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Learning Goals
By the end of this reading you should be able to:
- Identify the main characteristics of bryophytes
- Describe the distinguishing traits of liverworts, hornworts, and mosses
- Chart the development of land adaptations in the bryophytes
- Describe the events in the bryophyte lifecycle
Introduction
Seedless non-vascular plants all fall into the group Bryophyta, a group of plants that are the closest extant relative of early terrestrial plants. The first bryophytes (liverworts) most likely appeared about 450 million years ago. Because of the lack of lignin and other resistant structures, the likelihood of bryophytes forming fossils is rather small. Some spores protected by sporopollenin have survived and are attributed to early bryophytes.
More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They constitute the major flora of inhospitable environments like the tundra, where their small size and tolerance to desiccation offer distinct advantages. They generally lack lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more accurate, bryophytes are commonly called non-vascular plants.
In a bryophyte, all the conspicuous vegetative organs—including the photosynthetic leaf-like structures, the thallus, stem, and the rhizoid that anchors the plant to its substrate—belong to the haploid organism or gametophyte. The sporophyte is barely noticeable. The gametes formed by bryophytes swim with a flagellum, as do gametes in a few of the tracheophytes. The sporangium—the multicellular sexual reproductive structure—is present in bryophytes and absent in the majority of algae. The bryophyte embryo also remains attached to the parent plant, which protects and nourishes it. This is a characteristic of land plants (embryophytes).
The bryophytes are divided into three phyla: the liverworts (Hepaticophyta), the hornworts (Anthocerotophyta), and the mosses (true Bryophyta).
Liverworts:
Liverworts are viewed as the plants most closely related to the ancestor that moved to land. Liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species. The most obvious structure of the liverworts is the lobate flat thallus, this is the gametophyte. Since there is no cuticle the plant takes up water over its entire surface but lacks protection from desiccation. Openings that allow the movement of gases may be observed in liverworts but these are not stomata, because they do not actively open and close.
The liverwort life cycle starts with the release of haploid spores from the sporangium that developed on the sporophyte. Spores disseminated by wind or water germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Male and female gametangia develop on separate, individual plants. Once released, male gametes swim with the aid of their flagella to the female gametangium (the archegonium), and fertilization ensues. The zygote grows into a small sporophyte still attached to the parent gametophyte. Like all sporophytes in non-vascular plants, the sporophyte will remain dependent on the gametophyte for nutrients throughout its life. The sporophyte will give rise, by meiosis, to the next generation of spores.
Liverwort plants can also reproduce asexually, by the breaking of branches or the spreading of a single cell, or a mass of cells, called gemmae. This type of asexual reproduction is sometimes referred to as fragmentation. On the thallus, the gemmae are produced in a cup and are splashed out of the cup by raindrops. The gemmae then land nearby and develop into new gametophytes. This allows the organisms to spread when conditions (presence of water) are favorable.
Hornworts
The hornworts (Anthocerotophyta) have colonized a variety of habitats on land, although they are never far from a source of moisture. Like liverworts, the gametophyte is the dominant phase of the lifecycle. The blue-green hornwort gametophytes grow as flat thalli on the soil with embedded gametangia. In each of the photosynthetic cells of the thallus, there is a single chloroplast. At the base of the plant, there is a meristem, where cells continuously divide and add to the height.
The archegonia and antheridia on the gametophyte undergo mitosis to produce gametes and then the flagellated sperm swim to the archegonia and fertilize eggs. After fertilization, the zygote develops into a long narrow, pipe-like sporophyte, a defining characteristic of this group. Once spore formation begins, the tip of the sporophyte splits open, releasing spores. Thin cells called pseudoelaters surround the spores and help propel them further in the environment. Once they reach a favorable environment the haploid spores germinate and give rise to the next generation of gametophytes. Stomata appear in the hornworts and are abundant on the sporophyte. Because they are often found in nutrient-poor environments, many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.
Review Question:
Mosses
The habitats of the 10,000+ moss species vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. 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, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.
Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures—resembling true leaves, but lacking vascular tissue—are attached in a spiral to a central stalk. The plants absorb water and nutrients directly through these leaf-like structures. 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 clearly adaptations to dry land. For example, stomata are present on the stems of the sporophyte, and a primitive vascular system runs up the sporophyte’s stalk. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids that originate from the base of the gametophyte. These structures are not roots, as they are not involved in the uptake of water or nutrients. , 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.
In the moss lifecycle, the haploid gametophyte germinates from a haploid spore and forms first a protonema—usually, a tangle of single-celled filaments that hug the ground. Cells that function like an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Rhizoids form at the base of the gametophore. Gametangia of both sexes develop on separate gametophores. The male organ (the antheridium) produces many sperm cells, whereas the archegonium (the female organ) forms a single egg. At fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
A structure called a peristome increases the spread of spores after the tip of the capsule falls off at dispersal. The concentric tissue around the mouth of the capsule is made of triangular, close-fitting units, a little like “teeth”; these open and close depending on moisture levels, and periodically release spores.
Review Question:
Summary
Seedless nonvascular plants are small, having the gametophyte as the dominant stage of the lifecycle. Without a vascular system and roots, they absorb water and nutrients on all their exposed surfaces. Collectively known as bryophytes, the three main groups include the liverworts, the hornworts, and the mosses. Liverworts are the most primitive plants and are closely related to the first land plants. Hornworts developed stomata and possess a single chloroplast per cell. Mosses have simple conductive cells and are attached to the substrate by rhizoids. They colonize harsh habitats and can regain moisture after drying out. The moss sporangium is a complex structure that allows the release of spores away from the parent plant.
End of Section Review Questions:
Review: Shared features
1) What is a common feature that the liverworts, hornworts and mosses share? (Multiple Answers)
Review: Liverworts
2) What are gemmae? (Multiple Answers)
Review: Structures of Bryophytes
| 1) protonema | A) Liverworts |
| 2) rhizoids | B) Mosses |
| 3) gemmae | C) Hornworts and Mosses |
| 4) pseudoelators | D) Hornworts |
Review: Adaptations to Land
4) For each group indicate what adaptations they have that are related to terrestrial environments
| 1) pores for gas exchange | A) Liverworts |
| 2) stomata and pseudoelators | B) Hornworts |
| 3) rhizoids and peristome | C) Mosses |
Image Attribution:
Figure 1. Image courtesy of Avenue [GFDL (http://www.gnu.org/copyleft/fdl.html)]
Figure 2. Image courtesy of Mariana Ruiz (LadyofHats) [Public domain]
Figure 3. This image is not creative commons,Copyright ©2008 Pearson Education, Inc. Pearson Benjamin Cummings