This is a paper that I wrote my second year at NHTI in by biology (I) course on moss (yes you can write a final term paper on moss and enjoy it). It was an eye opening project and presentation that I really enjoyed. Written April 2nd 2016.
Nonvascular and Seedless Vascular Plants
Cassidy Brennan
Understanding the plants around us today is important, and understanding their history is equally important. Scientists today look to plants and their origins to evaluate health and biodiversity of forests as we will look at later on.
Similar to the animal kingdom it's thought to be true that all plants have a common ancestor, and this ancestor is believed to be green algae from the charophyte group for a variety of reasons. These include the fact that both land plants and algae have common characteristics such as the photosynthetic material Chlorophyll B, the cell wall made from cellulose, storage of carbon, and the formation of the cell plate during the process of cytokinesis are also characteristics shared between green algae and terrestrial plants. On a molecular level analysis of the evolution of the RNA and the DNA of both green algae and terrestrial plants proves that they have a common ancestor as well (Woodward, 2015). Modern plants have less in common with the ancestral plant than the charophyte group does.
Looking back at fossil remains we can see evidence of plants from at least 475 million years ago. It's possible that plants moved from aquatic environments to terrestrial environments to better adapt to a changing climate on Earth. The benefits of moving to land were better access to sunlight and less competition in the beginning. The increase in sunlight was hugely beneficial ancestral plants, who needed blue and red wavelengths to photosynthesize and produce enough molecular compounds such as proteins, carbohydrates, and lipids to live. At this point, with no competition and unlimited or restricted access to the most vital resource, terrestrial plants evolved. (Woodward, 2015). Because of the lack of any structures that would store or move water, the early plants were limited to moist environments that would always provide the water needed. The fossil evidence along with scientific thinking suggests that bryophytes were the first evolutionary step away from green algae.
Bryophytes are seedless nonvascular plants that include mosses, liverworts, and hornworts. The most basic of all plants, the nonvascular group are the only ones where the gametophyte stage is dominant. The structure consists of rhizoids that serve as structural support and resemble roots, although only in appearance. Small leaf looking green tissue makes up the body of the plant and this is where water is absorbed, photosynthesis occurs, and where the archegonium and antheridium are located. The haploid stage, in which a multicellular haploid gametophyte develops from a spore derived from the antheridium and produces haploid gametes in the archegonium, is the dominant stage in the bryophyte life cycle. The mature gametophyte produces both male and female gametes, which join to form a diploid zygote. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues. The sperm spores have to travel to the archegonium through water on the plant, which is one of the reasons why a moist environment is required for survival. The development of a protected embryo is an adaptation to terrestrial land, something not seen in water plants before this point (Woodward, 2015).
About 420 million years ago the fossil record starts to show evidence of vascular tissue trends in terrestrial plants, and during the Carboniferous period they were the dominant land plant type. There are now three classifications of seedless vascular plants: ferns, horsetails, and lycophytes. The biggest difference in the life cycle of seedless vascular plants from non-vascular plants is the at during the alternation of generations, the sporophyte stage is now the dominant and most visible stage. Haploid spores come from the sporangium on the dominant sporophyte and develop into a young gametophyte that supports both an antheridium and archegonium with haploid gametes. The gametes eventually meet and make a new diploid sporophyte. This fertilization process is still dependant on moisture to carry the spores to the archegonium which means the process is carried out closer to the ground for its facilitation (Schaeffer et al., 2009).
The biggest advantage of vascular tissue in plants is the ability to transport fluids throughout the body of the plant and structural support for upward growth. The two most important tissues are xylem (nonliving cells) and phloem (living cells). Xylem is used to transport water and minerals to the rest of the plant from the roots, and phloem transports sugars made during photosynthesis down to the roots from the leaves. The harder substance given to the body of the plant is primarily from lignin that is found in the matrix of the cell walls (Schaeffer et al, 2009). Terrestrial plants at this point now have protection for the elements as well. A waxy “cuticle” that remains a solid at all temperatures, now coats the exposed surfaces of the plant. The wax is made of lipids that are hydrophobic and protect the plant from losing or gaining unwanted fluids. To keep this beneficial to the plant, there are small openings on the bottom of leaves called stomata that are regulated by the plant and provide a passageway for gases to flow as waste product, and for necessary molecules to reach the plant's vascular system. These gasses are water vapor (going both ways depending on the humidity of the surrounding environment), carbon dioxide to be used in photosynthesis, and oxygen as a waste product of photosynthesis (Woodward, 2015). A less well known change in development is apical meristem shoot starts to be seen in seedless vascular plants with a cell known as the apical cell (inverted and pyramidal) that isn’t seen angiosperms (Frank et al., 2015).
It is well known that biodiversity is important among plant communities for a variety of reasons (health of the forest and inhabitants etc.) and there are different ways to measure the health and age of a forest with the amount of biodiversity within it. There was a long term study done that concluded that the older the forest was, the more diverse it was and the more rare species of plants it held. This study was done in the late nineteen nineties by scientists in Canada who were looking for evidence of the importance of biodiversity in the boreal balsam fir forest of eastern Canada. They achieved this by taking samples of plant material, determining its age and species, and then calculating the diversity by various means. It was determined that forests that have been undisturbed by natural events (such as forest fire, geologic events, etc) and also have limited to no human interference - classified as ‘old growth’ - have higher rates of biodiversity in plant material including nonvascular and seedless vascular varieties. “A total of 213 species of nonvascular plants, lichens, and fungi were identified, including 33 lichens, 43 mosses, 29 liverworts, and 108 saprophytic fungi” according to the article. In their conclusion on that matter of nonvascular and seedless vascular plants in the community the author states:
The study of the nonvascular plant communities has mainly demonstrated that old-growth forests harbour a large number of rare or uncommon species, most of which are not found in mature second-growth stands. These are mainly specialist species requiring very specific environmental conditions. The majority of these species colonize coarse woody debris...Very coarse woody debris have the advantage of conserving a minimum of humidity during periods of drought, offering a greater colonization surface and persisting over longer periods, favouring rare species sensitive to variations in humidity.
Understanding the plants we have around us is a huge part of understanding plant biodiversity and its importance in forest and ecosystem health. More studies specifically on how nonvascular and seedless vascular plants effect a forest and inhabitants biodiversity would provide more detail into their importance. Possibly even global effects of the nonvascular and seedless vascular populations, such as their role in climate change, fuel supplies and how those subjects themselves have influenced the nonvascular and seedless vascular populations in turn.
References
Desponts, M., Brunet, G., Bélanger, L., & Bouchard, M. (2004). The eastern boreal old-growth balsam fir forest: A distinct ecosystem. Canadian Journal of Botany. Retrieved March 18, 2016.
Frank, M. H., Edwards, M. B., Schultz, E. R., McKain, M. R., Fei, Z., Sørensen, I., & ... Scanlon, M. J. (2015). Dissecting the molecular signatures of apical cell-type shoot meristems from two ancient land plant lineages. New Phytologist, 207(3), 893-904. doi:10.1111/nph.13407.
Schaeffer, S. W. (2009, June 10). Plants II: Seedless Vascular Plants. Retrieved from https://wikispaces.psu.edu/display/110Master/Plants II - Seedless Vascular Plants .
Woodward, D. (2015, May 18). Plants I: Evolution and Diversity, Nonvascular Plants. Retrieved from https://wikispaces.psu.edu/display/BIOL110F2013/Plants I - Evolution and Diversity, Nonvascular Plants.
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