Bryophyta

(Greek, bryon: moss; phyton: plant)

Bryophytes are the simplest, primitive, non-vascular plants which occupy apposition intermediate between algae and pteridophytes from evolutionary point of view.

General Characteristics

1.       They usually grow in cool, moist and shady places.

2.       They complete their vegetative growth on land but require water for fertilization. Hence, they are called amphibians of the plant kingdom.

3.       Main plant body is gametophyte (n) which is independent.

4.       Plant body is thallus (not differentiated into root, stem and leaves) or foliose (differentiated into rhizoids, short stem axis and simple leaves).

5.       Vascular tissues (Xylem and Phloem) are absent.

6.       They reproduce by :

-          Vegetative reproduction takes place by fragmentation, gemma, protonema, tuber.

-          Sexual reproduction is of oogamous type. Male reproductive organ- Antheridium, Female reproductive organ – Archegonium

7.       Antheridium produces male gamete called antherozoids and archegonium produces female gamete called egg or oosphere.

8.       In presence of water, fusion of antherozoid and egg results in formation of a diploid zygote (Oospore) which develops into a sporophyte.

9.       They show two distinct heteromorphic alternation of generation:

-          Haploid gametophytic

-          Diploid sporophytic

Classification

The division bryophyte is divided into three classes.

Marchantia

Systematic Position

Kingdom             -             Plantae

Division               -             Bryophyta

Class                   -             Hepaticopsida

Order                   -             Marchantiales

Family                 -             Marchantiaceae

Genus                 -             Marchantia

Morphology

·       Marchantia is a green, dorsiventrally flattened, dichotomously branched, gametophytic thallus, commonly found in moist and shady places.

·       The tip of each thallus bears an apical notch which represents the growing point.

·       The dorsal surface contains a prominent midrib furnished with many gemma cups and ventral surface contains unicellular rhizoids and multicellular scales.

·       The rhizoids are of two types: Smooth walled (wide with smooth inner wall; helps in anchorage and absorption) and Tuberculated (narrow with peg like tubercles in the inner wall; helps in conduction).

·       The scales, which help in the retention of water, are also of two types: Ligulate (tongue shaped) and Appendiculate (with an appendage on the tip).

Reproduction

1.       Vegetative reproduction 

·       By fragmentation: As the process of death and decay starts from the posterior end and reaches the point of dichotomy, two apical parts of thallus separate to grow into a new plant.

·       By adventitious branches: The thallus gives rise to adventitious branches from the ventral surface which later separates from the parent plant and forms new thalli. 

·       By gemma: A gemma is a modified bud of the tissues that develops on the dorsal surface of the thallus. It detaches from the parent at maturity and develops into new thalli.

2.       Sexual reproduction

Sexual reproduction in Marchantia is oogamous. All species are dioecious. Male reproductive bodies are known as antheridiaand female as archegonia. Antheridia and archegonia are produced on special, erect modified lateral branches of thallus called antheridiophore and archegoniophore, respectively.  Antherozoids are present in the antheridium while the archegonium consists of several neck canal cells, a venter canal cell and an egg.

Water is needed for fertilization. The neck canal cells and the ventral canal cell disintegrate and form a mucilaginous mass, which oozes out as the archegonia swells after absorbing water. It consists of chemical substances, which triggers the chemotactic response. The mature antheridium dehiscence and antherozoids are set free. They get attracted and swim towards archegonia. One of the antherozoids fuses with egg and fertilization takes place. The male and female nuclei fuse together to form a diploid cell called the zygote. The zygote formation ends the gametophytic phase and represents the first stage of the sporophytic generation.

The diploid zygote does divides mitotically and develops into a multicellular structure called the sporophyte. The sporophyte is differentiated into foot, seta and capsule. It is not the free-living stage, it is dependent on the gametophyte for nourishment. Some of the cells of sporogenous tissue called spore mother cells (diploid) divide meiotically to produce haploid spores. These haploid spores are released by the dehiscence of the capsule. Under favourable conditions, they germinate to form the new haploid plant or gametophyte.

Ecological Importance of Bryophytes

·       Soil formers: Mosses grow in rocks, where their death and decay help in soil formation. 

·       Prevent soil erosion: Mosses grow densely on soil and prevent soil erosion by holding enough rain water and reducing the impact of run-off water.

·       Recycle nutrients: Bryophytes help in recycling of nutrients through biogeochemical cycle.

·       Indicators of air pollution: Bryophytes are very sensitive to air pollution and can indicate air pollution. Air pollution reduces photosynthesis eventually causing their death.

Economic Importance of Bryophytes

·       Medicine: Sphagnum has antiseptic and high absorptive property therefore used to make surgical bandage. Tea of Polytrichium commune helps to dissolve stone in the kidneys and gall bladder. Marchantia polymorpha is used in treatment of pulmonary tuberculosis.

·       Packing materials: Dried bryophytes are excellent packing material for fragile goods. They are also used as packing material for shipment of cut flowers, vegetables, etc due to their great water retention capacity.

·       Food: Bryophytes are a source of food for birds and mammals inhabiting cold regions.

·       Nitrogen fixation: Bryophytes like Anthoceros thalli has nitrogen fixing cyanobacteria.