In this latter type of reproduction, the gemmae small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes. The hornworts Anthocerotophyta belong to the broad bryophyte group that have colonized a variety of habitats on land, although they are never far from a source of moisture.
The short, blue-green gametophyte is the dominant phase of the lifecycle of a hornwort. The narrow, pipe-like sporophyte is the defining characteristic of the group. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant. Stomata appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus contain a single chloroplast.
Meristem cells at the base of the plant keep dividing and adding to its height. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment. Hornworts : Unlike liverworts, hornworts grow a tall and slender sporophyte. The life cycle of hornworts also follows the general pattern of alternation of generations and has a similar life cycle to liverworts. The gametophytes grow as flat thalli on the soil with embedded gametangia.
Flagellated sperm swim to the archegonia and fertilize eggs. However, unlike liverworts, the zygote develops into a long and slender sporophyte that eventually splits open, releasing spores. Additionally, thin cells called pseudoelaters surround the spores and help propel them further in the environment. Unlike the elaters observed in liverworts, the hornwort pseudoelaters are single-celled structures.
The haploid spores germinate and produce the next generation of gametophytes. Like liverworts, some hornworts may also produce asexually through fragmentation. Life Cycle of Hornworts : The life cycle of hornworts is similar to that of liverworts. Both follow the pattern of alternation of generations. However, liverworts develop a small sporophyte, whereas hornworts develop a long, slender sporophyte.
Liverworts also disperse their spores with the help of elaters, while hornworts utilize pseudoelaters to aid in spore dispersal. Mosses are bryophytes that live in many environments and are characterized by their short flat leaves, root-like rhizoids, and peristomes. More than 10, species of mosses have been cataloged. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox.
Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts. Such salts are a common ingredient of compounds marketed to eliminate mosses from lawns. Mosses form diminutive gametophytes, which are the dominant phase of the life cycle. Green, flat structures resembling true leaves, but lacking vascular tissue are attached in a spiral to a central stalk or seta.
The plants absorb water and nutrients directly through these leaf-like structures. The seta plural, setae contains tubular cells that transfer nutrients from the base of the sporophyte the foot to the sporangium. Some mosses have small branches. Some primitive traits of green algae, such as flagellated sperm, are still present in mosses that are dependent on water for reproduction.
Other features of mosses are adaptations to dry land. Additionally, mosses are anchored to the substrate, whether it is soil, rock, or roof tiles, by multicellular rhizoids. These structures are precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals.
The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. Setae : This photograph shows the long slender stems, called setae, connected to capsules of the moss Thamnobryum alopecurum.
The moss life cycle follows the pattern of alternation of generations. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema: usually, a tangle of single-celled filaments that hug the ground.
The tiny diploid sporophytes, which remain attrached to the parent plant, have a very simple structure. Meiosis within the sporophyte produces a number of haploid spores. These spores are surrounded by curious long and twisted moist cells called elaters. When the capsule dries and bursts, the elaters twist and jerk around in a way that scatters the spores in all directions.
Liverworts can also reproduce asexually by means of special structures called gemmae cups. These little cups can be easily seen on the surface of the plant. Each gemma cup contains a number of tiny plantlets called gemmae, and a single drop of water will disperse them. The green gametophytes of the hornwort look very much like a liverwort. But their small sporophytes more closely resemble those of mosses.
The sporophytes grow out of the gametophyte, and look like a little upright horn. Like mosses, hornworts have stomata, and so are probably more closely related to mosses and other plants than to the liverworts they mat resemble. These plants are symbiotic with the cyanobacteria Nostoc. The cyanobacteria fixes nitrogen for the hornwort. Division Hepaticophyta - liverworts Marchantia, Conocephalum, Porella; fr.
Examine the living mosses on display. Notice the small capsules on top of the tiny sporophytes. Mosses generally grow in one of two growth types: cushiony moss and feathery moss.
Examine slides of the antheridia and archegonia. The sausage shaped antheridia produce sperm, and the flask shaped archegonia produces eggs. Examine slides of the protonema. What type of algae does it remind you of? This resemblance is additional evidence that green algae gave rise to all higher plants.
Examine the terrestrial liverworts Marchantia and Conencephalum one or both should be on display. How does their growth habit differ from that of the mosses? Can you see any gemmae cups on the upper surface of these plants? Examine the aquatic liverworts like Porella and Riccia one or both should be on display. Notice how they differ from the more terrestrial forms of liverwort. Look at the preserved liverworts , and observe their distinct reproductive structures they look like little green umbrellas.
How does their life cycle differ from mosses? Hint: Be sure you understand the general life cycle of plants, and can tell which stages are haploid gametophytes 1N or diploid sporophytes 2N.
We'll learn several life cycles in lecture and in lab moss, fern, pine, flowering plant , but all of them are variations on the same basic theme. Just as the evolution of spores was the key to the invasion of the land surface by bryophytes, the invention of complex vascular tissues let tracheophytes complete the conquest of dry land.
There are about , species of vascular plants, grouped in nine divisions. Tracheophytes all have a well developed root-shoot system, with highly specialized roots, stems, and leaves, and specialized vascular tissue xylem and phloem that function like miniature tubes to conduct food, water, and nutrients throughout the plant.
Because ferns and fern allies posses true vascular tissues, they can grow to be much larger and thicker than the bryophytes. The ferns and fern allies non-seed tracheophytes mark two major evolutionary strides. In these and in all more advanced plants, the leafy green diploid sporophyte now becomes the dominant stage.
The tiny gametophyte may be either autotropophic like the fern prothallus or heterotrophic like the gametophytes of some lycopsids , and is generally free living and independent of the parental sporophyte. Unlike the vascular sporophytes, the gametophytes have no vascular tissue at all. These gametophytes are therefore very small, and develop best in moist areas, where they can absorb water directly from their surroundings. Like the bryophytes, ferns and fern allies are still restricted to moist habitats.
Their flagellated sperm need a thin film of water to swim between the antheridium and the archegonium. And when the baby sporophyte grows up from the gametophyte, it is exposed to desiccation drying up. This basic strategy of a free-swimming sperm and a non-motile egg is shared by plants, animals, and algae. It makes sense, because it means only one set of gametes has to make the perilous journey outside of the organism. The ferns and fern allies germinate from spores.
These plants are mostly homosporous - their spores are identical and you can't differentiate which will grow into male or female plants. They are also monoecious - both the archegonia and antheridia male and female reproductive structures are borne on the same plant.
Contrast these primitive vascular plants with the more advanced seed plants, the gymnosperms and angiosperms, which germinate from seeds rather than from spores.
Seed plants are all heterosporous. It is easy to differentiate the larger female megaspore from the smaller male microspore. The sperm of seed plants have no flagella. They lack antheridia, and only a few still have an archegonia. Unlike the more primitive ferns and fern allies, seed plants are mostly dioecious , having separate male and female plants. In many of these primitive plants, certain leaves are specialized for reproduction. These modified leaves, or sporophylls , bear the sporangia at their bases.
These sporophylls usually branch out from a shortened stem, forming a club shaped structure called a strobilus. The pine cone and the flower are elaborate variations on these primitive strobili.
There are four divisions of non-seed tracheophytes, vascular plants that reproduce by means of spores , the Psilophyta, Lycophyta, Sphenophyta, and Pterophyta.
Before these non-seed tracheophytes evolved, the bryophytes were the dominant form of plant life. The evolutionary edge of having a more efficient conducting system, and a well-developed root-shoot system enabled them to outcompete bryophytes.
There are only two living genera of whisk ferns, sole survivors of a large and widespread group of early land plants. In addition to the living Division Psilophyta, the psilopsids, there are two extinct divisions of primeval vascular plants. The primitive whisk ferns resemble these extinct pioneers in many ways. They are the only living vascular plants that lack a root-shoot system, a characteristic they share with both extinct Divisions of ancestral vascular plants.
Some recent molecular evidence suggests that one, or even both, of the living genera of psilopsids may actually be more closely related to ferns, like a fern that has reverted to more primitive traits.
If this is true, then Psilophyta will join the ranks of the numerous extinct Divisions of plants. Psilopsids are found in tropical and subtropical areas, and occurs throughout the southern US.
I once found one growing on my back porch under the leaves of a spider plant. Whisk ferns are a common weed in greenhouses all over the world. They are simple green upright stems, with dichotomous branching. They have no leaves, and no true roots.
The outer tissues of the stem do all the photosynthesizing. A portion of the stem called a rhizome runs along the ground, or just below it.
They also have true root systems, leaves and stems. However, ferns are unlike most plants because they do not produce flowers and seeds. Instead, they use spores to reproduce.
The spores that ferns produce are different than seeds. They do not store as many nutrients as seeds or have protective coverings. To compensate, ferns produce large amounts of spores to ensure that some will survive until germination.
Fern allies, such as quillworts, horsetails also called scouring rushes and club mosses, are similar to ferns but have enough genetic differences that ecologists have placed them in their own plant divisions. Additionally, bryophytes do not produce flowers and seeds.
Instead, like ferns, they use spores to reproduce. Of the bryophytes, mosses are the most prevalent. Forest Preserve District ecologists and volunteers have identified in DuPage County in a range of locations from dry surfaces, trees and rocks to underwater niches. Some grow almost anywhere while others only inhabit certain ecosystems. Mosses are typically soft plants that grow in clumps or mats. Like other plants, they produce chlorophyll and undergo photosynthesis, but they do not have true roots.
Because rhizoids are less efficient than roots, mosses generally prefer damp places with low light. When dry, they can go dormant, drawing moisture and nutrients from the green portion of the plant back in the rhizoids, which causes their leaflike structures to curl. When moist conditions return, they spring back to life, turn green and grow. Their spores grow in beaklike capsules and are usually dispersed by the wind.
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