1. Characteristics of Nonvascular Plants
The nonvascular plant species alive today-loosely referred to as bryophytes-are descendants of the first plants to survive the harsh conditions of a terrestrial environment.
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2. Liverworts, hornworts, and mosses share some characteristics.
Nonvascular Plants
Liverworts, hornworts, and mosses share some characteristics.
Bryophytes house their embryos inside specialized maternal tissues to protect them during the early stages of development.
Bryophytes have a rudimentary cuticle, which represents the first step in the development of the thick waxy layer that coats the leaves of many land plants.
Nonvascular plants lack specialized tissues to transport nutrients and water.
Most nonvascular plants grow in a dense, interconnected network close to the ground, which allows water and nutrients to travel from one part of the network to another through capillary action.
Bryophytes do not grow very tall partly because most of the structures produced by their gametophytes are too thin to support much weight.
As mosses grow from a haploid, unicellular spore into a haploid multicellular gametophyte, they produce specialized structures called protonema, absorbent strands that are only a single cell in thickness.
If protonema have access to enough water and minerals, they produce bud-like structures that eventually become gametophores, which contain sex organs.
The combination of protonema and gametophores are what makes up the moss' gametophyte, which has root-like structures called rhizoids.
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3. How are liverworts, hornworts, and mosses different?
Nonvascular Plants
How are liverworts, hornworts, and mosses different?
Nonvascular plants consist of three monophyletic groups: mosses, hornworts, and liverworts.
Hornworts are the most closely related to all other land plant species; liverworts are most closely related to charophytes, the green algae from which scientists propose terrestrial plants originally evolved.
Hornworts tend to form symbiotic relationships with cyanobacteria that colonize special hornwort internal cavities.
During the sporophyte stage, hornworts have stomata, small openings important to gas exchange as well as the movement of water and nutrients.
Liverworts tend to grow so close to the ground that they appear flat.
The gametes of these plants swim in a film of water along the surface of the cells and their sporophytes are too small to be seen without a magnifying glass.
All liverworts have a cuticle, although only some liverworts have stomata.
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4. Figure 1 Liverwort gametophytes.
Nonvascular Plants
Figure 1 Liverwort gametophytes.
The gametophyte is the most conspicious lifestage of bryophytes, including liverworts, as shown here. From the flat gametophyte thallus (leaf-like structure), the gametophores emerge. Haploid gametes are formed within.
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5. How are liverworts, hornworts, and mosses different?
Nonvascular Plants
How are liverworts, hornworts, and mosses different?
Mosses are by far the most diverse group of nonvascular plants.
Some species of moss have a cuticle, and moss sporophytes have stomata.
Some mosses have acquired a system for water transport that closely mimics the vascular system found in more complex land plants. In these organisms, water moves through a system of small central channels, no more than one cell in diameter. This allows mosses to grow taller than other nonvascular plants.
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6. Figure 2 Proposed phylogenetic tree for non-vascular plants.
The phylogenetic tree for non-vascular plants likely branched from an algae common ancestor into the liverworts (Marchantiophyta), the hornworts (Anthocerotophyta), mosses (Bryophyta), and the Polysporangiates. Polysporangiophyta include non-vascular plants from the fossil record such as Cooksonia, as shown, and they also share a common ancestor with all vascular plants including club mosses, horsetails, ferns, cone-bearing plants, and flowering plants.
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7. Nonvascular plants share similar life cycles.
The life cycle of a nonvascular plant involves an alternation of generations. The gametophyte stage dominates the life cycle of nonvascular plants.
Mature gametophytes have specialized tissues that can produce haploid male gametes, haploid female gametes, or both.
Typically, female gametes remain encased in protective tissue in the female gametophyte; sperm swim in through a film of water to fertilize the stationary egg.
As the embryo develops into a diploid sporophyte, it remains dependent on the gametophytes for water and nutrients, even as it becomes capable of photosynthesis.
The foot is the structure that anchors the sporophyte in the gametophyte, through which the sporophyte absorbs nutrients.
The foot connects to the stalk, or seta, through which nutrients travel to the capsule, where spores are produced.
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8. Figure 3 Life cycle of a nonvascular plant.
Nonvascular Plants
Figure 3 Life cycle of a nonvascular plant.
Many nonvascular plants, such as this moss, have diverse life cycles containing multicellular haploid phases, as well as diploid structures (sporophyte) that are supported by haploid structures (gametophytes) on the same plant.
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9. Mosses are diverse and ecologically important.
Nonvascular Plants
Mosses are diverse and ecologically important.
The major requirement for moss proliferation is moisture. Moss can lie dormant until enough moisture is present to revive the plant.
Mosses reduce soil erosion and maintain levels of soil nutrients, including nitrogen and important minerals. This can be critical in regions where few other plants grow.
Mosses can also absorb a certain amount of pollution without experiencing irreversible damage, which makes them good indicators of overall pollution levels.
The sphagnum mosses use the carbon dioxide released by the bacteria during photosynthesis.
Submerged sphagnum mosses do not have access to atmospheric carbon dioxide, so they are especially dependent on the methanotrophs.
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10. Figure 4 Ground pine moss.
Nonvascular Plants
Figure 4 Ground pine moss.
The ground pine moss gametophyte grows in the crack of a rock.
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11. Nonvascular Plants Figure 5 Moss growing.
Moss grows on a decaying tree.
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12. Figure 6 Moss in the desert.
Nonvascular Plants
Figure 6 Moss in the desert.
This thick carpet of moss is growing on sandstone, a pourous rocky surface. In desert environments, there are shady canyons with moss-covered sandstone like this, where the moss can stay semi-dehydrated for years until water returns.
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13. Future perspectives. Nonvascular Plants Principles of Biology
Scientists have recently discovered that many species of methane-oxidizing bacteria-known as methanotrophs-live symbiotically with peat mosses.
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14. Figure 7 Sphagnum moss gametophytes.
Nonvascular Plants
Figure 7 Sphagnum moss gametophytes.
White sphagnum grows in a network.
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15. Mosses Can Be Bioindicators
Nonvascular Plants
BIOSKILL
Mosses Can Be Bioindicators
Ecologists have a few options to determine whether an ecosystem is healthy. One way is to pay close attention to bioindicators, organisms that have specific, measurable responses to environmental stressors.
Mosses are useful as bioindicators because most of their cells are typically in very close contact with the external environment.
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16. Nonvascular Plants
Figure 8 Distance from a road correlates with levels of metal contaminants in moss.
The concentration of metal pollutants in moss plants declines with distance from either side of a heavily-traveled road.
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