All pictures below courtesy Ralph. E Taggert, Michigan State University|
A moss, showing sporophyte and gametophyte
A liverwort, showing sporophyte and gametophyte
Division Bryophyta, the bryophytes, comprises the mosses (Class Musci), as well as liverworts (Class Hepaticae) and hornworts (Class Anthocerotae). The bryophytes are thought to have been the first true plants, evolving from charophytes almost 500 million years ago.
The bryophytes exhibit several hallmarks of plants that are key adaptations to living on land, including a waxy cuticle to protect the leaves, stomata to allow gaseous exchange, and gametangia that protect developing embryos. Despite these traits, bryophytes are the only non-vascular plants, meaning that they do not contain a system of vascular tissue to conduct water and other nutrients through the body of the plant; some bryophytes, however, have water-conducting tubes. All bryophytes can absorb water through the entire body surface. This lack of vascular tissue equates to a lack of internal support, which is supplied to other plants by the rigid cell walls in vascular tissue.
Unlike other plants, bryophytes do not have true organs, such as leaves, stems, or roots. In place of roots, most bryophytes have thin, hairy tubes called rhizoids that provide anchorage and nutrient uptake from the soil. The bryophyte life cycle is unique in having a dominant gametophyte generation. The actual green plant in mosses and worts is the gametophyte plant, while the sporophyte consists of simply an enclosed sporangium, typically atop a stalk.
In the life cycle, gametangia in the gametophytes produce haploid gametes. When the plant is wet, the flagellated sperm swim to gametangia containing eggs, and fertilize the eggs to produce diploid zygotes. These divide mitotically, and eventually grow into sporophytes; each sporophyte remains attached to a gametophyte. In the sporangia atop the sporophyte stalks, meiosis produces haploid spores, which are released, and soon germinate and begin to mitotically divide into gametophytes.
As a result, bryophytes do not expand their territory very quickly, and they have a strong tendency to grow together, often as mats that blanket the ground in forests (see Mosses). Because bryophytes have flagellated sperm, they tend to inhabit moist environments that facilitate sperm movement. Bryophytes also prefer living in shady places such as ravines and forest. Bryophytes, especially mosses, inhabit almost all parts of the world, including tundra and desert habitats, as well as tropical forests.
1. Diagram life cycle of moss.
2. Classify to division.