Themes > Science > Botanical Sciences > Classification of Plants > Cryptogamia (Seedless Plants) > Bryophyta Mosses and Liverworts > Nonvascular Plants
Nonvascular plants are the simplest of all
land dwelling plants. Like their closest ancestors, the green algae,
they lack an internal means for water transportation. They also do
not produce seeds or flowers. They generally only reach a height of
one to two centimeters, because they lack the woody tissue necessary for
support on land.
Evolutionary History
Scientists possess fossil records that lead them to believe that plants evolved during four distinct periods. Nonvascular plants arose first, during the late Ordovician period of the Paleozoic Era, approximately 460 million years ago. Their closest non plant ancestor is a type of green algae called charophytes. Nonvascular plants exhibit several homologies to charophytes, such as
- Homologous chloroplasts - Both nonvascular plants and charophytes use chlorophyll and beta carotene as accessory pigments in their chloroplasts. Also, the thylakoid membranes of both are stacked as grana.
- Biochemical Similarities - Charophytes and plants both have cellulose in their cell walls, and their peroxisomes have the same enzyme composition.
- Similarities in the mechanisms of mitosis and cytokinesis - During mitosis in both nonvascular plants and charophytes, the nuclear envelope completely disperses during late prophase. In cytokinesis, the formation of a cell plate involves the cooperation of microtubules, actin microfilaments, and vesicles.
- Similarity in sperm ultrastructure - In the details of sperm ultrastructure, charophytes are more similar to plants than to other green algae.
- Genetic relationship - Nonvascular plant nuclear genes and ribosomal RNA most closely resemble those of charophytes.
Reproduction
Nonvascular plants can
reproduce both sexually and asexually. Asexual reproduction is the
less common method. It basically consists of the regeneration of
plant material, leaves or other parts, that fall to the ground and
generate secondary plants which bear new buds.
Most nonvascular plants, though, reproduce sexually. Their gametes
develop within structures called gametangia, which are organs that have
protective jackets of sterile cells that prevent the gametes from drying
out during development. The male gametangium is called the
antheridium, and it produces flagellated sperm. The female
gametangium, or archegonium, produces a single egg (ovum). As in
algae, the flagellated sperm require water to swim from the antheridium to
the archegonium and fertilize the egg. For most species, a film of
rainwater or dew is sufficient for fertilization.
Nonvascular plants, along with all other members of the plant world,
engage in a life cycle known as alternation of generations. During
this cycle, the plant forms both a multicellular diploid generation, the
sporophyte, and a multicellular haploid generation, the gametophyte.
In nonvascular plants, the dominant generation is the gametophyte, whereas
in most plants the sporophyte is dominant. The sporophyte in nonvascular
plants is smaller and shorter lived than the gametophyte, and it depends
on the gametophyte for survival.
After the sperm swims to the archegonium and fertilizes the egg, the
diploid zygote divides by mitosis and develops into an embryonic
sporophyte within the archegonium. This sporophyte grows into a long
stalk whose base remains attached to the archegonium as the top emerges.
A sporangium forms at the tip of the stalk, and haploid spores develop
within it by meiosis. Eventually, the sporangium bursts and the
spores scatter. They germinate by mitotic division and form
structures known as protonema, which eventually develop into mature
gametophytes, completing the life cycle.
Habitats
Nonvascular plants are almost always found in damp, shady places.
They have little or no resistance to drying, and because they lack
vascular tissue they cannot carry water from the ground to the aerial
parts of the plant. Like sponges, they must imbibe the water lying
on their surfaces and distribute it by the relatively slow means of
diffusion, capillary action, and cytoplasmic streaming. Therefore,
they cannot survive for very long in areas that are not constantly moist.
Some can survive in alternative habitats such as sand dunes, but the
majority thrive in dark, dank places. Because of their limited range
of terrestrial habitats, nonvascular plants have never dominated much of
the earth's landscape.
Energy Acquisition
Like most plants, nonvascular plants acquire energy through photosynthesis. During this process, the plant converts light energy into chemical energy, then proceeds to store it in the form of glucose or other organic compounds. Photosynthesis usually occurs in the upper parts of nonvascular plants, where they produce many small stemlike and leaflike appendages.Divisions
There are three divisions of
nonvascular plants: Bryophyta, Hepatophyta, and Anthocerophyta.
Until recently, scientists grouped all three together as one division,
Bryophyta, but the current view is that they are probably not related.
They do share some key characteristics, such as the presence of a waxy
cuticle and gametangia, but they all have distinct characteristics that
warrant separate divisions.
Bryophyta
Division Bryophyta consists of the mosses, and it includes approximately
10,000 species. Mosses are the most common and familiar nonvascular
plants. They usually grow in a mat formation, which consists of many
plants growing in a tight pack to hold one another up. The mat
usually has a spongy quality which enables it to retain water, thus aiding
in reproduction and preventing the plant from drying out. Mosses
possess multicellular, rootlike structures known as rhizoids which they
use for attachment and water absorption. All mosses consist of
"stems", either branched or unbranched, that bear leaflike
structures. It is important to note that these "stems",
"roots", and "leaves" are not homologous to those of
vascular plants.
In mosses, the capsules of the sporophytes (sporangia) are much more complex than those of the other nonvascular plants. In many cases the sporangia will possess a lid, or operculum, which is separated from the rest of the capsule by a ring. This lid will eventually detach, releasing spores. Spore release is sometimes aided by the movement of peristome teeth, which are arranged radially around the mouth of the capsule.
Hepatophyta
Division Hepato
phyta
is the home of the liverworts: it contains approximately 6,500 species.
These plants differ from mosses in that many do not have their
characteristic stem/leaf structure. Instead, their bodies are
divided into deeply grooved lobes. Some have coil shaped cells in
their sporangia which spring out of the capsule when it opens, helping to
disperse the spores. Their capsules are usually much simpler than
those of the mosses, consisting of simple spheres that split
longitudinally into four sections when mature. Some also develop
structures called gemmae, which are bundles of cells that reside in cups
on the surface of the plant and are dispersed by raindrops. Their
rhizoids are composed of single, elongated cells, not multiple cells as in
moss. Anthocerophyta The third and final division, Anthocerophyta,
consists of approximately one hundred species of hornworts. These
plants resemble liverworts in their gametophytes, but they can be easily
distinguished by their sporophytes. Hornworts possess elongated
capsules that grow like horns from the gametophytes, arising from a group
of cells at the base of the horn that divide continuously throughout the
sporophyte's life span. This feature is unique among plants:
hornworts are the only known plants to posses a continuously dividing
group of cells. Another unique feature of hornworts is their
photosynthetic cells: each possess a single large chloroplast instead of
the many smaller chloroplasts that most plants have.
Societal Impact
Due
to their limited range of habitats, nonvascular plants have not had that
great of an impact on society. Because of their sensitivity to the
world around them, though, they can be useful indicators of environmental
conditions. For example, some mosses are strict calcicoles, and they will
only grow where calcium is freely available in the substrate.
Nonvascular plants are also particularly susceptible to air and water
pollution, which makes them good indicators of the purity of the
environment. For example, in the countryside, trees generally have
large numbers of mosses and liverworts growing on them, but in towns and
cities the trees are generally bare. Mosses are also used in a lot
of decorative gardening, and they also serve as food for small animals and
insects.
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