Learning Objectives

By the finish of this section, you will be able to carry out the following:

Identify the main characteristics of bryophytesDescribe the differentiating traits of liverworts, hornworts, and mossesChart the advance of land also adaptations in the bryophytesDescribe the occasions in the bryophyte lifecycle

Bryophytes are the closest extant relatives of at an early stage terrestrial plants. The first bryophytes (liverworts) the majority of most likely showed up in the Ordovician period, about 450 million years ago. Because they lack lignin and other resistant frameworks, the likelihood of bryophytes developing fossils is rather tiny. Some spores defended by sporopollenin have endured and are attributed to early bryophytes. By the Silurian duration (435 MYA), however, vascular plants had spcheck out with the continents. This compelling fact is used as proof that non-vascular plants have to have actually preyielded the Silurian duration.

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More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They constitute the significant flora of inhospitable environments like the tundra, wright here their small dimension and tolerance to desiccation offer distinctive advantages. They generally absence lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and also nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more specific, bryophytes are generally called non-vascular plants.

In a bryophyte, all the conspicuous vegetative organs—including the photoman-made leaf-favor structures, the thallus (“plant body”), stem, and also the rhizoid that anchors the plant to its substrate—belengthy to the haploid organism or gametophyte. The male gametes formed by bryophytes swim with a flagellum, so fertilization is dependent on the existence of water. The bryophyte embryo likewise remains attached to the parent plant, which protects and also nourishes it. The sporophyte that develops from the embryo is badepend noticeable. The sporangium—the multicellular sex-related refertile structure in which meiosis produces haploid spores—is present in bryophytes and also missing in the majority of algae. This is also a characteristic of land also plants.

The bryophytes are split into 3 phyla: the liverworts or Hepaticophyta, the hornworts or Anthocerotophyta, and also the mosses or true Bryophyta.


Liverworts

Liverworts (Hepaticophyta) are presently classified as the plants a lot of very closely concerned the ancestor of vascular plants that adjusted to terrestrial atmospheres. In reality, liverworts have actually conquered eincredibly terrestrial habitat on Earth and also diversified to more than 7000 existing species ((Figure)). Lobate liverworts develop a level thallus, via lobes that have actually a vague resemblance to the lobes of the liver ((Figure)), which accounts for the name offered to the phylum. Leafy liverworts have tiny leafchoose frameworks attached to a stalk. Several leafy liverworts are shown in (Figure).


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Openings in the thallus that allow the movement of gases may be observed in liverworts ((Figure)). However before, these are not stomata, bereason they do not actively open and also cshed by the activity of guard cells. Instead, the thallus takes up water over its entire surface and also has no cuticle to proccasion desiccation, which describes their wanted wet habitats. (Figure) represents the lifecycle of a lobate liverwort. Haploid spores germinate into flattened thalli attached to the substprice by thin, single-celled filaments. Stalk-like structures (gametophores) thrive from the thallus and lug male and female gametangia, which may construct on separate, individual plants, or on the same plant, depending upon the species. Flagellated male gametes construct within antheridia (male gametangia). The female gametes build within archegonia (female gametangia). Once released, the male gametes swim with the help of their flagella to an archegonium, and also fertilization ensues. The zygote grows into a small sporophyte still contained in the archegonium. The diploid zygote will give rise, by meiosis, to the following generation of haploid spores, which deserve to be disseminated by wind or water. In many liverworts, spore dispersal is promoted by elaters—long single cells that unexpectedly readjust form as they dry out and also throw surrounding spores out of the spore capsule. Liverwort plants have the right to additionally recreate asexually, by the breaking of “branches” or the spanalysis of leaf fragments referred to as gemmae. In this last kind of remanufacturing, the gemmae—tiny, intact, complete pieces of plant that are produced in a cup on the surconfront of the thallus (presented in (Figure) and also (Figure))—are splashed out of the cup by raindrops. The gemmae then land also adjacent and also develop into gametophytes.


Refertile cycle of liverworts. The life cycle of a typical lobate liverwort is shown. This picture reflects a liverwort in which antheridia and also archegonia are produced on separate gametophytes. (credit: modification of work-related by Mariana Ruiz Villareal)

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Stomata (air pores that can be opened up and closed) appear in the hornworts and also are abundant on the sporophyte. Photosynthetic cells in the thallus each contain a single chloroplast. Meristem cells at the base of the plant save splitting and including to the height of the sporophyte. This growth pattern is distinctive to the hornworts. Many type of hornworts establish symbiotic relationships via cyanobacteria that deal with nitrogen from the setting.

The lifecycle of hornworts ((Figure)) follows the general pattern of altercountry of generations. The gametophytes flourish as flat thalli on the soil via installed male and female gametangia. Flagellated sperm swim to the archegonia and also fertilize eggs. The zygote establishes right into a long and also slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and assist propel them farther in the environment. The haploid spores germinate and provide climb to the next generation of gametophytes.


Refertile cycle of hornworts. The alternation of generation in hornworts is displayed. (credit: modification of work-related by “Smith609”/Wikimedia Commons based upon original work by Mariana Ruiz Villareal)

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Mosses

The mosses are the most many of the non-vascular plants. More than 10,000 species of mosses have been catalogued. Their habitats differ from the tundra, where they are the major vegetation, to the understory of tropical woodlands. In the tundra, the mosses’ shpermit rhizoids permit them to fasten to a substprice without penetrating the frozen soil. Mosses sluggish dvery own erosion, save moisture and soil nutrients, and also carry out shelter for little animals and also food for bigger herbivores, such as the musk ox. Mosses are very sensitive to air air pollution and also are supplied to monitor air high quality. They are also sensitive to copper salts, so these salts are a prevalent ingredient of compounds marketed to get rid of mosses from lawns.

Mosses develop diminutive gametophytes, which are the leading phase of the lifecycle. Green, level structures through an easy midrib—resembling true leaves, but doing not have stomata and vascular tissue—are attached in a spiral to a central stalk. Mosses have actually stomata just on the sporophyte. Water and nutrients are soaked up directly through the leafchoose structures of the gametophyte. Some mosses have small branches. A primitive conductive device that carries water and nutrients runs up the gametophyte’s stalk, yet does not extend into the leaves. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids, precursors of roots. They originate from the base of the gametophyte, however are not the major path for the absorption of water and minerals. The absence of a true root system describes why it is so basic to rip moss mats from a tree trunk. The mosses therefore occupy a thresorganize position in between various other bryophytes and also the vascular plants.

The moss lifecycle follows the pattern of altercountry of generations as shown in (Figure). The most acquainted structure is the haploid gametophyte, which germinates from a haploid spore and also creates initially a protonema—commonly, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem proactively divide and give increase to a gametophore, consisting of a photoartificial stem and also foliage-prefer frameworks. Male and also female gametangia construct at the pointer of sepaprice gametophores. The antheridia (male organs) create many sperm, whereas the archegonia (the female organs) each form a solitary egg at the base (venter) of a flask-shaped framework. The archegonium produces attractant substances and also at fertilization, the sperm swims dvery own the neck to the venter and also unites via the egg inside the archegonium. The zygote, safeguarded by the archegonium, divides and also grows into a sporophyte, still attached by its foot to the gametophyte.


Reproductive cycle of mosses. This illustration reflects the life cycle of mosses. (credit: modification of work by Mariana Ruiz Villareal)
Which of the adhering to statements around the moss life cycle is false?

The mature gametophyte is haploid.The sporophyte produces haploid spores.The calyptra buds to create a mature gametophyte.The zygote is housed in the venter.

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The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae), as checked out in (Figure), includes tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule.


Moss sporophyte. This photograph mirrors the lengthy slender stems, called setae, linked to capsules of the moss Thamnobryum alopecurum. The operculum and remnants of the calyptra are visible in some capsules. (credit: modification of job-related by Hermann Schachner)
Spore mommy cells in the sporangium undergo meiosis to create haploid spores. The sporophyte has numerous attributes that safeguard the emerging spores and aid in their dispersal. The calyptra, derived from the walls of the archegonium, covers the sporangium. A structure referred to as the operculum is at the guideline of the spore capsule. The calyptra and operculum fall off once the spores are all set for dispersal. The peristome, tproblem about the mouth of the capsule, is made of triangular, close-fitting systems like little “teeth.” The peristome opens up and closes, relying on moisture levels, and periodically releases spores.


Section Summary

Seedless non-vascular plants are small, having the gametophyte as the leading stage of the lifecycle. Without a vascular system and also roots, they absorb water and also nutrients on all their exposed surdeals with. Collectively well-known as bryophytes, the three primary groups encompass the liverworts, the hornworts, and the mosses. Liverworts are the most primitive plants and are closely regarded the initially land plants. Hornworts arisen stomata and possess a single chloroplast per cell. Mosses have easy conductive cells and are attached to the substprice by rhizoids. They colonize harsh habitats and also can reget moisture after drying out. The moss sporangium is a facility framework that enables release of spores away from the parent plant.

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(Figure) Which of the following statements about the moss life cycle is false?

The mature gametophyte is haploid.The sporophyte produces haploid spores.The calyptra buds to form a mature gametophyte.The zygote is hooffered in the venter.

Which of the following frameworks is not discovered in bryophytes?