GENERAL CHARACTERS AND LIFE CYCLE OF BRYOPHYTES

 Bryophytes are the first land plants. It is believed that, they originated from aquatic plant.

Bryophytes are not considered as the successful land plants because vascular tissue is absent and they need water for fertilisation.

Bryophytes are known as amphibians of the plant kingdom, because these plants can live in soil but are dependent on water for fertilisation.

They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures.

Bryophytes are Sciophytes, i.e. bryophytes prefer to grow in cool, moist (wet) and shady places.

Bryophytes in general are of little economic importance, but some mosses provide food for herbaceous mammals, birds and other animals.

The water conducting tissue in Bryophyta is parenchyma. 

Bryophytes include the various mosses and liverworts that are found commonly growing in moist and shaded areas in the hills.

Life Cycle of Bryophytes : 

• The main plant body of bryophyte is haploid. It produces gametes, hence is called a gametophyte.
• Sex organs are formed on gametophyte.
•  Sex organs are multicellular and jacketed in bryophytes. Male sex organ is called antheridium and female sex organ is called archegonium. An archegonium is flask shaped and produce single egg.
• The male gametes of bryophytes are motile. These motile male gametes are called as antherozoids. Antherozoids are usually comma shaped and biflagellate. Female gamete is called egg or ovum.
• In Bryophyta, fertilization is performed by zoidogamy i.e. male gamete swims into water to reach the female gametes and fertilizes it.
• As a result of the fertilization, a diploid zygote is formed. Zygotes do not undergo reduction division (meiosis) immediately.
• Zygote forms embryo and then sporophyte by mitosis. The zygote initiates the sporophyte generation. Sporophyte generation is a diploid stage.
• The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. The sporophyte of Bryophyta is not made of root, stem and leaves, but it is made of foot, seta and capsule, so it is known as sporogonium. Some cells present in capsule of sporophyte function as spore mother cells. Now meiosis takes place in spore mother cells. to form haploid spores.
  In Bryophyta the sporophyte is dependent on gametophyte. This is a unique character of Bryophyta.
• The germination of spores is direct or indirect. In liverworts the germination of spore is direct i.e. each spore forms a gametophyte or a thallus after germination. The germination of spores in mosses is indirect. In mosses a multicellular filament is formed after the germination of spore. This filament is known as protonema. Protonema developed from spores is called primary protonema and the protonema developed from parts other than spores are known as secondary protonema.
• Protonema is creeping, green, branched and filamentous. Now lateral buds are formed on protonema. Each bud develops and forms a gametophyte plant. Indirect germination is best for survival. Mosses are gregarious in nature because they appear in group.

LIVERWORTS :  

• Bryophytes included in this class have shape like liver (e.g., Marchantia) or flat (e.g. Riccia) so they are known as liverworts.
• Plant body of this group is thallus like. The thallus is dorsiventral and closely appressed to the substrate.. Rhizoids and scales are present on thallus. Rhizoids are unicellular and unbranched. Scales are multicellular and protective in functions.
• The leafy members (e.g. Porella) have tiny leaf like appendages in two rows on the stem like structures.
• The sporophyte of Liverworts is completely dependent on gametophyte i.e. it is dependent on gametophyte for food, water and habitat.
• The sporophyte of Liverworts is made up of foot, seta and capsule, after meiosis, spores are produced within the capsule. These spores germinate to form free- living gametophytes. True elaters are present in sporophyte of some members of liverworts. Elaters are hygroscopic and they help in dispersal of spores.
• Asexual (vegetative) reproduction in Liverworts takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (sing. Gemma). Gemmae are green, multicellular, asexual buds, which develop in small receptacles called Gemma cups located on the thalli. The gemmae become detached from the parental body and germinate to form new individuals. e.g. Marchantia.
• During sexual reproduction male and female sex organs are produced either on same (e.g. Riccia) or on different thallus (e.g. Marchantia).

MOSSES :  

• The predominant stage of the life cycle of a moss is the gametophyte which consist of two stages. The first stage is the protonema stage, which develops directly from a spore. It is creeping, green, branched and frequently filamentous stage.
• The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axes bearing spirally arranged leaves.
• The main plant body or gametophyte of mosses is made up of stem like, leaf like and rhizoids (roots like). They are attached to the soil through rhizoids. The rhizoids present in the class are multicellular, branched and obliquely septate.
• Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema.In sexual reproduction, the sex organs antheridia and archegonia are produced at the apex of the leafy shoots. After fertilization, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. The capsule contains spores. Spores are formed after meiosis.
• The sporophyte in mosses is more elaborated (developed) than that in liverworts.
• The sporophyte of mosses is also partially dependent (semi parasite) like that of hornworts. i.e. it is photosynthetic. The Mosses has an elaborate mechanism of spore dispersal.
• Peristomial teeth are present in moss sporophyte which help in spore dispersal.
• Common example of mosses are Funaria, Polytrichum and Sphagnum.