BIO 5 GENERAL BIOLOGY

BRYOPHYTES: MOSSES

1. The term "bryophytes" refers to a group of land plants that lack vascular tissue and share several other characteristics including a life cycle in which the gametophyte (1N multicellular adult plant) is the dominant portion. The gametophyte is dominant in the sense that it leads an independent existence, is photosynthetic, and lives for several years. The most familiar of the bryophytes are the mosses.

2. Mosses and other bryophytes have dealt with the problems of life on land by mostly avoiding them. For example, the plants generally live in wet environments and remain small in size. The moss gametophytes shown in the image ( bio5_lab\bryophytes_mosses\Male_MC.jpg) are no more than one centimeter tall. Note that some of the plants have a flower-like arrangement of brown colored leaves at the top. These plants are male moss gametophytes. Other plants have green "leaves" at the top. These gametophytes are female.

3. If the flower-like portion of the male gametophytes is sliced in longitudinal section and examined with a microscope, a cluster of club-shaped reproductive structures is visible ( bio5_lab\bryophytes_mosses\Antheridia_MC.jpg). These multicellular structures are called antheridia, and each one produces several hundred 1N sperm cells. If you switch to high power and examine one antheridium, you can see the sperm cells within the protective covering of the antheridium ( bio5_lab\bryophytes_mosses\Antheridium_MC.jpg). The sperm cells stain darker staining, and the covering layer stains lighter. When it rains, rain drops land in the flower-like structure at the top of the male plants, and sperm cells are splashed from the male plant to (hopefully) one of the nearby female gametophyte plants.

4. If the top of the female gametophyte plant is sectioned longitudinally and examined with a microscope, several egg-producing structures called archegonia are present. The image shows two archegonia (bio5_lab\bryophytes_mosses\Mnium_archegonia130.jpg ). Each produces one 1N egg cell. The egg is clearly visible in the archegonium at the left. Only part of the egg cell is visible in the archegonium on the right. Once sperm have been splashed to the female plants, sperm swim into the archegonium and fuse with the egg. This process is called fertilization. A single 2N cell called the zygote (or fertilized egg) is formed. The zygote then undergoes mitosis many times to eventually form a multicellular 2N plant called the sporophyte.

5. It's important to keep in mind that the sporophyte develops in the location of the archegonium. This means that 1N female gametophyte moss plants have 2N sporophytes attached to them ( bio5_lab\bryophytes_mosses\Atrichum_moss_130.jpg). The stalked reddish or tan colored structures in the image are sporophytes. These are attached to green, photosynthetic female gametophytes. It is the female gametophytes that provide food, water and minerals for the non-photosynthetic, short-lived sporophytes.

6. The sporophytes have a thin stalk that grows several centimeters tall (bio5_lab\bryophytes_mosses\Hairy-cap_moss.jpg). As the sporophytes mature, the top portion enlarges (bio5_lab\bryophytes_mosses\sporophytes_of_Polytricum.jpg ). The enlarged portion is called the sporangium. It is in the sporangium that spores are produced by meiosis. Meiosis results in the production of 1N spores. When the spores are mature, the sporangium breaks open, and spores are released into the air. The stalk portion of the sporophyte ensures that the sporangium is elevated above the gametophyte, and this maximizes the chances for spore dispersal.

7. If a spore (1N) lands in a suitable environment, it undergoes mitosis many times and forms a multicellular 1N gametophyte. If the spore's nucleus has a Y chromosome, the gametophyte produced is male. If the spore's nucleus contains and X chromosomes, the gametophyte produced is female.