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Haploid or diploid. Which would you say is dominant?
That may depend on the plant. Start with moss. The typical nonvascular plant. But such a simple plant has a very interesting life cycle. Whereas most kinds of plants have two sets of chromosomes in their vegetative cells, mosses have only a single set of chromosomes. So, how does meiosis occur?
Life Cycle of Nonvascular Plants
Nonvascular plants include mosses, liverworts, and hornworts. They are the only plants with a life cycle in which the gametophyte generation is dominant. Figure below shows the life cycle of moss. The familiar, green, photosynthetic moss plants are gametophytes. The sporophytegeneration is very small and dependent on the gametophyte plant.
Like other bryophytes, moss plants spend most of their life cycle as gametophytes. Find the sporophyte in the diagram. Do you see how it is growing on the gametophyte plant?
The gametophytes of nonvascular plants have distinct male or female reproductive organs (see Figure below). Male reproductive organs, called antheridia (singular, antheridium), produce motile sperm with two flagella. Female reproductive organs, called archegonia(singular, archegonium), produce eggs.
The reproductive organs of bryophytes like this liverwort are male antheridia and female archegonia.
In order for fertilization to occur, sperm must swim in a drop of water from an antheridium to an egg in an archegonium. If fertilization takes place, it results in a zygote that develops into a tiny sporophyte on the parent gametophyte plant. The sporophyte produces haploid spores, and these develop into the next generation of gametophyte plants. Then the cycle repeats.
- In nonvascular plants, the gametophyte generation is dominant. The tiny sporophyte grows on the gametophyte plant.
- Describe antheridia and archegonia and their functions.
- Create your own cycle diagram to represent the moss life cycle.
Characteristics of Mosses and Other Non-Vascular Plants
Non-vascular plants, or bryophytes, include the most primitive forms of land vegetation. These plants lack the vascular tissue system needed for transporting water and nutrients. Unlike angiosperms, non-vascular plants do not produce flowers, fruit, or seeds. They also lack true leaves, roots, and stems. Non-vascular plants typically appear as small, green mats of vegetation found in damp habitats. The lack of vascular tissue means that these plants must remain in moist environments. Like other plants, non-vascular plants exhibit alternation of generations and cycle between sexual and asexual reproductive phases. There are three main divisions of bryophytes: Bryophyta (mosses), Hapatophyta (liverworts), and Anthocerotophyta (hornworts).
Life Cycle of a Moss
Life Cycle of Seedless Vascular Plants
Unlike nonvascular plants, all vascular plants—including seedless vascular plants—have a dominant sporophyte generation. Seedless vascular plants include clubmosses and ferns. Figure below shows a typical fern life cycle.
A mature sporophyte fern has the familiar leafy fronds. The undersides of the leaves are dotted with clusters of sporangia. Sporangia produce spores that develop into tiny, heart-shaped gametophytes. Gametophytes have antheridia and archegonia. Antheridia produce sperm with many cilia archegonia produce eggs. Fertilization occurs when sperm swim to an egg inside an archegonium. The resulting zygote develops into an embryo that becomes a new sporophyte plant. Then the cycle repeats.
Life Cycle of Gymnosperms
Gymnosperms are vascular plants that produce seeds in cones. Examples include conifers such as pine and spruce trees. The gymnosperm life cycle has a very dominant sporophyte generation. Both gametophytes and the next generation’s new sporophytes develop on the sporophyte parent plant. Figure below is a diagram of a gymnosperm life cycle.
Cones form on a mature sporophyte plant. Inside male cones, male spores develop into male gametophytes. Each male gametophyte consists of several cells enclosed within a grain of pollen. Inside female cones, female spores develop into female gametophytes. Each female gametophyte produces an egg inside an ovule.
Pollination occurs when pollen is transferred from a male to female cone. If sperm then travel from the pollen to an egg so fertilization can occur, a diploid zygote results. The zygote develops into an embryo within a seed, which forms from the ovule inside the female cone. If the seed germinates, it may grow into a mature sporophyte tree, which repeats the cycle.
Reproductive Cycle of Pines / The Amazing Lives of Plants (15 min.)
Life Cycle of Angiosperms
Angiosperms, or flowering plants, are the most abundant and diverse plants on Earth. Like all vascular plants, their life cycle is dominated by the sporophyte generation. A typical angiosperm life cycle is shown in Figure below.
The flower in Figure above is obviously an innovation in the angiosperm life cycle. Flowers form on the dominant sporophyte plant. They consist of highly specialized male and female reproductive organs. Flowers produce spores that develop into gametophytes. Male gametophytes consist of just a few cells within a pollen grain and produce sperm. Female gametophytes produce eggs inside the ovaries of flowers. Flowers also attract animal pollinators.
If pollination and fertilization occur, a diploid zygote forms within an ovule in the ovary. The zygote develops into an embryo inside a seed, which forms from the ovule and also contains food to nourish the embryo. The ovary surrounding the seed may develop into a fruit. Fruits attract animals that may disperse the seeds they contain. If a seed germinates, it may grow into a mature sporophyte plant and repeat the cycle.
Roots: Support for the Plant
Roots are not well-preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution than vascular tissue. The development of an extensive network of roots represented a significant new feature of vascular plants. Thin rhizoids attached bryophytes to the substrate, but these rather flimsy filaments did not provide a strong anchor for the plant nor did they absorb substantial amounts of water and nutrients. In contrast, roots, with their prominent vascular tissue system, transfer water and minerals from the soil to the rest of the plant. The extensive network of roots that penetrates deep into the soil to reach sources of water also stabilizes plants by acting as a ballast or anchor. The majority of roots establish a symbiotic relationship with fungi, forming mutualistic mycorrhizae, which benefit the plant by greatly increasing the surface area for absorption of water, soil minerals, and nutrients.
Bryophytes reproduce through both asexual and sexual life cycles.
Certain bryophytes are only capable of asexual reproduction or cannot achieve fertilization, due to loss of functional sexuality. Asexual reproduction occurs vegetatively via small, reproductive structures gemmae that disperse easily. Gemmae comprise either a single cell or cluster of cells that are having undifferentiated growth. A gemma can bring about the formation of a new vegetative structure or gametophyte when it dissociates from the parent plant body.
Fragmentation is another way of asexual reproduction, by which bryophytes can regenerate via leaf and stem fragments. The mechanism of spore dispersal and fragmentation is aided by water, wind, and animal movement.
Its life cycle exists two phases that are explained below:
This stage begins, when the dormant spores form a germ tube. The spores first develop a green, filiform complex called a protonema. A protonema goes through several maturation phases, after which it starts developing rhizoids and aerial filaments. Red light and kinetin stimulate the cells that promoting the growth of shoots, which later enlarge to modify into mature gametophytes.
Then, the gametophytes develop a specialized reproductive structure (gametangia) at the apex of main shoot that holds the gametes (eggs and sperms). The leafy gametophyte develops separate branches for the male gametangium (antheridium) and the female gametangium (archegonium).
The male antheridia produce on the upper branches that hold antherozoids or sperms, while the female archegonium carries a single egg or archaegonia. Before fertilization, the anthrezoids of the male gametangium of same or different plant goes down to the archaegonium via water droplets. There is a unique feature, where both the gametangia of bryophytes are enclosed by a sterile layer of non-reproductive tissue that is absent in algal gametangia.
This stage begins with the fusion of a biflagellate sperm with a single ovum inside the archegonium. After fusion, nuclear and cytoplasmic exchange occurs between anthreaoids and archegonia, resulting in the formation of the diploid zygote (2n), and the process is called fertilization. In bryophytes, a biflagellate sperm goes down to the archegonium via a passage of water film on the plant surface.
A diploid zygote in the archegonium develops into a multicellular, diploid embryo by undergoing repeated mitotic divisions. An embryonic sporophyte highly relies on the gametophyte that provides nutrition sources like sugars and minerals etc.
An embryonic sporophyte differentiates into three distinct structures, namely foot, stalk, and capsule. The foot provides a base to anchor the young sporophyte. A stalk appears as a slender filament that attaches to the sporangium and originates from the foot cell.
A spore capsule appears as a tight cap (calyptra) that comprises a layer of sterile protective cells around an embryonic sporophyte, at the opposite end of foot. A spore capsule encloses a sporangium sac that holds the heterospores, which function as sexual dispersal units. The spore-producing cells go through meiotic cell division to form spores (haploid).
The spore formation occurs, when the foot cell uptakes the nutrients from the gametophyte, and then conducts it to the spore capsule via long stalk or seta. A spore capsule comprises a layer of sterile cells, second layer of elaters that are hygroscopic or absorbs moisture from the surrounding.
The hydrogroscopic movement of the cells within the spore capsule results in flicking of the spores upward and outward. Unfortunately, the spore capsule ruptures, as the spores attain maturity and move freely in the environment as dormant spores. The life cycle of bryophyte continues once the dormant spore starts germinating into protonema.
Therefore, we can conclude that the gametophyte phase contains haploid cells that possess an unpaired number of chromosomes. In contrast, the saprophyte phase includes diploid cells that possess two sets of paired chromosomes. Gametophyte phase occurs predominantly in all the bryophytes and lasts longer.
Gymnosperm Life Cycle
Gymnosperms reproduce with an alteration of generations, meaning their reproductive cycle has both haploid and diploid phases.
As in all other vascular plants, gymnosperms have a sporophyte dominant life cycle (the sporophyte is the diploid multicellular stage, which comprises the body of the plant, i.e., a leafy tree). The gametophyte phase is relatively short, and sees gametes produced on the reproductive organs, which are usually cones.
The female ovulate cone, or megasporophyll, bear the megasporangium, diploid cells, which undergo meiosis to produce four haploid spores. Of these haploid spores, only one survives as the megaspore. The surviving megaspore then, through mitosis, develops into the female gametophyte. Within the female gametophyte there is an egg and an endosperm mother cell the endosperm mother cell creates endosperm, which eventually ‘feeds’ the embryo.
The male cone, called the microsporophyll, is a small, spongy, leaf-like organ which bears the microsporangium. The microsporangium contains the male microspores, which undergo meiosis to generate the male gametophyte, pollen. The pollen grain contains the pollen tube cell and the generative cell (which contains two sperm, although one dies).
When the pollen reaches the egg cell, either by wind or by animal through pollination, the pollen grain releases the single sperm. The nuclei of the female and the male gametophytes then fuse to create a diploid zygote. The endosperm, a haploid nutritional tissue, is released from the endosperm mother cell, and surrounds the zygote to form a seed. The seeds appear as the ‘scales’, which are visible on the cones of gymnosperms these scales are then dispersed to form a new sapling sporophyte, which grows into a mature sporophyte, and the cycle continues.
Female cones are larger and woodier than male cones and are usually positioned higher up on the tree, although in dioecious species, such as the cycads, the male and female cones are borne on separate trees.
Seedless Vascular Plants
Seedless vascular plants have a waxy cuticle, stomata, and well-developed vascular tissue. Their vasculature allows them to grow to larger sizes than the nonvascular plants, but they still largely occupy moist habitats. While this lineage is more well adapted to drier habitats than are the nonvascular plants, they still require moisture for reproduction. Although the developing diploid embryo is dependent on the haploid gametophyte for survival (like mosses), the diploid sporophyte is more conspicuous and is the prominent generation of seedless vascular plants. Phylogenetically, seedless vascular plants are basal to the seed plants. The seedless vascular plants include species such as ferns and horsetails.
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)
Heating up a cold case: Applications of analytical pyrolysis GC/MS to assess molecular biomarkers in peat
Kristy Klein , . Jens Leifeld , in Advances in Agronomy , 2021
4.1 Non-vascular plants (Sphagnum mosses)
When using Py-GC/MS to screen for an overall contribution of non-vascular plants in peat, bryophytes can be identified through the presence of non-hydrolyzable aliphatic biopolymers in the C23–25 length range. However, for bogs dominated by Sphagnum spp., 4-isopropenylphenol is a well-tested and highly specific biomarker for the presence of Sphagnum moss and is highly amenable to Py-GC/MS analysis. ( McClymont et al., 2011 Schellekens et al., 2009 Stankiewicz et al., 1997 Van der Heijden et al., 1997 ). This marker is useful not only due to its specificity to Sphagnum spp., but also because of the compound's apparent sensitivity to changes in the water table ( Schellekens et al., 2015a ), enabling its abundance to be used as an indicator of changes in hydrological regime and degradation status. ( Schellekens et al., 2015a ) measured 4-isopropenylphenol in Sphagnum-dominated peatlands, (with primary contributions from S. centrale and S. subsecundum and minor contributions from S. palustre and S. magellanicum). Due to the historical dominance of Sphagnum moss in the peat profile core, they concluded that observed variations (0.21%–2.85% of the total ion current) in the biomarker abundance were unlikely to be caused by changes in the contribution of moss to the peat, but rather from variations in aerobic decomposition. As a result of their findings, these authors also concluded that inhibition of decomposition in Sphagnum-dominated ecosystems might not be due to the contribution of Sphagnum-contributed phenolics (such as 4-isopropenylphenol), as had been previously thought but rather, that abundant presence of these phenolics suggested that water-logged conditions and anaerobic decomposition were already present in the peat profile.
In addition to climate/hydrological-derived shifts, 4-isopropenylphenol has also been used as a biomarker to reflect the influence of regional ecosystem events. 4-isopropenylphenol was used to confirm data demonstrating that airborne nutrient-depositing mineral dust corresponded with a seven-fold increase in net accumulation in Store Mosse (the “Great Bog”), a nutrient-limited ombrotrophic bog in southern Sweden ( Kylander et al., 2018 ).
Seedless Vascular Plants
By the Late Devonian period (385 million years ago), plants had evolved vascular tissue, well-defined leaves, and root systems. With these advantages, plants increased in height and size. During the Carboniferous period (359–299 million years ago), swamp forests of club mosses and horsetails, with some specimens reaching more than 30 meters tall, covered most of the land. These forests gave rise to the extensive coal deposits that gave the Carboniferous its name. In seedless vascular plants, the sporophyte became the dominant phase of the lifecycle.
Water is still required for fertilization of seedless vascular plants, and most favor a moist environment. Modern-day seedless vascular plants include club mosses, horsetails, ferns, and whisk ferns.
The club mosses , or Lycophyta, are the earliest group of seedless vascular plants. They dominated the landscape of the Carboniferous period, growing into tall trees and forming large swamp forests. Today’s club mosses are diminutive, evergreen plants consisting of a stem (which may be branched) and small leaves called microphylls (Figure 5). The division Lycophyta consists of close to 1,000 species, including quillworts (Isoetales), club mosses (Lycopodiales), and spike mosses (Selaginellales): none of which is a true moss.
Figure 5: Lycopodium clavatum is a club moss. (credit: Cory Zanker)
Ferns and whisk ferns belong to the division Pterophyta. A third group of plants in the Pterophyta, the horsetails, is sometimes classified separately from ferns. Horsetails have a single genus, Equisetum. They are the survivors of a large group of plants, known as Arthrophyta, which produced large trees and entire swamp forests in the Carboniferous. The plants are usually found in damp environments and marshes (Figure 6).
Figure 6: Horsetails thrive in a marsh. (credit: Myriam Feldman)
The stem of a horsetail is characterized by the presence of joints, or nodes: hence the name Arthrophyta, which means “jointed plant”. Leaves and branches come out as whorls from the evenly spaced rings. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem (Figure 7).
Figure 7: Thin leaves originating at the joints are noticeable on the horsetail plant. (credit: Myriam Feldman)
Ferns and Whisk Ferns
Ferns are considered the most advanced seedless vascular plants and display characteristics commonly observed in seed plants. Ferns form large leaves and branching roots. In contrast, whisk ferns , the psilophytes, lack both roots and leaves, which were probably lost by evolutionary reduction. Evolutionary reduction is a process by which natural selection reduces the size of a structure that is no longer favorable in a particular environment. Photosynthesis takes place in the green stem of a whisk fern. Small yellow knobs form at the tip of the branch stem and contain the sporangia. Whisk ferns have been classified outside the true ferns however, recent comparative analysis of DNA suggests that this group may have lost both vascular tissue and roots through evolution, and is actually closely related to ferns.
With their large fronds, ferns are the most readily recognizable seedless vascular plants (Figure 8). About 12,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist and shaded places. They made their appearance in the fossil record during the Devonian period (416–359 million years ago) and expanded during the Carboniferous period, 359–299 million years ago (Figure 9).
Figure 8: Some specimens of this short tree-fern species can grow very tall. (credit: Adrian Pingstone) Figure 9: This chart shows the geological time scale, beginning with the Pre-Archean eon 3800 million years ago and ending with the Quaternary period in present time. (credit: modification of work by USGS)