BRYOPHYTES Feb 29

Bryophytes are commonly called mosses.

Mosses have gametophytes with 2 phases.

When spore initially germinates(i.e., mitosis begins and cells enlarge).

-first forms structure called protonema.

Major division of mosses in 2 groups follows form of protonema.

- (1)Sphagnum (peat mosses) and its relatives have thalloid protonema,

- (2) other mosses have a filamentous, branched protonema.

Second phase of moss gametophyte arises when protonema forms a bud to initiate the "leafy" phase.

-all mosses have leafy phase in life cycle.

-Body of "leafy" phase has elongate, cylindrical axis called caulid.

-"leaves" said not to be true leaves because they don't develop or have structure of leaves of more complex plants.

-"leaves" of mosses called phyllidia (sing. phyllid).

-Phyllids are single cell layer thick.

-Phyllids may have diverse shapes and sizes.

-Phyllids are helically arranged on caulid.

Rhizoids of mosses are always multicellular (those of hornworts and liverworts were always unicellular).

Sphagnum (peat moss) a moss of boggy areas somewhat unique body.

-e.g., mature gametophyte does not form rhizoids.

-the caulids of Sphagnum form cluster of branches are nodes.

-Cluster of branches concentrated nodes near apex of the caulid.

-This gives the plant a tufted appearance.

"Leaves" of Sphagnum quite unique.

-They consist of two forms of cells.

-Dead cells (hyaline cells) have pores and thickened areas of cell wall.

-living cells with little or no chlorophyll surround the dead cells.

Dead cells of phyllid take up water through pores.

-The plant has the ability to take up about 20X it dry weight in water because of dead cells in leaves.

-This water retention makes Sphagnum quite useful as a packing material for shipping flowers and whole plants or for starting cutting of new plants.

Sphagnum one of few economically important mosses.

-in addition to use as packing material:

-it is harvested and dried in some areas such as Ireland where it is used as fuel for cooking and heating.

-Fossil peat bogs form rocky material called lignite that is mined and sold as a coal-like fuel.

Sphagnum mosses form peat bogs.

-often found in colder parts of world.

-The mosses release H+ ions into water giving entire bog and acidic pH-often less than 4.

-Acidic environment of peat bog prevents bacterial growth.

-This is one reason why archeologists often exhume animal, including human, bodies with exceptional preservation from bogs.

-Humans that are hundreds of years old have been exhumed from bogs with skin and clothes nearly intact.

-Acidic nature of Sphagnum that prevents bacterial growth led to the use of the moss as a wound dressing during WWI.

-When you are reading the Hemingway novels about ambulance teams during WWI that were racing around to collect wounded; they were probably applying Sphagnum to wounds rather than sterile cloth bandages.

Conducting tissue is found in the axes of some moss gametophytes.

-This conducting tissue includes cells for conducting water (hydroids)

-and cells for conducting photosynthetic products (leptoids)

-These conducting elements are considered to have evolved within the mosses as a derived feature.

Discuss evolution terms:

Mosses are generally not considered to be vascular plants.

-The "vascular plants" are an evolutionary group called Tracheophytes (i.e., plants with tracheary elements for conducting water.).

-Tracheophytes generally considered to include all extant land plants except liverworts, hornworts and mosses.

-This point of view supposes that one gorup of mosses has evolved a conducting system in parallel with that found among Tracheophytes.

-When similar features evolve in parallel in two different groups they are called parallelisms.

-Parallelisms are a form of confounding evolutionary events that we call homoplasy.

-so we could say that parallelism is form of homoplasy.

Let's suppose for moment, that vascular system of mosses is not parallelism.

-i.e., it represents a cellular system that is forerunner of the vascular system of tracheophytes.

-when corresponding attributes have the same evolutionary origin in two groups they are called homologies vs. analogy

Recent analyses of relationships of bryophytes and tracheophytes have suggested that we should perhaps consider the vascular system of mosses and tracheophytes to be homologous.

- such and interpretation would suggest that mosses primitively had obtained a vascular system (which they share with tracheophytes and them the more derived mosses lost the simple vascular system.

Gametangia of mosses usually in cluster at apex of gametophyte axis.

-Archegonia and antheridia may occur together at apex of single gametophyte in some kinds of mosses. (monoecious)

-More than half of mosses have antheridia and archegonia on separate gametophyte axes

-this condition of separate sexes called dioecious.

-At apex of moss axis, numerous sterile filaments found among gametangia-these called paraphyses.

Sperm must swim in pool of water or be splashed to archegonial plants.

-cup-like area at apex of gametophyte where antheridia located called splash cup.

Only a single sporophyte develops on a single gametophyte axis.

-Sporophyte has some photosynthetic tissue but still largely dependent upon gametophyte for nutrition and water.

-Sporophyte is never separate form gametophyte.

-it is more diminutive and shorter-lived than gametophyte (also true of liverworts and hornworts).

Sporophytes develop from zygote that forms in archegonium following fertilization.

Foot of sporophyte is the region that anchors the body in gametophyte.

Elongate stalk of sporophyte is called seta.

Sporangium at apex of sporoph.-often called capsule.

In Sphagnum, the operculum is forcefully popped off.

-Spores forced out at same time.

-This is caused by build-up of air pressure as drying sporangium shrinks.

In other mosses, operculum not forcefully popped off.

-It simply falls away

-Some mosses have teeth below operculum

-called peristome teeth

-Peristome teeth are sensitive to moisture.

-Air in capsule dries when operculum removed and this causes teeth to open.

-spores are released when teeth open.

Let's consider some aspects of reproductive ecology of mosses.

1. As we have discussed life cycles, descriptions of life cycles have been schematic and showing general set of events.

2. But what happens when mosses actually reproduce in nature?

3. We discussed that motile sperm create a problem for land plants

4. For mosses, liverworts and hornworts this problem limits the individuals to living in very moist environments where free water is at least available during the time when sperm must reach the egg.

We should recognize that this problem created by the fact that sperm must swim to eggs affects the dynamics of populations in nature.

1. If sperm fail to reach the archegonia, then have no sex to create individuals with new gene combinations

2. Lack of sexual reproduction in a population will diminish the amount of variation n the population.

3. Lack of variation may limit potential of the organism to respond to environmental changes--may make population more susceptible to extinction or extreme bottlenecks in size.

Let's ask important question: Do sperm fail to reach eggs in natural populations which could lead to diminished sexual reproduction?

Moss sperm that has been released from antheridium lives about 6 hours in water.

1. Sperm can swim about 1 cm in standing water in that 6 hr. time.

2. So we need to recognize that sperm don't get too far in standing water.

3. Populations studies show that sperm travel a maximum of 4-5 cm, although may get farther if splashed.

4. Mean distance of sperm dispersal shown to be 1-2 cm.

5. Some sperm shown to be transported by insects and spiders.

--sticky sperm adhere to these crawling organisms.

6. This limited dispersal ability of sperm shows that gene dispersal itself may be very limited.

Bryophytes exist with gametophytes tightly clumped in populations.

1. These populations form cushions or mats of individuals.

2. Among most plants, as population density increases the death rate of individuals also increases

--This is not true among mosses: studies of populations have shown that death rate decreases as density increases. (probably true only up some extremely dense threshold).

This tight clumping means that sperm simply do not need to travel as far to find an egg to fertilize.

1. Given that they appear to travel only 1-2 cm, dense packing of individuals increases possibility of encountering gametophytes with archegonia in that range.

But dispersal ability of sperm is only one part of reproductive biology.

1. Population structure also affects potential for sexual reproduction.

2. In moss populations more female gametophytes than male have been found.

3. This means that a sperm that gets splashed from a male will be more likely to land on a female than another male.

No estimates of success for sperm from particular males.

Surveys of populations have shown that 10-20% of gametophytes with archegonia never have sexual reproduction.

Combining the population attributes of extreme density of individuals with the presence of more females than males makes it possible for these motile sperm to function adequately in sexual reproduction despite the limitations created by being a swimming cell in a terrestrial environment.