Craig
Illustrated Guide Comparing the Somatic and Reproductive Structures of Botanical Organisms
Comparative Botany – BIOL 4404
May 8, 2011
PREFACE:
In the studying of botanical sciences, there are several organizational hierarchies to consider than “just plants.” In light of recent taxonomic efforts, the notion of a Kingdom Plantae is a bit obsolete. First of all, one must consider the super-group Archiplastidia. This group consists of the glaucophytes, red algae, green algae, and land plants. For the most part, all of the organisms in these sub-groups would be easily distinguishable as plants. However, in addition to Archiplastidia, botanical science also deals with a specific sub-set of prokaryotic life forms: the cyanobacteria. This guide will cover the following botanical organisms: cyanobacteria, red algae, bryophytes, filicalean ferns, gymnosperms, and angiosperms.
Cyanobacteria, as previously mentioned, are prokaryotic organisms and thus are not grouped into Archiplastidia. Regardless of their out-group status, cyanobacteria are vital to all photosynthetic organisms. Current research has provided evidence that supports that the chloroplasts found within the cells of the members of Archiplastidia are derived from a past endosymbiosis between plants and cyanobacteria. Therefore, without these prokaryotic beings, plants would not be able to harness energy from sunlight. Although cyanobacteria are simple organisms, the different species can come in several forms. Some cyanobacteria are unicellular. They can either exist as individual, free-living cells or can live in a colony of similar unicells. Other cyanobacteria exist as multicellular filaments. These filaments can be unbranched, pseudobranched, branched, or tapered. Some filamentous cyanobacteria have specialized cells called heterocysts that allow them to fix nitrogen. Like unicellular cyanobacteria, filamentous strains can either live as individuals or exist as colonies of like species. This guide will specifically cover details on Merismopedia sp., a non-filamentous, colonial cyanobacterium.
Red algae, or Rhodophyta, is a large group that consists of thousands of species. Reds primary exists in marine environments; however, some varieties can be found in freshwater areas. Some red algae are independent, free living, and unicellular. These varieties have very simple life cycles. Other reds have multicellular, filamentous growth habits with complex life cycles consisting of multiple phases. Polysiphonia sp. is one instance of a complex red alga and will be the example for that group.
Bryophytes are an assemblage of three sub-groups: bryophyta, hepatophyta, and anthocerophyta. This group of embryophytes is considered to have the most complex gametophyte phase and the least complex sporophyte phase of all land plants. They are non-vascular, and thus require moist conditions to be active. Bryophyta includes mosses, hepatophyta is composed of thallose and leafy liverworts, and anthocerophyta includes the hornworts. This guide will focus on the thallose liverwort Marchantia polymorpha.
Ferns, or Pteridiophyta, are true vascular land plants. They are directly opposite of bryophytes in that they have simple gametophytes and complex sporophytes. There are several groups of ferns, but the most familiar of these are the leptosporangiate, or Filicalean, ferns. Of the leptosporangiates, this guide will primarily focus on Pteris sp.
Gymnosperms are vascular, seed-bearing plants. Most gymnosperms can be described as tree-like; however, this description does not fit all members. Most gymnosperms also consist of woody stems and branches. Gymnosperms possess the simple gametophyte phase as well as the most complex sporophyte phase of all land plants. Gymnosperms include the cycads, Ginkgo, gnetophytes, and conifers. In this guide, the conifer Pinus sp. will be utilized as the specific example of gymnosperms.
Angiosperms, like gymnosperms, are also vascular, seed-bearing plants. Angiosperms are comprised of the flowering land plants and contain the most individual species of the seed plants. They are similar to the gymnosperms in their having a complex sporophyte and simple gametophyte. The main difference between the two groups is that angiosperms produce an ovule that is completely surrounded by extra tissue and is thus isolated from the outside world, while gymnosperms make ovules that are in contact with the open air. Additionally, angiosperms are by no means considered to be woody plants. While there are many different species of flowering plants, Erodium cicutarium, or Crane’s Bill, will be the example angiosperm in this guide.
While this guide is fairly comprehensive in its coverage of examples of cyanobacteria and Archiplastidia, it is not quite complete. Due to time constraints and lack of adequate specimens, two noteworthy groups have been left out.
First of all, the green algae have been neglected. Green algae is made up of three groups: the Cholorphyceae, the Charophyceae, and the Ulvophyceae. There is a lot of variety amongst the differing species of green algae. The unicellular types of greens can be free living or colonial. Furthermore, there are both flagellated and non-flagellated versions of green algae unicells and colonies. Green algae can also come in branched and unbranched filamentous forms. Finally, some greens are very large and almost appear plantlike in contrast to their almost microscopic counterparts.
Secondly, a general group called the fern allies has been left out. The fern allies are vascular plants that share a very similar life cycle and growth phases with ferns. This group consists of the groups Lycophyta and Sphenophyta. The Lycophytes are further subdivided into homosporous nonligulates, which produce only one type of spore and thus have only one type of gametophyte, and heterosporous ligulates, which produce male and female spores and gametophytes. Sphenophytes are also known as horsetails. All horsetails that currently exist are classified together into a single genus: Equisetum.
The purpose of this guide is to compare the aforementioned botanical groups in respect to certain aspects of their lifecycles and reproductive methods and structures. The somatic structures of each example will first be compared. Any structures associated with asexual reproduction will then be examined. Then, the structures in which meiosis occurs of the examples as well as what happens to the meiotic products will be compared. Finally, fertilization structures and products will be compared. Cyanobacteria and the sub-groups of Archiplastidia have such a wide variety of differences in their life cycles, and as such, not every example will be represented in each comparative section.
CHAPTER 1: Somatic Structures-
Merismopedia sp.:
Merismopedia sp. is a colonial cyanobacterium consisting of several sister cells living within a common sheath. Colonies of Merismopedia sp. are typically very small, although few macroscopic varieties exist. In its somatic stage, the cells of a colony exist in a single plane, that is, the colony is only one cell thick. Colonies typically have a rectangular overall shape, while individual cells of Merismopedia sp. are round. The edges of colonies are often irregular or wavy. Although each colony is an individual ‘sheet’ of cells, multiple colonies can aggregate into a larger, more three dimensional colony. (Komarek, 1998)
****habit photo OF MERISMOPEDIA****
Polysiphonia sp.:
In Polysiphonia sp., vegetative growth occurs within four somatic structures. The somatic diploid stages of Polysiphonia sp. include a free living structure and a dependent structure. The tetrasporophyte is a free living structure that arises from a carpospore and will produce tetrasporangia when it becomes reproductive. The dependent somatic diploid structure of Polysiphonia sp. is the carposporophyte. The carposporophyte arises from a zygote and develops on a female gametophyte within an old cystocarp. The carposporophyte will eventually give rise to several carposporangia. There are two different somatic haploid variants of Polysiphonia sp. These are the male gametophyte and the female gametophyte. Both ‘genders’ are free living and arise from a corresponding male or female tetraspore. When reproductive, the male gametophyte will produce spermatangial branches whereas, the female will produce a cystocarp.
It should be noted that, the tetrasporophyte, male gametophyte, and female gametophyte of Polysiphonia sp. all look identical until they become reproductive and thus begin forming their respective reproductive structures. These structures appear as long, branching filaments to the naked eye. When viewed under microscope, however, further detail of the growing and dividing cells can be seen.
***INSERT PHOTO OF POLYSIPHONIA SP. HABIT AND CARPOSPOROPHYTE***
In the free living somatic structures, length increases via growth that occurs through cell division at an apical cell. When the apical cell divides, it produces a new axial cell below it. The axial cell can then undergo mitosis and divide laterally to form pericental cells around it.
***PHOTO OF CLOSE UP OF POLYSIPHONIA SP.***
Marchantia polymorpha:
In liverworts, specifically Marchantia polymorpha, there are three structures associated with somatic growth. Marchantia polymorpha undergoes vegetative growth in the free living, haploid gametophyte. Since Marchantia polymorpha is dioecious, it has a separate male and female gametophyte. These gametophytes are similar in appearance until they produce reproductive structures. They are composed of a flat leaf-like structure with rhizoids and scales on its underside. The gametophyte also has a midrib and several pores along its surface. Cell division in the gametophyte occurs at an apical notch. From here, the leaf expands laterally as cells are added.
****GAMETOPHYTE PHOTO*****
Marchantia polymorpha also has a dependent somatic diploid structure: the sporophyte. The sporophyte of Marchantia polymorpha consists of a foot, which is anchored into the old archegonia, a seta, and a capsule. At the base of the capsule, where it meets the seta, are several stomata. While the capsule contains sporogenous tissue, it is still considered a somatic structure up until meiosis begins within the capsule.
*****SPOROPHYTE*****
Pteris sp.:
Ferns have two different somatic stages. In Pteris sp., there is a free living, haploid gametophyte (as is the case in Marchantia polymorpha). Since ferns are monecious, both male and female reproductive structures are produced on the same gametophyte. The gametophyte of Pteris sp. consists of a flat, leaf like body, called a prothallus, which is somewhat heart shaped. The prothallus is comprised mostly of chloroplast-containing photosynthetic cells. In the ‘notch’ of the heart shape of the gametophyte is a meristem where cell division occurs which, in turn, causes the gametophyte to grow. On the underside of the gametophyte are several rhizoids which anchor the structure to its substrate.
****GAMETOPHYTE ****
Pteris sp. also has a haploid sporophyte phase. The sporophyte has a foot that is imbedded in the gametophyte. The sporophyte is not totally dependent, however, because it also has roots that anchor it to its substrate. The foot and roots are connected to the main body, or shoot, of the sporophyte. The shoot typically runs parallel to the surface upon which it is growing. Along the shoot are branches of microphyllus leaves. Ferns are somewhat unique in that their leaves unfurl from the shoot. Young branches of leaves that have yet to fully unroll are commonly referred to as fiddleheads, due to their superficial similarity to the scroll of a violin.
****SPOROPHYTE, FIDDLEHEAD***
Pinus sp.:
The primary somatic structure associated with Pinus sp. is the diploid sporophyte stage. The sporophyte consists of a woody stem, or trunk, that contains both xylem and phloem. The trunk is anchored into the ground via a taproot system. Growing out from the trunk are numerous woody branches. Upon each branch are several clusters of needle like, microphyllous leaves. Each cluster of needles is referred to as a fascicle. The sporophyte arises from a germinating seed.
*****PINE HABIT, BRANCH CLOSE UP***
On the branches of Pinus sp. both male and female diploid cones can be found. While the cones contain the sites of fertilization and reproduction, they can still be considered a somatic structure, much in the same way that the sporophyte capsule in Marchantia polymorpha is, due to the fact that the cones increase in size due to mitosis. Each cone is composed of multiple scales surrounding a central core in a tight, spiral pattern.
The male and female gametophytes of Pinus sp., the microgametophyte and megagametophyte respectively, are also somatic structures. The haploid microgametophyte is an individual pollen grain that arises from a microspore via cell division. The haploid megagametophyte arises from a single megaspore that undergoes mitosis to form the innermost part of the ovule.
****POLLEN & MEGAGAM*
Erodium cicutarium:
The somatic structures of Crane’s Bill are very similar to that of Pinus sp. Crane’s Bill has a somatic diploid sporophyte that arises from a germinating seed. The sporophyte has a fibrous-like root system which anchors the herbaceous, vascular stem to the ground. Erodium cicutarium has multiple megaphyllous leaves emanating from the stem. Also branching from the stem are buds, flowers, and fruits; however, these are not somatic in and of themselves and will thus be covered in later sections.
***HABIT PHOTO***
Erodium cicutarium, like Pinus sp., also has somatic haploid microgametophytes and megagametophytes. Respectively, these are the pollen grain and the embryo sac. Pollen grains in Erodium cicutarium arise in a similar manner to those in Pinus sp.; however, they look slightly different. As in Pinus sp., the megagametophyte arises from a megaspore within the ovule. The key difference in these two megagametophytes is that those in Pinus sp. are exposed to the outside world, while those of Crane’s Bill are completely sealed off.
****POLLEN & MEGGAM****
CHAPTER 2: Asexual Reproductive Structures-
Merismopedia sp.:
There are no gametes in Merismopedia sp., so asexual binary fission is the only method of reproduction available to it. In Merismopedia sp., cell division occurs as it would in any cyanobacteria: a single cell clones replicates its genetic material and each copy is transferred into a forming daughter hemi-cell. The mother cell will then cleave into two identical cells. However, because Merismopedia sp. exists as a flat sheet, the cells within the colony’s sheath will only divide perpendicularly to the sheath. (John, 2002) Merismopedia sp. can also create new colonies by dissolving the sheathes of existing colonies. This allows the individual cells to regroup and form new colonies. However, this is not technically reproduction, as the number of cells remains the same.
****DIVIDING CELL*****
Polysiphonia sp.:
In Polysiphonia sp., asexual reproduction occurs solely within the carposporangia. Cell division occurs in the carposporangia to give rise to multiple diploid carpospores. Each carpospore is essentially a clone of the initial carpospore produced in a carposporangium. Therefore, there is no genetic variation between carpospores that arise from the same carposporangium. The carpospores are eventually shed and will germinate to give rise to a tetrasporophyte.
*****CARPOSPORE & SHIT*****
Marchantia polymorpha:
The gametophyte of Marchantia polymorpha is capable of asexual reproduction through gemmae. Gemmae are haploid, spore-like materials that are formed by mitotic cellular division in gemma cups. These cups are located across the surface of the gametophyte. Asexual reproduction occurs when water splashes into, and then out of the cups, thus spreading the gemmae. A gemma will then germinate and form a new gametophyte that is identical in gender and genetic composition to the one from which it came.
****GEMMA CUP + GEMMAE PHOTO****
Pteris sp.:
In most Filicalean ferns, such as the example in this guide, asexual reproduction does not occur. However, it is possible for some ferns to reproduce without fertilization. A few species of ferns have been successfully regenerated by culturing cuttings of the rhizomes of a sporophyte. When given adequate nutrition, these severed rhizomes regenerated into separate sporophytes. (Fernandez, 1997) In another study, it was found that some species of leptosporangiate ferns have gemma cups on their gametophytes. (Farrar, 1990) In these instances, asexual reproduction occurs in the exact same way as it does in bryophytes such as Marchantia polymorpha.
Pinus sp.:
Asexual reproduction did not occur in any observed specimens of Pinus sp. This is due to the fact that, in nature, Pinus sp. will only reproduce via sexual fertilization. However, it is possible to force some species to reproduce without using gametes. By amputating a fascicular shoot (with needles) from a branch, one can treat the cutting with supplements and it will take root into a substrate, thus eventually growing into a new sporophyte. (Mergen, 1964)
Erodium cicutarium:
Erodium cicutarium does not undergo any sort of asexual reproduction or cloning. It is strictly a sexual organism. However, other angiosperms (mostly aquatic varieties) can asexually reproduce. These asexual-capable angiosperms can reproduce in a number of ways. Some species can produce and disperse seeds without fertilization. These seeds will then germinate into a new plant. Other species can undergo a similar process to ferns and produce new growth from portions of their vegetative structures. (Philbrick, 1996)
CHAPTER 3: Structures Associated with Meiosis in Eukaryotes-
Polysiphonia sp.:
Meiosis in Polysiphonia sp. occurs only within the tetrasporangia, which is associated with the free living tetrasporophyte. Before meiosis, the tetrasporangia look similar to the other filaments of the tetrasporophyte; however, once meiosis begins, each tetrasporangium is filled with a tetrad of tetraspores and swells up.
***TETRASPORANGIUM PHOTO***
Marchantia polymorpha:
In Marchantia polymorpha, meiosis only occurs in the capsule of the sporophyte. When the capsule begins to mature, the diploid sporogenous tissue within the capsule, which is now the sporangium, will begin to differentiate. Upon further development of the capsule, the sporogenous tissue will become even more differentiated from the cells of the capsule wall. (Durand, 1908) When fully mature, the sporogenous tissue will have fully developed into spores and elaters.
*****SPORANGIUM********
Pteris sp.:
On the underside of the leaves of the sporophyte lie numerous sori. These structures consist of multiple sporangia clustered within a covering called the indusium. Each individual consists of an outer jacket called the annulus, an inner layer called the tapetum, and a region of sporogenous tissue in the center.
****SORI & SPORANGIA***
Pinus sp.:
Erodium cicutarium:
CHAPTER 4: Fate of Meiotic Products in Eukaryotes-
Polysiphonia sp.:
Meiosis in the tetrasporangia will yield tetraspores. These spores are formed in clusters of four haploid spores that are collectively called tetrads. In each tetrad there are two male spores and two female spores. When shed, each tetraspore will give rise to a male or female gametophyte.
*****TETRASPORES****
Marchantia polymorpha:
After meiosis, the capsule will be filled with male spores, female spores, and elaters. The elaters are specialized filaments that will help disperse the spores once the capsule opens. When the spores are shed, they will germinate into their corresponding male or female gametophyte.
****CAPSULE W/ SPORES, ELATERS.***
Pteris sp.:
Within each sporangium, the sporogenous tissue will give rise to spore mother cells. The spore mother cells will differentiate into spores. The spores of ferns are semi-rounded with a rough texture. The spores also have a triradiate ridge along their surfaces. The spores are provided nutrients from their host sporangium via the tapetum until they are ready to be shed. When ready, the sporangia will dehis and release the spores. At this point, the indusiums of the sori will have also opened and the spores can freely fall from the sporophyte leaves. After dispersal, the surviving spores will give rise to a gametophyte.
****SPORES******
Pinus sp.:
Erodium cicutarium:
CHAPTER 5: Structures Associated with Fertilization in Eukaryotes-
Polysiphonia sp.:
In Polysiphonia sp., the structures associated with gamete production are attached to their respective male and female gametophytes. Male gametophytes will form specialized protrusions called spermatangial branches. These branches are comprised of numerous spermatangial which each contain several spermatia.
***SPERMATANGIAL BRANCH PHOTO****
Female gametophytes have a slightly more complex structure called a cystocarp. Within the cystocarp is a row of specialized cells that is called the carpogonial branch. At the end of the carpogonial branch is a specialized cell that contains the egg. This cell is called the carpogonium. At the end of the carpogonium is a long filament that is called the trichogyne.
****CYSTOCARP PHOTO****
During fertilization, spermatia are released from the male gametophyte’s spermatangial branches. Because the spermatia are not flagellated, they must rely on movement in the surrounding water to reach their female counterparts. In the event of a successful fertilization, one or more spermatia will come into contact with the trichogyne. The spermatia will then be directed down the trichogyne to the carpogonium, where the egg is located. Once a spermatium reaches the carpogonium and egg, a zygote is formed within the cystocarp.
Marchantia polymorpha:
The male gametophyte of Marchantia polymorpha will produce antheridiophores when it is time to attempt fertilization. These stalked structures rise above the surface of the gametophyte and have a flat head at their apex. Within this head are multiple antheridia. Each antheridium is comprised of a short stalk that is set below a sterile layer of cells that surround a region of spermatogenous tissue. The tissue will develop into spermatogenous cells, and each cell will yield a single, bi-flagellated sperm.
***ANTHERIDIOPHORE, ANTHERIDIA***
The female gametophyte also carries its sexual reproductive organs on a stalked structure called the archegoniaphore. At the top of the archegoniaphore is a structure that can be described best as a lobed umbrella, as it has several rounded arms that hang from a central mass. Under each arm and towards the stalk lie several archegonia. The archegonium, as it is in other organisms, is composed of an enlarged portion called the venter, which is anchored to the archegoniaphore. The cells of the venter continue down to a long, narrow portion of the archegonium called the neck. The neck is usually sealed off, but when Marchantia polymorpha is sexually reproductive, a canal is formed up the neck and into the interior of the venter, where an egg is contained.
***ARCHEGONIAPHORE + ARCHEGONIA***
Pteris sp.:
Although ferns only produce one type of gametophyte, both male and female reproductive structures are present. As with some other plants, the female reproductive structure is the archegonia. It is composed of a neck with neck canal and a venter. The venter is affixed to the surface of the gametophyte and contains an egg within it. The male reproductive structure is the antheridia. Antheridia in ferns appear as small hemispherical growths on the surface of the gametophyte. Each antheridium contains numerous sperm. The sperm in ferns are helical in shape and have multiple flagella running most of the way down their lengths. While in the antheridia, the sperm are compressed into a flat disk.
***ARCHEGONIA, ANTHERIDIA, SPERM***
Pinus sp.:
Erodium cicutarium:
CHAPTER 6: Products of Fertilization and Their Fates in Eukaryotes-
Polysiphonia sp.:
After a period of dormancy, the diploid zygote will begin to divide again. From the zygote, the carposporophyte develops. The carposporophyte grows within the pericarp of the old cystocarp and is thus dependent on the female gametophyte. When mature, the carposporophyte will produce carpospores.
****CARPOSPOROPHYTE PHOTOS***
Marchantia polymorpha:
Sperm is typically delivered from an antheridiophore to an archegoniaphore by rainwater. A raindrop will splash off of an antheridiophore and pick up multiple sperm. When the droplet hits the head of an archegoniaphore, it will create a liquid layer over the arms, thus giving the sperm a surface in which to swim to the archegonia and eggs.
When a sperm reaches an archegonium, it will swim up the neck canal and fuse with the egg to form a diploid zygote. At this time, the other archegonia on that particular arm of the archegoniaphore will abort so that no resources are wasted; however, the archegonia on other arms are unaffected. The zygote will undergo mitosis to form a multi-cellular embryo. The cells of the embryo, or young sporophyte, will eventually differentiate into the three parts of the sporophyte. At the basal portion of the old archegonium, the foot of the young sporophyte will form. The foot will be permanently affixed to the archegoniaphore. Attached to the foot, the seta and capsule will develop and grow away from the old archegonium, which now forms a layer over the capsule called a calyptra.
****Young SPOROPHYTE******
Pteris sp.:
When sperm are released from the antheridia, each one elongates from its initial disk shape into a helical filament. The sperm then use their flagella to swim towards the archegonia that also lie on the surface of the gametophyte. A sperm will swim down the neck canal of an archegonium and into the venter. The sperm will then fuse with the egg and form a zygote. At this point, the neck canal of the archegonia will close and the other archegonium surrounding it will abort. After dormancy, the zygote will divide into an embryo which will eventually become a sporophyte. The developing sporophyte remains attached to the old archegonia through a foot. The foot will branch off into an initial root and also into a primary shoot and leaves.
****ZYGOTE, YOUNG SPOROPHYTE****
Pinus sp.:
Erodium cicutarium:
CONCLUSION:
![:\](https://bluelight.org/xf/BL_Images/Orig_Emoji/wrygrin.gif "wry grin :\")
