11th Class Biology Plant Kingdom Bryophyta

(Gk. Bryon = moss; phyton = plants)

Bryophyta includes the simplest and primitive land plants. Which are characterised by the persence of independent gametophyte and parasitic sporophyte. The term bryophyta was coined by Braun (1864) but bryophytes were delimited in its present form by Schimper (1879). It occupies a position intermediate between algae and pteridophyta. Due to peculiar type of their habitats, they are regarded as 'the amphibians of the plant kingdom'. The science connected with the study of bryophytes called bryology.

Habitats

Bryophytes usually grow in moist and shady places. The plants grow densely together and form green carpets or mats on damp soil, rock, walls, barks of trees and on decaying logs in forests, especially during the rainy season.

Specialized habitats : Some bryophytes grow in diverse habitats such as – aquatic (e.g., Riccia fluitans, Ricciocarpus natans, Riella), epiphytes (e.g., Dendroceros, Radula protensa and many mosses), saprophytes (e.g., Buxbaumia aphylla, Cryptothallus mirabilis), and in dry habitats such as dry heaths (e.g., Polytrichum juniperinum), deserts (e.g., Tortula desertorum) and dry rocks (e.g., Porella platyphylla).

Gametophytic plant body

(1) The life cycle of bryophytes consists of two distinct phases – the gametophytic phase and the sporophytic phase. The haploid gametophyte is dominant, long lived, green and independent whereas the diploid sporophyte is short lived and dependent upon the gametophyte. The two phases are morphologically distinct.

(2) The plants are small, range from few millimetres (e.g., Zoopsis) to 30–40 centimetres. The tallest species may reach upto 70 cm in length (e.g., Dawsonia).

(3) The gametophytes are either thalloid (i.e., not differentiated into true roots, true stem and true leaves) or leafy shoot having stem-like central axis and leaf-like appendages.

(4) The roots are completely absent and they are replaced by unicellular or multicellular thread like rhizoids. In some higher forms the multicellular rhizoids form cords.

(5) The vascular tissue (i.e., xylem and phloem) are completely absent.

Reproduction

(1) Vegetative reproduction : The bryophytes reproduce vegetatively by following methods :

Death and Decay : Most of these plants reproduce vegetatively by gradual death and decay of the older part of the plant body.

Adventitious branches : Many plants like Riccia fluitans, Reboulia, Asterella, Pellia etc. reproduce by adventitious branches. They separate and produce new plants.

Tubers : Several species of Riccia, Anthoceros, Sewardiella, Asterella etc. produce tubers which give rise to new plants on the arrival of favourable conditions.

Gemmae : Several members, reproduce vegetatively by forming multicelled gemmae. In Marchantia, Lunularia, the gemmae are produced in gemma cups. Gemmae are also produced on the thallus of Anthoceros. Several mosses also produce gemmae on the 'leaves' (Bryum), or axis or rhizoids or on the protonema (Funaria).

Primary protonema : The mosses generally reproduce vegetatively by breaking of the primary protonema. New gametophores now arise from the buds differentiated on it.

Secondary protonema : In several mossess a secondary protonema may arise from the rhizoids or primary protonema or even from the injured sporophyte. It may produce buds which give rise to new gametophores.

Rhizoids : Mosses may also reproduce vegetatively from the rhizoids e.g., Leucobryum.

(2) Sexual reproduction : It is oogamous, between a flagellate sperm or antherozoid and non motile egg or oosphere. The male sex organs is called as antheridium and the female as archegonium. The antheridial stalk is very distinct whereas the archegonial stalk is generally short. They may be of embedded type e.g., Riccia, Anthoceros or of projecting type e.g., Marchantia, mosses.

Antheridia : They are generally borne on the dorsal surface of the thallus. Each antheridium is distinguishable into a stalk and the body. The antheridial body consists of a mass of androgonial cells covered by a 1-cell thick sterile jacket. The terminal cell of the jacket, when distinct, is called as operculum. Each androgonial cell finally behaves as androcyte mother cell. The androcyte mother cell then forms two androcytes (antherozoid mother cell), each of which is metamorphosed into a biflagellate antherozoid.

Archegonia : These are also borne on the dorsal surface of the thallus. The archegonia are flask shaped structures distinguishable into a long neck and a globular, swollen venter. A multicelled stalk is also present in mosses but in others it is very short. The neck is one-cell thick. It is generally made up of six vertical rows of cells. The neck is capped by four cover cells and contain varying number of neck canal cells inside. The venter is also 1-cell thick in most of the plant. The venter contains an egg and a ventral canal cell.

Importance of water in fertilization

The bryophytes are fundamentally terrestrial plants but require presence of water to complete their life cycle. The water is needed for dehiscence of antheridia, liberation of antherozoids, transfer of antherozoids from antheridia to archegonia, opening of archegonial neck, and the movement of antherozoids into the archegonial neck.

Before fertilization the walls of androgonial cells disorganise to form a mucilagenous mass. The opercular cell is removed and the antherozoids are liberated. The neck canal cells and the ventral canal cell also disorganise. The cover cells split apart giving a free passage to incoming antherozoids. The antherozoids are attracted towards the egg by chemotactic stimulus, which in bryophytes, is provided in the form of sugars. Antherozoids enter in to archegonia and fertilized the egg.

Sporophyte

(1) The diploid fertilized egg (zygote) is the first cell of sporophytic generation. It divides and develops into a sporophytic plant body called sporogonium.

(2) The wall of venter forms calyptra, which provides a protective covering to the developing sporogonium.

(3) The sporogonium, in most of the cases, is differentiated into foot, seta and capsule.

(4) The sporogonium is completely dependent on the gametophyte for water and mineral supply and, in most of the cases, partly or wholly for organic nutrition. The sporogonium remains attached to the gametophytic plant body throughout its life.

(5) The sporogonium is mainly concerned with the production of asexually formed haploid spores (or meiospores). The spores are produced inside the capsule of sporogonia as a result of meiosis in the spore mother cells.

(6) The spores are the first cells of gametophytic generation. They germinate to produce the gametophytic plant body either directly or through a juvenile filamentous stage, called protonema.

Important features of classes

Campbell (1940), Smith (1955), Takhtajan (1953) divided bryophyta into three classes namely Hepaticae, Anthocerotae and Musci. Proskauer (1957) changed the names of these classes in accordance with the recommendations of the code, into Hepaticopsida, Anthocerotopsida and Bryopsida.

Hepaticopsida : The latin word Hepatica means liver. Thus the members of hepticopsida are popularly known as liverworts.

The gametophytic plant body is small, dorsiventral, thallose or leaf axis (foliose). Chlorophyllous cells contain many chloroplasts, one to several oil bodies, pyrenoids are absent. Rhizoids are unicellular. Sex organs develop from single superficial cells.

Sporogonium has little or no chlorophyllous tissue and stomata. The capsule is not linear. It lacks columella and intercalary meristem. Capsule dehisces by drying of capsule-wall, usually by more than two valves.

Anthocerotopsida : Gametophyte is thalloid. Thalli are lobed, dorsiventral, internally homogenous without any differentiation of tissues. Air chambers and air pores are absent but mucilage cavities may be present. Rhizoids are only smooth walled and scales are absent. Each cell possesses single (some times more) large chloroplast with central pyrenoid and oil bodies are absent. Antheridia are endogenous in origin, borne singly or in groups inside the closed cavities.

Sporogonium is differentiated into foot, meristematic zone and capsule (the seta is absent). Capsule has central sterile columella. The capsule dehisces basipetally by two valves and shows hygroscopic twisting.

Bryopsida : The members of bryopsida are commonly known as mosses. Gametophyte is differentiated into two stages – prostrate protonema and erect radial leafy shoot. Leaf-like appandages are spirally arranged on stem like axis. Rhizoids are multicellular with oblique septa. Sex organs develop from superficial cells.

Sporogonium is differentiated into foot, seta and capsule. Wall of capsule is several layered with stomata on epidermis. The capsule has central columella and elaters are absent.

Economic importance

(1) Soil conservation : Mosses grow in dense mats over the soil surface. They bind the soil particles and prevent soil erosion by running water.

(2) Formation of soil : Mosses along with lichens play a very important role in the formation of soil over the bare rocky surface. They grow on rocks and add organic matter to the substratum after their death. It makes the rock surface suitable for the growth of higher plants.

(3) Use in nursery : The Sphagnum plants have magnificent property of retaining water. They can with hold water two hundred times more than their own weight. Hence they are widely used by gardeners to keep cut plant parts moist during transportation and propagation.

(4) Peat : Sphagnum plants grow as semiaquatic or submerged in acidic marshes. The older portions of plants die but do not decay due to peculiar germicidal properties. Constantly increasing mass of dead remains accumulate year after year. These dead remains are slowly compressed and become hardened due to weight and forms a compact dark coloured peat rich in carbon.

(5) Other uses : Certain bryophytes are used to obtain a number of antibiotic substances. Some bryophytes have important medicinal uses. For example – The tea prepared from Polytrichum commune is used to dissolve kidney and gall bladder stones.