Life cycles and water considerations in the bryophytes:
Life cycles:
The bryophyte life-cycle is comprised of two separate generations: the primary food producing 1N gametophytic generation and the shorter-lived 2N sporophytic generation. Unlike the more advanced alga discussed previously, the IN generation predominates.
The moss life cycle: compare this cycle with the liverworts life program below. Are there significant differences in which stage predominates, or is it a case of morphological differences
The liverwort life cycle below:
One factor that may phase you is the terminology... words you should have a feel for:
antherida (plural).antheridium (single) = sperm producing structures
archegonia (pl) archegonium (s) = egg producing structure
protenema = spores germinate forming simple filamentous bodies which further develop
sporophyte = 2N means 2X the # of chromosomes of the gametophyte but not necessarily diploid as many plants are polyploid and have many copies of chromosomes ( we will see in the ferns, some may have hundreds.. redundant or what?)
gametophyte = 1N containing only 1 set of chromosomes... under what set of circumstances is 1N better than 2N? in 2N you have a second set which may compensate for mutated genes in the one set or give you more ability to form alternative forms of enzymes/proteins which are functional under different environmental conditions (ie. temperature or pH conditions).
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As we noted from the diagram above, the gametophyte gives rise to the sexual structures, the antheridia and archegonia:
Antheridia are the male sexual organs of the gametophyte generation of mosses and liverworts. They consist of 1-2 cell-thick walled sacs either enclosed by differentiated leaves, or in some thalloid liverworts, enveloped in a cavity in the thallus.
Antherocytes form in great numbers and are released to give rise to antherozoids - the motile male gametes. These always require a film of water in which to "swim" to the female archegonia in order to fertilize the egg.
Photograph by Brian Steer./info Hale
Archegonia (the female structures) consist of flask-like structures with an elongated neck, and at their base develops an egg. Above the egg cell run a line of canal cells, enclosed by a single layer of neck cells. When the archegonium is mature, the canal cells disintegrate leaving a mucus-like substance through which the male antherozoids swim to reach the egg cell.
After fertilization, a the sporophyte develops, which remains attached to the gametophyte, unlike as we shall see later the support of the gametophyte by the sporophyte in the more advanced plants.
The sporophyte includes the capsule borne upon a thin stalk known as a seta. When mature, this capsule releases thousands of spores which may with luck germinate into a new gametophyte.
Asexual Reproduction
The gametophyte can also reproduces asexually through fragmentation or in some species a specialized structure known as gemmae cups, which contains pieces of vegetative cells which can develop into a new plant.
Under what circumstance would asexual reproduction make more sense? well, if conditions are good, resources abundant..then according to human logic at least, the cheapest means would be to form progules which wouldn't spread far necessarily, and would maintain their 'adapted' genetic constitution.
However, if conditions were crowded or if the environmental conditions were to change, then sexual reproduction forming light easily moved spores which would carry new genetic recombinations would make more sense.Other reasons for sexual recombinations? think of some possibilities and bring them to class.
Water absorption and movement....
As bryophytes are generally plants of humid places, water is essential for their life cycles and metabolism.
Bryophytes lack roots to absorb water from superficial or deep seated areas of the substrate. They have instead, rhizoids, structures composed of one to many cells (uni- or pluricellular) that attach them to the substrate but do not yet have the capacity to uptake water. Why not? remember these critters have yet to evolve a true vascular system.. not differentiated xylem or phloem cells.
Water conduction is mostly external ( through the cell walls) although there are groups in some of the thalloid hepatics and mosses, that possess a developed a simple conducting system for the translocation of water, minerals and photosynthates. Their water content depends to a large extent on the ambient humidity.
Life cycles and features of the Bryophytes The liverworts Differences between the mosses and liverworts Return to introductory Bryophyte page
