Plant Diversity

The heart of the plant kingdom lies in the bed of diversity.Three main ideas were examined in the second unit of plant biology: autotrophic land plants,fungi, and the protist taxa, the main characteristics of these taxa, and how the groups are interrelated. Protists and fungi, while not considered land plants, must be discussed to grasp the evolutionary development of land plants. Plants can be divided into two sections: vascular and non-vascular plants. Additionally, vascular plants can be further subdivided into seedless vascular and seed vascular plants. The main groups studied under the plantae kingdom were bryophytes, lycophytes, pterophytes, gymnosperm, angiosperms, and charophytes. Each of these share certain characteristics and all taxons display novel characteristics that make them different from the other groups. As mentioned, the taxa share characteristics, thus demonstrating that there is an ancestral relationship between all of the groups discussed above. The diversity of plants can be seen in the variety of taxa which display common and novel characteristics among the groups, which illustrate the phylogenic relationships between all groups of land plants, fungi, and protists.

Before digging into the land plants, a discussion on lesser related organisms, protists, charophyceans, and fungi must be had. Protists are a complex kingdom of unicellular, colonial,and multicellular organisms that are autotrophic, heterotrophic, or mixotrophic. There are numerous clades of protists, yet there are several that provide a link to to the evolution of land plants. This can be explained by the endosymbiotic theory, which suggests that a heterotrophic protist ingested plastids and then later evolved into red and green algae(which we know land plants are closely related to green algae). Four clades demonstrate the diversity and evolutionary relationship of protists, plants, and fungi. First, the alveolata characteristically have sacs that regulate the cell’s ion and water content. Moreover, the dinolagellates are armored with cellulose plated walls, which is a similar component in land plants(perhaps a relationship?) . The second clade, the stramenopila contain groups that are most similar to the red and green algae. In Sara’s lecture on stramenopila, the clade exhibited many charactersitcs that can be found in plants and fungi. For example, the diatoms have hyphae to absorb nutrients much like fungi and the brown algae are multicellular with some alternation of generations much like land plants(5). There are interesting connections between the kingdoms and these similarities highlight the ancestral relationships(or convergent evolution) between the many clades. The last two clades, rhodophyta and chlorophyta are most closely related to the land plants. Rhodophyta, red algae, is known for its secondary pigment phycoerythrin and has no flagellated stages(5). Chlorophyta, green algae, is most similar to land plants and has chloroplasts similar to those found in plants. Most chlorophytes have complex life cycles with alternation of generations. According to the phylogenic tree, the charophyceans and land plants share the most recent common ancestor with chlorophyta.

Beside the protist kingdom, the Charophyceans are especially important to land plants because they are the closest living relative of land plants. Prior to the evolution of charophyceans, most plant life was aquatic. The charophyceans inhabit the shallow waters around lakes where they are exposed to the above water elements. Unlike its ancestors that were fully aquatic, the charophyceans are able to avoid drying out, which gives a clue to biologists how the development of land plants may have occurred. The plants have a layer of sporopollenin, which coats the outer plant tissue to prevent it from drying out. Scientists believe that the charophyceans developed derived traits that would enable the first land plants to survive without being submerged in water.
Fungi, a diverse and mysterious kingdom, that seem similar to land plants, but are remarkably different. Actually, fungi are closer related to animals, than they are to plants; for example, evidence shows that animals and fungi descended from a flagellated, unicellular organism(Campbell 612). There are five phyla of fungi: chytrids, zygomycetes, glomeromycetes, ascomycetes, and basidiomycetes. All phyla share similar characteristics such as being heterotrophic, are symbionts, and participate in asexual and sexual reproduction. Each phylum possesses a trait that discriminates it from the rest. The chytrids, the most primitive of the fungi illustrate a trait that supports the hypothesis that fungi evolved from a flagellated organism. All chytrids have motile sperm(4). The phyla zygomycetes and glomeromycetes are similar to one another, except few variations. The zygomycota, or molds, have a resistant zygosporangium that is multinucleate(Campbell 614). On the other hand, the glomeromycota exhibit similar traits, yet they are involved in a mutualistic relationship with trees and plants via the arbuscular mychorrizae. The arbuscular mychorrizae invade the living roots of plants and trees to absorb water and minerals(4,8). The remaining phyla, ascomycota and basidiomycota are sister groups that developed fruiting bodies, but differ in complexity and main function. The ascomycota range from microscopic yeasts to complex cup fungi and morels. Most ascomycota reproduce sexually by releasing conidospores that are dispersed by the wind, yet some do asexually reproduce(Campbell). Furthermore, the most common function that ascomycota participate in, is a mutualistic relationship with algae and cyanobacteria, known as lichen. Finally, the basidiomycota, the most commonly recognized fungi specialize in decomposition of wood and other plant materials. Basidiomycota are known for their elaborate “toadstool” bodies that are involved in sexual and asexual reproduction.

The main taxa of land plants, bryophytes, lycophytes, pterophytes, gymnosperms, angiosperms, and charophytes(and fungi will be included as well) comprise the taxa under the plantae kingdom. The groups are categorized as vascular(seed and seedless) and non-vascular, which highlight the common characteristics, as well as the differences between the land plants. Non-vascular land plants, bryophytes, lack a vascular tissue system, namely phloem and xylem. The bryophytes include the hornworts, liverworts, and mosses, but it is not clear if they form a clade or not(certain evidence illustrates that they are not monophyletic, as some mosses display a simple vascular system). The bryophytes are gametophyte dominant in the life cycle, unlike the vascular land plants(1). The gametophyte is comprised of protonemata and gametophore. The protonemata have large surface area for water and mineral absorption, plus it forms a bud with each apical meristem that will produce a gametophore. The gametophore produces the gamete which will turn into the mature gametophyte. The bryophytes “hug” the ground, often forming “carpets on the forest floor”, which coincides with the idea that structurally, it cannot grow upwards(Campbell 580). The lack of vascular tissue prevents it from growing tall since it could not nourish itself without the use of vascular systems. Moreover, the bryophytes anchor themselves to the ground using rhizoids, rather than roots. Unlike the roots of vascular plants which help with absorption, rhizoids do not take part in absorption.

As previously mentioned, the evolutionary relationship between the mosses, liverworts, and hornworts are under scrutiny. The hornworts and mosses are more complex than the liverworts; for example, the liverworts lack a stomata. Stomata are found in all vascular plants, meaning that the stomata evolved in the ancestor of the hornworts, mosses, and vascular plants, but not in the liverworts. There are several thought pools on the evolutionary relationship between these plant groups. For example, if the hornworts are the oldest of the plants, then perhaps it developed stomata, lost it in evolution, and then regained it, or the hornworts evolved separately from the mosses and vascular plants(Campbell 583). Again, the evolutionary relationship is partially understood with the respect to the remaining vascular plants.

The remaining plant groups, lycophytes, pterophytes, gymnosperms, and angiosperms are vascular land plants. The vascular land plants are divided into seedless and seed plants. Seedless vascular plants are lycophytes(club mosses, spike mosses, and quillworts) and pterophytes(ferns and fern allies), which have similar characteristics of nonvascular plants and novel characteristics that illustrate the process of evolution(2). All vascular seedless plants exhibit alternation of generations and reduced gametophytes. Most importantly, pterophytes and lycophytes have vascular systems. A vascular system contains xylem to conduct water and phloem to conduct sugars to the far stretches of the plant structures. This enables these groups to grow in height, unlike its bryophyte relatives. Furthermore, these groups have roots to anchor and absorb nutrients, as well as leaves to capture solar energy. Both of these traits are not seen in nonvascular plants. While the ferns and fern allies have bryophytic traits,they posses novel traits that connect them to the vascular seed plants. For example, pterophytes have flagellated sperm and require water to get to the eggs for fertilization, much like the bryophytes(3). On the other hand, the ferns and fern allies display reduced gametophytes and grow much larger than bryophytes. The lycophytes are the most primitive vascular plant, which are small, herbaceaous plants that look similar to true mosses. Plus, lycophytes tend to grow on tropical trees as epiphytes(3).

Lastly, the evolution of vascular seed plants paved the road for the evolution of animals. Seed plants developed 320 million years ago with remarkably different traits that would change the fate of the planet forever. Four main traits evolved to produce the vascular seed plants. For example, seed plants have microscopic gametophytes and are sporophyte dominant(Campbell 591). Furthermore, seed plants are heterosporous, meaning that there are bisexual gametophytes(unlike most seedless plants, which are homosporous). Thirdly, seed parents retain the megaspore within the mature sporophyte to protect it from damage. Lastly, microspores are contained in pollen grains, which can be dispersed via the wind or animals to fertilize a megaspore(Campbell 592). There are two clade of seed plants, the gymnosperms and the angiosperms. The gymnosperms have seeds that are not enclosed in ovaries. The four phyla are conferophyta, ginkgophyta, gnetophyta, and cycadophyta. Coniferophyta bear cones that release the seeds for fertilization. This is the largest phyla of gymnosperms. On the other hand, the cycadophyta, gingkophyta, and gnetophyta have a small number of species, which can be found in tropical regions and deserts. The second clade, the angiosperms are seed plants that produce flowers and fruits. The flower and fruit have key roles in the angiosperm life cycle. For example, the flower is the sexual reprodcuction structure, which for most angiosperms relies on fertilization by an insect or other animal(9). Secondly, the fruit is a mature ovary that encases the seed. Angiosperms are involved in double fertilization, meaning that one sperm fertilizes the egg to make a diploid zygote, while the other sperm fuses with the multinucleate female gametophyte cell. Only then, can the ovule mature into a seed.

The plant kingdom is vast, thus a variety of learning tools were used to turn the information into learned knowledge. Educational web sites, the Campbell text, student lectures, and lab activities provided the basis for understanding the plant, protist, and fungi taxa. First, the sexual cycles of all land plants are similar with slight variations, making it hard to differentiate between the differing cycles. However, two labs, the coniferophyta lab and the fern lab, taught me the structures of the megaspore, microspore, and the mechanisms for fertilization to occur. During the coniferophyta lab, the slides that I viewed gave a clear picture of what the megaspores and microspores looked like and how the seeds contain ovules and spores(6). After this I was able to explain to others what they look like, and how they are contained within the male and female cones. Moreover, the fern growing lab provided the platform to learn how the egg and sperm meet in fertilization. Fern sperm requires water to meet the egg, therefore we grew male and hermaphroditic (heterosporous) gametophytes in agar dishes and introduced them to one another in a water medium(7). Under the microscope, I was able to see the male release its sperm and migrate toward the hermaphroditic gametophyte. In addition to the lab slides, the student lectures, particularly, Tyler Waterman’s on fungi and Sara Barnum’s on protists provided a great toolbox to learn about taxa that seemed “all over the place” in my mind. For example, Tyler’s presentation on fungi structure clarified the many structures of the fungus, such as differentiating between the fruiting body, hyphae, and mycelium(4). Another lecture that helped start the learning process of plants, was the protist lecture by Sara Barnum. Her lecture was so thorough that I have not needed to reference the text book to compare and contrast the differing groups of protists, i.e. stramenopiles, alveolates, red and green algae(5). It was this place when I fully comprehended the endosymbiotic hypothesis and the evolution of land plants. Moreover, to understand the relationship between plants, I developed my own phylogenic tree, which can be seen on page 7.

Outside of the classroom, educational web sites and the Campbell text closed the gaps in my understanding of the information and provided clarification with the respect to the differing characteristics of the plant taxa. In the text book the most useful pages were, page 571, page 579, and page 624. Page 571, table 28.1 provided a table that categorized the groups of protists and their distinguishing characteristics. The straight-froward table made it easy to integrate the information in my mind. Secondly, the evolutionary tree on page 579 gave me a clear understanding of the relationships between all land plants. Lastly, page 624 provided the distinguishing characteristics of fungi phyla. Two websites gave me further understanding of the plant diversity unit. First, the bryophyte website clearly explained the key traits in bryophytes and how they are similar to some traits still maintained in the vascular plants. Second, the seedless vascular plant website from Ohio State gave a full discussion on the CO2 uptake by ferns and the possible connections that seedless vascular plants have to other plant life.

Learning about plants means nothing until one can say that it made a difference in their thinking about the diversity of plants. I have always valued plants and their contribution to my life, but it wasn’t until we began discussing the seedless vascular plants and fungi did I gain a newfound perspective on plants. For example, I am vegan and have always paid my homage to the plants that provide nutrition to my life, but I never looked at the plants that were responsible for the evolution of many of the land plants that we see today. For example, the role of seedless vascular plants in harboring the carbon dioxide to provide a pristine environment for the development of seed plants went unnoticed until David discussed it in lecture. Furthermore, I was blind to the prevalence of symbiotic relationships between most plant life and fungi until I read the Campbell text and listened to Tyler Waterman’s presentation. After I realized that plants work together in so many ways, I was able to put the evolutionary tree together in my mind without having to struggle with understanding why they are related in the ways that they are. Plant life is diverse. It covers the world in beautiful colors, it serves itself in economical and ecological ways, and it has evolved over the last 600 million years to produce an earth that animals can inhabit. The plantae kingdom continues to awe its followers as more relationships are being discovered between the phyla and as new species arise.

Sources:
1. http://bryophytes.plant.siu.edu/
2. http://www.hcs.ohio-state.edu/hcs300/svp1.htm
3. Fern Lecture- David
4. Fungi Lecture- Tyler Waterman
5. Sara’s Protist Lecture
6. Coniferophyta Lab- ponderosa pine slides
7. Fern Growing Lab- agar dishes
8. Campbell Textbook, 7th edition
9. Nic’s Angiosperm Lecture