Understanding True Biological Diversity: A Comparison of the Life Cycles of Common Mosses and Ferns
The human life cycle, especially from the time of birth onwards, is actually relatively simple despite its seeming complexities. In the womb, the human life cycle is actually more complex, with the fetus going through many radically different physical shapes and developmental stages; once born, however, human beings retain the same basic shape and physiological functions throughout the rest of their lives. There are different stages of development, of course, with the growth and life of a human being broken into several different periods of psychological and physical development but here is not a true change in form accompanying these stages, such as the changes that have been observed in many other organisms’ life cycles.
The butterfly is commonly used in both science and the humanities in a metaphoric sense to illustrate truly different stages in an organism’s life cycle. Most second graders know that butterflies begin life as caterpillars emerging from eggs; that these caterpillars then build cocoons and enter the pupa stage of their life cycle, in which they are entirely sedentary but for the metamorphosis they are undergoing; and finally, that they emerge as butterflies, in the final adult stage of their life cycle wherein they are ready to lay (or fertilize) a new batch of eggs. Though perhaps simplistic and without a doubt somewhat hackneyed at this point, the example of the butterfly’s life cycle is still a valid illustration of what truly different life cycles look like, in one that is highly approachable and readily observable and thus an excellent educational tool. There are other lifecycle differences, however, that are more interesting and less commonly known due to their complexities.
This paper will provide a brief comparison of the life cycles of common mosses and ferns. The fact that these organisms are more closely identified as plants rather than animals is perhaps misleading, giving the impression that their life cycle stages would be less complex and the differences between them would be less extreme. This is not the case, however, and mosses and ferns both have unique life cycles that are tied directly to their reproductive modes and other unique features of these organisms. A literature review was conducted to obtain a broader understanding of the life cycles of mosses and ferns, and to ensure that information came from scholarly sources that could explain the true science behind the observed life cycles in addition to being more trustworthy than other sources. By analyzing and comparing the similarities and differences between the life cycles of mosses and ferns, it is hoped that the research contained below will lead to a deeper appreciation for the true degree of biological diversity that exists in the world.
Mosses are seed plants, and so although they have alternating generations like all plants, their life cycle is significantly different in both its mechanisms and its outward physical developments (Capon 2005; Cavendish 2000). Mosses begin their life cycle (or, at least they are typically said to “begin,” although it is technically incorrect to identify a “beginning” in an cyclical pattern) as masses of haploid cells — cells that have only one copy of each chromosome rather than two as in most other organisms including other plants (Cavendish 2000). Cells with two sets of chromosomes are referred to as diploid, and are generally the result sexual reproduction leading to genetic as well as phenotypic variations that create intra-species (Capon 2005; Cavendish 2000).
The haploid cells in mosses are the result of meiotic divisions that occur in the diploid cells that comprise only one of the stages in the moss life cycle (Mader 1987; Capon 2000). The haploid masses that we commonly know as moss exist solely to allow sexually fertilization to take place, and they actually contain make and female sex organs — sometimes on the same plant — that produce gametes and use the water in their environment as a medium allowing the movement of sperm from the male organs to the eggs produced by the female organs (Cavendish 2000; Mader 1987). Once fertilization occurs, sporophytes are formed and constitute the second stage of the moss life cycle (Capon 2005). The sporophytes grow while attached to the female gametophyte — the haploid mass of cells we know as moss (Cavendish 2000, Mader 1987). Both the gametophyte and the sporophyte developmental forms of most mosses grow through the process of mitosis, despite the fact that the gametophyte calls are haploid while the sporophyte cells are diploid (Caopn 2005). Sporophytes, as their name implies, produce the haploid spores that will eventually (or so it is hoped) attach to other damp surfaces and grow gametophyte masses to start the process of the moss life cycle all over again (Capon 2005; Cavendish 2000; Mader 1987). Because mosses do not grown from seeds or ever truly flower, they are not considered true plants. Their genetic sequence and its connection to the moss life cycle, however, has been noted to have serious implications with the genetic sequencing and life cycle development of other plants (Rensing et al. 2002). It has even been suggested that there is much to be learned about the development and evolution of animal species and even human beings from a study of the life cycles of mosses and their relation to the life cycles of animal organisms (During 1979). Understanding the moss life cycle could potentially lead to e better understanding of evolution itself.
There is also much that can be learned from mosses about the life cycle of ferns, which have very similar life cycles and are also non-flowering and seedless in nature (Mehltreter et al. 2010; Sadava et al. 2011). Beginning at the same stage that was used as the ‘beginning” of the moss life cycle, ferns “begin” as haploid spores themselves, and can grow into gametophyte masses as long as the conditions are correct (Mehltreter et al. 2010; Sadava et al. 2011). The difference, however, is that these gametophyte masses are not generally as long lived as the gametophyte moss masses simply because they are not needed for as long as the moss gametophytes are (Mehltreter et al. 2010). While the haploid gametophyte stage is a dominant physical form in the moss life cycle, fern sporophytes are more dominant and can persists for hundreds of years while continuing to asexually reproduce through spore deposition (Sadava et al. 2011; Evans 1964).
The fern sporophytes are the leafy “plants” that are commonly thought of as ferns, just as the gametophyte stage in the moss life cycle is the form typically thought of as “moss.” Both mosses and ferns produce sporophytes through sexual reproduction; in mosses, sporophytes remain attached to and are dependent on female gametophytes, while the more advanced root and leaf structures of fern sporophytes allow them to grow more independently and to continue reproducing asexually for a longer period of time (Evans 1964; Sheffield & Bell 1987). It has even been suggested that this is due to a collection of RNA in the cytoplasm of cells that somehow reactivates sporophytic growth, making the continued longevity of ferns in the sporophyte stage almost certainly a rather strange mutation that served the organisms quite well (Sheffield & Bell 1987).
This is only one of the ways in which an understanding of the fern life cycle can help provide a deeper understanding of the nature and progress of evolution. As one of the least changed plant-like organisms still in existence, ferns provide a window into the evolutionary past of life on this planet unlike any other type of organism — others are older, certainly, but they cannot provide the same glimpse of genetic history that ferns can (Mehltreter et al. 2010; Evans 1964). Some researchers have even developed and tested hypothesis regarding the genetic link of true plants to fern-like predecessors, suggesting that that the generally more advanced sexual organs of flowering plants evolved out of the analogous structures that exist on ferns (Munster et al. 1997). The basis of all plants’ alternating generations and complex life cycles could be found in a common ancestor shared with fern species, even though ferns are no better reproducing sexually than moss are fully dependent on a saturated enough environment to perform a task that flowers have developed innumerable methods of getting done (Munster et al. 1997; Mehltreter et al. 2010).
The life cycles of moss and ferns are highly similar, with both developing from haploid cells and gametophytes that sexually reproduce to create sporophytes, which in turn asexually reproduce by producing haploid spores that start the cycle all over again. Mosses, however, are more typically found in their haploid gametophyte stage, which the sporophytes are dependent on, whereas ferns are most often seen in the sporophyte stage, in which they can survive for hundreds of years. Both types of organisms are still being studied today to provide clues to our own evolution and other genetic mysteries.
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