Plants in the division Bryophyta have features that are considered to be rather primitive. These are plants with little specialization of tissue, which are not well-adapted to life in a relatively dry land environment. They also have comparatively simple reproductive processes, and are the only plants which have a dominant gametophyte generation. A study of the features of mosses will illustrate the major characteristics of this plant division. Here are some examples of mosses
In mosses, the gametophyte is small and at least partially erect, with very little specialization of cells and tissues, specifically, no true leaves, stems, or roots. The moss gametophyte has a shoot portion that appears leafy, and has rhizoids which emerge from its base to attach it to the substratum upon which it grows. The gametophyte is generally green and photosynthetic, and obtains water and other nutrients from the soil by direct absorption into its cells. It contains no cells specializing in the transport of water and/or nutrients (vascular tissue) and therefore cannot grow so large as to prevent contact between the soil and the majority of its cells.
At maturity, the moss gametophyte is capable of developing gametangia on its surface. sperm-producing antheridia can arise amongst the leaf-like structures along the length of the thallus; egg-producing archegonia most often develop at the tip of the erect gametophyte. When fully developed, flagellated sperm are released from an antheridium and swim through a film of water to reach an egg-containing archegonium.
Syngamy of the egg and sperm produce a zygote within the archegonium. This zygote undergoes mitosis to produce an embryo, again retained within the archegonium. Finally, the embryo matures into a sporophyte, consisting of a sporangium (capsule), a seta (stalk), and a foot which remains embedded in the gametophyte tissue. The continued attachment of the sporophyte to the gametophyte allows the sporophyte to absorb most of its needed nutrients from the gametophyte.
Meiosis occurring within the sporangium produces spores. Following spore production, the capsule opens up to release the spores, which germinate to produce new moss gametophytes.
The life history of mosses can be viewed here, 2.
1. Obtain a dried or live specimen of a moss, 2 from your instructor. Identify the rhizoids and shoot of the moss gametophyte. Your specimen should also include an attached sporophyte. Label its seta (stalk) and sporangium (capsule).
2. Place the moss capsule on a microscope slide in a drop of water. Using a dissecting microscope, examine and draw the capsule in greater detail. At the tip of the capsule a cap-like structure called an operculum should be present. Using a dissecting needle, gently pry the operculum off the capsule to reveal a row of structures called peristome teeth. The peristome teeth help regulate the release of spores from the capsule. Draw the capsule again, labeling these structures. Finally, crush the capsule to observe the large number of small, green spores contained within.
3. Examine a prepared slide of moss archegonia attached to the tip of a gametophyte branch. Each archegonium consists of an elongate neck, with a canal in it leading down to the egg-containing venter. Draw an archegonium, and label its neck, venter, and egg, and the stalk that attaches it to the rest of the gametophyte.
4. Examine a prepared slide of moss antheridia, also attached to the tip of a gametophyte branch. Each antheridium has an external jacket of sterile cells, and a large mass of spermatogenous cells which will develop into sperm. Draw an antheridium and label its sterile jacket and spermatogenous cells, and the stalk that attaches it to the rest of the gametophyte.
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