Plant Kingdom
Bryophytes
Habits & Habitats
Bryophytes include the various mosses and liverworts which are usually found in moist shaded areas in the hills. Bryophytes are also called amphibians of the plant kingdom because they can live on soil but need water for sexual reproduction. They are usually found in damp, humid and shaded localities.
Characteristics
The plant body is thallus-like and can be prostrate or erect. The plant body is attached to the substratum by unicellular or multicellular rhizoids. True roots, stem or leaves are absent. However, root-like, leaf-like or stem-like structures may be present. Vascular tissue is absent.
Reproduction
The main plant body is haploid in bryophytes. The main plant body is called gametophyte because it produces gametes. Sex organs are multicellular. Male sex organ is called antheridium and produces biflagellate antherozoids. The female sex organ is called archegonium and produces a single egg. Antherozoids are released into water for fertilization. After fertilization, zygote does not immediately divide meiotically. Zygote produces a multicellular body called sporophyte. Sporophyte is attached to the photosynthetic gametophyte so that it can get nourishment from the gametophyte. Some cells of the sporophyte undergo meiosis to produce haploid spores. The spores germinate to produce gametophyte.
Economic Importance of Bryophytes:
- Peat is obtained from species of Sphagnum which is a moss. Peat has been in use as fuel. Peat is also used as packing material for shipment of living materials; because peat has good capacity to hold water.
- Mosses are among the first organisms to colonise rocks and thus play an important role in soil formation. Mosses hasten the speed of breaking of rocks into smaller pieces, and thus pave the way for soil formation from rocks. Some mosses also prevent soil erosion which may happen because of rains; as they form dense mats on soil.
- The bryophytes are divided into two classes, viz. liverworts and mosses.
Liverworts
Characteristics
They usually grow in moist and shady habitats. The plant body is thalloid. Thallus is dorsiventral and closely appressed to the substrate. Some members have tiny leaf-like appandages in two rows on the stem-like structures.
Reproduction
Asexual reproduction takes place by fragmentation, or by formation of specialized structures called gemmae. A gemma is a green, multicellular, asexual bud. A gemma develops in small receptacles called gemma cup. The gemma gets detached from the parent body and germinates to form a new individual.
In case of sexual reproduction, male and female sex organs are produced either on the same or on different thalli. The sporophyte is differentiated into a foot, seta and capsule. Spores are produced after meiosis. These spores germinate to produce free-living gametophytes.
Mosses
Characteristics
Gametophyte is the predominant stage of the life cycle of moss. The gametophyte consists of two stages, viz. protonema stage and leafy stage.
Protonema develops from the spore. It is creeping, branched and usually filamentous.
The leafy stage develops from the protonema as a lateral bud. The leafy stage consists of upright, slender axes bearing spirally arranged leaves. It is attached to the soil through multicellular and branched rhizoids. The leafy stage bears the sex organs.
Reproduction
Vegetative reproduction takes place by fragmentation and budding in the secondary protonema.
In case of sexual reproduction, antheridia and archegonia are produced at the apex of the leafy shoots. After fertilization, the zygote develops into a sporophyte. The sporophyte consists of a foot, seta and capsule. The capsule contains spores. Spores are formed after meiosis. There is an elaborate mechanism of spore dispersal in mosses.
Common Examples: Funaria, Polytrichum and Sphagnum
Alternation of Generation
In case of bryophytes, gametophyte is the predominant stage but length of sporophyte can vary. Although sporophyte grows on gametophyte but sporophyte is multicellular and lives for a considerable time period. This type of alternation of generation is known as haplo-diplontic.