Division Bryophytes

It is considered as a phylum and also as a group or division.

General Characteristics

The bryophyta is a group of plants comprising over 15,000 species of liverworts, hornworts and mosses are the only nonvascular plants. Bryophytes are typically quite small and a few exceed 2 centimeters in length. They generally require a moist environment for active growth and reproduction, but some bryophytes tolerate dry areas. In the Arctic and the Antarctic, these are the most abundant plants in respect of their numbers and species. The gametophytes of bryophytes are green and manufacture their own food. They are relatively large and evident as compared to sporophytes. Some of their sporophytes are completely enclosed within gametophyte tissue, others which are not enclosed, turn brownish or straw colored at maturity. The three main features of bryophytes are:

They lack specialized vascular tissues

Multicellular sex organs produce embryo.

Sporophytes are always smaller and obtain their food from the gametophyte.

Their life cycles are similar to seed plants.

Bryophytes are also called amphibious plants because they need water for development, existence and reproduction.

Mosses cover several rocks  

Adaptive Characters in the Bryophytes for Life on the Land

Following are the adaptive characteristics exhibited by non-vascular plants:

A compact multicellular plant body to conserve water.

Some modifications in photosynthetic tissues for absorption of CO₂ Gas.

Special structures for the absorption of water.

Heterogamy, the production of egg and sperm.

Protection of reproductive cells.

Formation of embryos.

Alternation of generations.

The Multicellular Plant Body and Conservation of Water

The plant body of liverworts is called thallus and is multicellular e.g. Marchantia.  The thallus consists of hundreds of cells. Only the cells of the upper layer are exposed to the atmosphere. Some cells are photosynthetic and some are storage cells. Water cannot evaporate from these inner cells because the upper epidermis has covering of cutin, which is a wax like substance. It reduces the evaporation of water in some mosses and liverworts. The layer of cutin is called cuticle.

 

Absorption of Carbon dioxide

The upper epidermis in Marchantia has many pores. The pores open into the air chamber. The air chamber is surrounded with photosynthetic cells. CO2 is absorbed by large amount of wet surfaces of the photosynthetic cells of the air chambers. CO2 then diffuses into the cytoplasm. When CO2 is being absorbed, evaporation of water may occur through the pores.

 

Absorption of Water

The structures for absorption of water in moss and liverworts are rhizoids. These are present on the lower surface of the Marchantia thallus. Rhizoids are long filamentous structures. They are unicellular and are extensions of the cell of the lower epidermis. Rhizoids increase the surface area for absorption of water from the soil and also help in anchorage.

 

Heterogamy

 When two types of gametes are produced, it is called Heterogamy. Sperms and ova are produced by the nonvascular plants e.g. Moss, Marchantia etc. The sperms are flagellated and motile; require water for reaching the egg.  The egg is non-motile and large. It contains large amount of food. The food is used to nourish the early stages of the developing embryo after the fertilization of egg. Due to the water requirement for fertilization they cannot live away from water and are thus called amphibious plants.

 

Protection of Reproductive Cells

The Moss, Marchantia etc. can be distinguished as male and female plants. The sex organs are multicellular, (whereas in algae the sex organs are unicellular). In the moss plants the sex organs are at the tip of the green shoot. The male sex organ is called antheridium and it produces sperms. The female sex organ is called archegonium and it produces eggs. The sex organs are covered by sterile hairs to prevent the drying of sex organs. Most of the cells of the sex organs are sterile which form a protective coat around the egg and sperms. Protection of spore is performed by sporangium.

 

Embryo Formation

Fertilization is inside the archegonium. The zygote divides to form the embryo and is retained inside the archegonium. The chances of survival of embryo are increased as it is protected by the wall of the archegonium. Embryo is present in all bryophytes and vascular plants.

 

Alternation of Generation

The mosses and liverworts have a life cycle with alternating gametophyte and sporophyte generations. It increases the chances of survival of the plants on land.