specifically to a life cycle that includes the alternation between multicellular haploid life stages and multicellular diploid life stages. Diploid forms (sporophytes) produce haploid spores, which divide and develop directly into multicellular haploid structures (gametophytes). Gametophytes then produce haploid gametes, which join with other gametes through fertilization to form diploid sporophytes once again. The diploid and haploid generations alternate, over and over. In some organisms the diploid stage is the dominant form that is responsible for the majority of growth and resource acquisition, whereas in others the haploid stage is dominant. We will consider the life cycle of each of the four major groups of plants, in particular the prominence of the gametophyte versus the sporophyte, and the dependence of these forms on each other.
Figure 3. Life cycles of the groups of multicellular eukaryotes. (Click to enlarge)
Geologic Time Scale for Plant Evolution
It is important to remember that ancestral plants had many more shared features with charophytes than those of modern plants. They would have been transitional between the green algae and the plants in existence today. The environment they occupied was most likely subject to periods of drying, such that they slowly evolved features that allowed them to exist in a terrestrial rather than aquatic environment. This theme of adaptation to environmental conditions, via natural selection, is very important in the study of plant evolution.
Fossil plant remains that clearly show features of true plants date back to at least 475 million years ago (Fig. 4). The fossil record provides small windows into the past, and it aids scientists in approximating what life forms existed during a certain period in time. Because it can take millions of years for evolutionary changes to become established and because it is more rare than common for fossilization to occur, scientists estimate that the earliest plants began to evolve prior to the first evidence of plants in the fossil record. In fact, some molecular evidence indicates that land plants could have appeared as early as 600 million years ago.
Figure 4. Major events in plant lineages and their approximate times, based on the fossil record. (Click to enlarge)
The Transition From Aquatic to Terrestrial Environments
Although it is not certain when plants first arose, it appears that they did so during a time when the Earth's climate was changing. Likely, those areas where plants evolved was subject to periods of flooding and periods of drying, and characteristics that enabled some species to better survive during the dry periods evolved slowly. Adaptation to the drier conditions eventually enabled early plants to colonize the land. To fully appreciate the huge advantages that terrestrial migration had for plant development, it is necessary to understand the differences between aquatic and terrestrial environments with respect to requirements for plant growth.
Advantages of Moving to a Terrestrial Environment: Increased Photosynthesis and Decreased Competition
In order for plants to photosynthesize and produce the proteins, lipids, and carbohydrates necessary for growth, they require light energy. Light energy available to organisms living beneath the water's surface is greatly reduced (Fig 5). The blue and especially red wavelengths of light that are absorbed by photosynthetic pigments do not penetrate deep beyond the surface of the water; therefore, photosynthetic organisms living in an aquatic environment do not receive the full amount of light energy radiated from the sun. Photosynthetic organisms growing on land do not face this problem. Photons emitted from the sun can directly strike light-absorbing surfaces and the full range of useful wavelengths are available for photosynthesis. In the aquatic environment, many large algae compete for sunlight (similar to the competition for sunlight in modern forests). For early plants first moving into terrestrial environments, there was no competition for access to light.
Figure 5. The photic zone. (Click to enlarge) ([http://www.seattle.gov/transportation/seawall_glossary.htm#photic)
It is important to remember the fundamental role that plants play within an ecosystem. Plants and other autotrophs are the basis for supporting heterotrophic life. Prior to colonization of the land by plants, there was little basis for support of animal or fungal life. As mentioned above, the vast majority of life existed in the ocean, including herbivores that depended on algae for food. Another great advantage to the terrestrial migration of early plants was the lack of herbivores on land. Compared to life in the ocean, the terrestrial environment provided free access to sunlight and freedom from damage by larger organisms that could crush or eat the developing plants. However, the conditions on land were not completely hospitable for early plants.
Challenges of Terrestrial Environments: Desiccation and Upright Growth
The major challenge for early plants first migrating onto land was the lack of water. In an aquatic environment, desiccation is generally not a problem and there is no need for any protective covering to prevent water loss. Lacking any protection from the dry terrestrial environment, early plants probably dried out very quickly and would have been limited to very moist environments.
The ancestors of early plants were dependent on water, not only to maintain their moisture content but also for structural support. The buoyancy of water supports upright growth of giant marine seaweeds (e.g., kelp, Fig. 6) Consider the seaweeds that are often found washed up on the beach. Although these algae are no longer alive, when held beneath the water their upright form is restored. In a terrestrial environment, the surrounding media is air rather than water. Air does not provide any support for upright growth. The transition to land required changes in structural features, and, as will be discussed later in this tutorial, adaptations for structural support are key features used in plant classification.
Figure 6. Kelp forest off California coast (http://bio.research.ucsc.edu/people/carr/nereo-lit.htm)
Adaptive Features of Plants
During the course of their evolution, plants have adapted to a land-based existence. Because modern plants occupy numerous, often specialized, ecological niches (e.g., deserts, rainforests, and even aquatic environments), there are many more specific adaptations than those that will be covered in this tutorial.
We will focus on those adaptations that have allowed plants to overcome the challenges of a dry environment and the lack of support for upright growth. These adaptive features include: cuticles, stomata, vascular tissue, gametangia, and seeds. As each of these adaptive features is discussed, keep in mind the transition of early plants from an aquatic to a terrestrial environment and how each feature could enhance the success of plants on land.
Waxy Cuticles and Stomata
A major adaptation to the dry terrestrial environment is the waxy cuticle. Cuticles, composed of wax, are found on the surface of all above-ground parts of the plant. Waxes are a class of lipids (discussed in Tutorial 3) that, due to their chemical properties, are maintained as a solid, even at the highest temperatures found in extreme conditions (e.g., deserts). Like all lipids, waxes are hydrophobic and impermeable to water (Fig. 7). Of course, plant roots are not covered by cuticle because they are the structures responsible for water uptake and have less exposure to the air than parts of the plant that are above-ground..
The waxy cuticle covering the surface of the plant shoot is an effective barrier to desiccation because it prevents loss of water to the air. Not surprisingly, desert plants have a much thicker cuticle layer than plants growing in wet environments.
Figure 7. Water beads on the waxy surface of a kale leaf. (Click to enlarge) (http://en.wikipedia.org/wiki/Plant_cuticle)
Figure 8. Stomatum opened (left) and closed (right). http://ksj.mit.edu/tracker/2007/08/telegraph-few-others-high-co2-may-mean-m
Stomata are also an important adaptive feature to the terrestrial environment. Because the cuticle is impermeable, it is necessary for plants to have pores through which gasses can be exchanged with the environment. Carbon dioxide is required for photosynthesis and oxygen is produced during this process. These gasses enter and exit the plant through the stomata (Fig. 8). Water can also be lost through stomata, so their opening is regulated by the plant.
Vasculature describes a system of specialized cells found throughout the body of the plant. Vasculature has two functions. First, the specialized cells of vascular tissue allow transport of water and nutrients throughout the plant. This adaptation enables water, absorbed by the roots of the plant, to reach the stem and leaves, and the sugars from photosynthesis, produced in the shoots, to be transported to the roots. Plants with vasculature do not have to depend on living in a moist environment to maintain adequate water throughout the plant.
The second function of vasculature is structural support. Cells of the vascular tissue have secondarily reinforced cell walls that make the tissue rigid (Fig. 9). The vascular tissue that runs throughout the plant body, circulating water and nutrients, also forms a "skeleton" that strengthens the roots, shoots, and leaves. Vascular tissue enables plants to grow upright (some to very great heights), while maintaining moisture levels in all parts of the plant.
Figure 9. Plant cell wall structure. (Click to enlarge) (http://en.wikipedia.org/wiki/File:Plant_cell_wall_diagram.svg)
The evolution of vasculature was a major event in plant history. Plants with vascular tissue do not appear in the fossil record until approximately 425 million years ago, well after the origin of land plants. After this date there was an explosion of plant life, indicating that vascular tissue is a highly successful adaptation to life on land.
The transition from an aquatic to a terrestrial environment was also marked by adaptations in plant reproduction. In the charophyte ancestor of modern plants, gamete production, fertilization, and development of the embryo were highly dependent on the aquatic environment. Gametes were dispersed by water currents and were maintained in a hydrated state until fertilization occurred. The zygote and growing embryo developed free from the parent organism because there was no threat of drying out. The move to land required protection from desiccation of gametes and embryos, as well as a new means of gamete and embryo dispersal.The major adaptation of plants to the terrestrial environment (with respect to reproduction) was the production of gametes and the development of embryos within gametangia. The gametangium (-ium, singular; -ia, plural, from Latin) can be male or female, and is the site of gamete production. The female gametangium produces egg cells and the male gametangium produces sperm. A protective chamber, formed by a single layer of sterile cells, prevents the gametes from drying out by reducing or eliminating their exposure to air. Egg cells are maintained in the female gametangium, but the sperm must leave the male gametangium and travel to the egg for fertilization to occur (Fig. 10). Some groups of modern plants have retained the primitive characteristic of flagellated sperm and still are dependent on water for dispersal of male gametes; however, the majority of modern plants do not have motile sperm and have developed nonwater-based methods of dispersal (e.g., wind and insect pollination).In all plants, fertilization occurs within the female gametangium, where the zygote begins to develop into the embryo. Because all plants retain the developing embryo within the gametangium, they are referred to as embryophytes. Protection of the growing embryo is especially important in the terrestrial environment because the waxy cuticles, stomata, and vascular tissue present in mature plants are not well developed in the embryonic plant.
Figure 10. Egg in archegonium on moss plant. Fertilization will take place within the female tissues and the embryo will be retained within this gametangium (Click to enlarge) (http://www.usca.edu/biogeo/zelmer/122/plantdiv/).
Protection of the Developing Multicellular Embryo, and Seeds and Their Dispersal
Protection of the developing multicellular embryo varies among the different plant lineages. The most primitive group of plants retains the developing embryo through sexual maturity. The diploid embryo is completely dependent on the haploid gametophyte generation. This life cycle is typical of the nonvascular plants, and will be described in detail in Tutorial 13. The more-derived plant lineages have further adapted to the terrestrial environment by producing specialized structures for protection and nutrition of the developing embryo. The embryo is enclosed in a seed, which is dispersed from the parent plant long before the embryo reaches maturity. In the derived plant lineages, the haploid gametophyte is greatly reduced because it no longer plays a dominant role in protecting the embryo; in these groups of plants, the haploid gametophyte has become completely dependent on the diploid generation. As you will learn in tutorials discussing seed plants, seeds are a highly successful adaptation to the variable environmental conditions on land. Independent of the parent plant, the seed-enclosed embryo can withstand drying and temperature fluctuations, even the digestive tract of some animals, until conditions are suitable for germination and growth of the embryo to maturity (Fig. 11)
Figure 11. The germination and early development of a seed plant. http://www.extension.org/pages/10516/cotton-germination-and-seedling-development
The most recent adaptations to the terrestrial environment were the evolution of flowering plants and the production of fruit as a means for seed dispersal. Flowering plants produce their seeds within a fruit that provides a functional "packaging" around the seed(s). The fruit can be edible, such that the digested seeds are then deposited with the feces of the animal that consumed the fruit. Other fruits are suitable for transport on air currents, water currents, or on the fur of different animals. You will learn more about flowering seed plants (and their remarkable adaptations to life on land) in Tutorial 15.
The first evidence of seed plants in the fossil record occurs approximately 305 million years ago. Seed production enabled plants to reproduce more successfully because the embryos had a much better chance of surviving the dry terrestrial environment than did the embryos of more primitive plants that were still dependent on the parent plant body. Just think about the advantage to plants whose offspring could be widely dispersed and were protected (within the seed) until conditions were suitable for growth. Seed plants are so dominant in the world today that we have to remind ourselves that there are numerous plants in existence that do not produce seeds.
Fruit production by flowering plants is a more specialized adaptation to life on land because it reflects not only the environment, but also the other life forms that exist there. Although the first flowering plants occur in the fossil record only 175 million years ago, the success of fruit production is marked by the huge radiation of flowering plants.
Plant Phylogeny: Lineages are Defined by Major Adaptive Features
The various adaptations to the terrestrial environment (e.g., waxy cuticles, stomata, vasculature, gametangia, seeds, and fruit) have evolved slowly during the 475 million-year history of plants. With these adaptations in mind, we will move on to a discussion of plant phylogeny and begin our review of the major characteristics of each of the plant lineages (Fig. 12).
Figure 12. Overview of Plant Evolution. (Click to enlarge)
Before we begin to discuss each of the plant lineages, it is important to understand the phylogenetic relationships among them. According to this figure, are nonvascular plants "older" than nonflowering seed plants? The correct way to interpret a phylogenetic tree is to read which groups are more closely related to one another, and which groups are more primitive or more highly diverged. Living nonvascular plants are not "older" than nonflowering seed plants, but they possess a greater number of primitive character states than do nonflowering seed plants. Also, the origin of the nonflowering seed plant lineage occurred later in time than the origin of nonvascular plants, but this does not mean that currently living nonvascular plants are any older than currently living nonflowering seed plants. Flowering seed plants are the most derived lineage of plants.Now that you have a working knowledge of the major adaptations present throughout the plant kingdom and understand the evolutionary relationships among them, you will be introduced to the four lineages: (1) nonvascular plants, (2) seedless vascular plants (3) nonflowering seed plants, and (4) flowering seed plants. Always keep in mind how the adaptations found in each lineage of plants reflect the environmental conditions in which each lineage developed.
Nonvascular Seedless Plants
As the name of this group indicates, plants in this lineage do not have vascular tissue (or if present, it is very reduced). Because they lack substantial vasculature, plants in this lineage are generally small in size, lack significant structural support, grow close to the ground in moist areas, and lack significant water-conducting cells. Plants first evolved in environments that were transitional between the land and the sea, and although modern nonvascular plants are dependent on water to complete their life cycles, they are able to withstand long periods of desiccation. Nonvascular plants include mosses, liverworts, and hornworts.
Nonvascular Plants: Environment and Morphology
Of all the plant lineages, nonvascular plants are the most basal. This means that these lineages diverged before that remaing groups of land plants diversified. Current information suggest that the liverworts diverged first, followed by the mosses then the hornworts (Fig. 13). They are less derived than seedless vascular plants and seed plants, but the nonvascular plants are highly successful in the environments they inhabit.
Figure 13. Phylogenetic tree showing the hypothesized relationships among the lineages of land plants. The three groups that comprise the nonvascular plants are not members of a monophyletic groups, with hornworts more closely related to vascular plants. (Click to enlarge)
As you walk through a wooded area, you will likely find mosses growing on rocks, rotting wood, trees, or on the ground. Nonvascular plants are generally small and do not extend much more than a few inches above the surface they are growing on (Fig. 14). Their appearance can best be described as a "carpet of green." The plant body that is most obvious is the gametophyte generation, which is haploid.Nonvascular plants typically grow in moist environments. Their lack of vascular tissue requires them to maintain close contact with water to prevent desiccation. They do not have true roots, true stems, or true leaves (which are distinguished by the presence of vascular tissue). Rhizoids are the root-like structures that function to anchor them to the surface they are growing on, however, they are not capable of water uptake. Water is absorbed throughout the "leafy" plant body of the gametophyte. They also require a moist environment for successful fertilization. They do not produce pollen grains and have retained the primitive condition of a flagellated sperm. The male gametes are motile in water and must be released into a moist environment so that the sperm can swim to the female gametangium (where the egg cells are located).
Figure 14. Liverworts with sporocarps arising from thalli (http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/liverwort/sporocarp_page.html)
Alternation of Generations: The Moss Life Cycle
To better understand alternation of generations in nonvascular plants, study the moss (bryophyte) life cycle in Figure 15. It depicts the life cycle of a moss and helps to distinguish the haploid and diploid stages. Note the prominent form of the moss, the gametophyte, which is haploid (1n). This is the plant body that is most often observed. In the figure there are separate male and female gametophytes; however, the gametophyte can also be bisexual (male and female gametangia are located on the same plant body). In step 1, the gametophyte is the generation that produces gametes; sperm are produced in the male gametangium, the antheridium (plural, antheridia), and eggs are produced in the female gametangium, the archegonium (plural, archegonia). In step 2, the motile sperm has reached the egg, which is retained in the archegonium and fertilization takes place. (Remember, water is required for fertilization because the flagellated sperm must swim to the egg.) In step 3, the diploid (2n) zygote undergoes mitosis and begins to develop into the embryo (also 2n). In step 4, the embryo matures into the sporophyte, the diploid (2n) plant body. The sporophyte is the small, brown, stalked structure that one sometimes sees held above the main body of the moss. In step 5, meiosis takes place in the sporangium of the mature sporophyte and haploid spores are produced. In steps 6 and 7, the haploid spores are dispersed and each spore undergoes mitotic cell division to create a haploid multicellular gametophyte. The prominent haploid gametophyte is then ready to produce gametes (back to step 1).
An important feature of the moss life cycle is that the developing embryo is retained on the gametophyte plant body. This is an adaptation to the terrestrial environment because the embryo is protected from desiccation throughout its development into the sporophyte. (Remember the description of plants as embryophytes?) If you think about haploid and diploid stages in terms of the animal life cycle, it will be difficult to make sense of the plant life cycle. As noted above, the plant life cycle includes alternation of generations, with a multicellular haploid stage. Review again, step 5 of the life cycle (where meiosis takes place). Note that meiosis does not occur again when gametes are produced. Once you recognize these differences, you should begin to feel more comfortable thinking about plant life cycles.
Figure 15. Bryophyte Life Cycle. (Click image to enlarge)
The diversity of plants existing today is the result of 475-700 million years of evolution and adaptation to the terrestrial environment. Plants arose from a lineage of green algae and their closest protest relatives are the Charophytes. Both have cellulosic cell walls, form cell plates during cytokinesis, store carbon in the form of starch, have chlorophyll b as an accessory pigment, and share similar RNA and DNA sequences for particular genes. Charophytes are aquatic organisms, and it is highly likely that the earliest plants occupied transitional environments between the sea and the land. The transition to the terrestrial environment was advantageous for plants because there was direct access to sunlight and little to no herbivore activity. Early plants were ill-equipped for life out of the water, and desiccation was a major challenge to a land-based existence.
Adaptations to the terrestrial environment enabled generation after generation of plants to successfully live out of the water. The waxy cuticle and stomata are effective in reducing water loss and preventing desiccation. Vascular tissue further reduces the problem of desiccation because it allows transport of water and nutrients throughout the plant. Upright growth for improved access to sunlight is also an advantage conferred by vascular tissue because it also functions in internal support of the plant body. Protection of gametes and developing embryos is the role of jacketed gametangia, and later, the seed. More recently in plant history, adaptive features have been influenced by other organisms in the terrestrial environment; some plants produce specialized flower structures and fruits that attract insects and other animals that aid in pollination and seed dispersal.
The nonvascular plants (including mosses, liverworts and hornworts) are highly successful and can be found the world over. They are resistant to desiccation, but prefer a moist environment due to their lack of vascular tissue and motile gametes. Nonvascular plants, like other plants, are embryophytes, and their life cycles are based on alternation of generations. The prominent generation of nonvascular plants is the multicellular haploid gametophyte. The diploid sporophyte generation is completely dependent on the gametophyte for its survival. In the more derived plant lineages, the gametophyte is greatly reduced. You should also know that the nonvascular plants do not have vascular tissue or seeds, however, they do have a stomata, a protected embryo, and most have a waxy cuticle. Seedless vascular plants are more derived than nonvascular plants and are defined by their lack of seed production and presence of vascular tissue. The more derived lineages, nonflowering seed plants and flowering seed plants, both produce seeds, but only the flowering seed plants produce flowers and fruits. Life cycles and the major characteristics of the vascular plants will be covered in the next two tutorials.
After reading this tutorial, you should have a working knowledge of the following terms:
A Real Life Example: Other types of photosynthesis in plants
The Calvin cycle is also referred to as C3 metabolism because fixed CO2 first appears in the form of a three-carbon molecule. CO2 and other gases enter and exit the cell through tiny pores called stomata. Plants are able to regulate the movement of gases into and out of cells by controlling the opening and closing of stomata. The opening and closing of the stomata is a balance between taking up CO2 and losing water. All terrestrial plants use the Calvin cycle, but some plants initially fix carbon using alternate routes.
C4 and CAM metabolism are more derived photosynthetic pathways and, in certain environments, increase the fitness of plants that utilize these pathways. In both C4 and CAM plants, CO2 is initially fixed into four-carbon compounds; eventually these four carbon compounds release CO2 to RUBP and the Calvin-cycle enzymes act as usual.
Many plants that are adapted to xeric (dry) environments, such as cacti, use a CAM pathway. In the case of CAM, carbon dioxide enters the open stomata during the night, is fixed, and the stomata are closed during the day when the Calvin-cycle operates.
- How does the CAM pathway better adapt plants to xeric conditions?
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