NEET Biology Notes plant kingdam Bryophytes
These include various mosses and liverworts, commonly growing in moist shaded areas in the hills. These are also called amphibians of plant kingdom because they live in soil but dependent on water for sexual reproduction.
They are found commonly in damp, humid and shaded places and play an important role in plant succession on bare rock/soil. Plant body is thallus-like, attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves. The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte. The male sex organ is called antheridium. They produce biflagellate antherozoids. The female sex organ is called archegonium, which produces a single egg.
An antherozoid fuses with the egg to produce a zygote. Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it.
Bryophytes in general are of little economic importance. Some mosses provide food for herbaceous mammals, birds and other animals. Mosses along with lichens are the first organisms to colonise rocks. They decompose rocks making the substrate suitable for the growth of higher plants. Species of Sphagnum, a moss, provide a peat that have long been used’as fuel, and as packing meterial for trans-shipment of living meterial because of their capacity to hold water.
The bryophytes are of following two types:
It grow in moist and shady places, like Marchantia. The thallus is dorsiventral and closely appressed to the substrate.
Gemmae are green, multicellular, asexual buds, which develop in small receptacles called gemma cups which are present on furrow of dorsal side of the thallus. The sporophyte is differentiated into a foot, seta and capsule. Spores are produced within the capsule. These spores germinate to form free-living gametophytes.
Plant body is thalloid, green, dorsiventral and dichotomously branched. On the dorsal side of the thallus, a median longitudinal groove is present. On dorsal side, rhomboidal or polygonal areas are present. Scales and rhizoids are borne on the ventral side of the thallus. Rhizoids are colourless, unicellular structures. These are of two types, i.e. smooth walled and tuberculate.
Sex organs are dioecious with similar male and female thalli. Sex organs are borne on gametophores. These are developed • in localised areas called receptacles. The mature antheridium has an ovoid body supported on a short, multicellular stalk. The body of the antheridium has a jacket layer, i.e. one cell in thickness. It encloses a mass of androcytes. Each androcyte gives rise to a biflagellate sperm.
The archegonia of Marchantia has a short but distinct stalk, which attaches the venter to the receptacle. There is a collar-like structure at the base of the venter of each archegonium, called perigynium.
In addition, there is a two-lipped curtain-like perichaetium with the fimbriated margins around each series of archegonia.
The sporophyte is differentiated into foot, seta and capsule. It hangs freely from the undersurface of the female receptacle surrounded by the perigynium and the perichaetium. The foot is well-developed and broad. It functions as an attaching and absorbing organ.
The short seta elongates rapidly but slightly, just to push the mature capsule through and beyond the surrounding protective sheaths. At maturity, the capsule is an oval-shaped yellow body concerned both in the production and distribution of spores.
In the life cycle of mosses, the predominant stage is protonema, which directly develops from a spore. It is creeping, green, branched and frequently filamentous stage.
The second stage is leafy stage, which develops from the secondary protonema as a lateral bud. Mosses are attached to the soil through multicellular and branched rhizoids. Reproduction occurs by vegetative and sexual means. The sporophyte in mosses is more elaborate than in liverworts. Common examples of mosses are Funaria, Polytrichum and Sphagnum.
- The main plant body is small yellowish green gametophore. The gametophore consists of slender, erect, branched or unbranched stem-like structures called axis, small spirally arranged leaf-like structures called phylloids and multicellular, branched, thin root-like structure called rhizoids, which have oblique cross walls.
- Anatomically the axis has single layered epidermis made up of thick walled chloroplast containing cells, cortex of chloroplast containing parenchymatous cells and the central cylinder (medulla) made up of a group of thick walled cells conducting tissue.
- Vegetative reproduction takes place by die and decay of older parts, that arise of branched filamentous structure (secondary protonema) from injured portion of axis, leaves, rhizoids, by bulbils (resting bud arise from protonema), gemmae (bud-like structure formed from the terminal cells of protonema), or by apospory. The gametes are produced in highly differentiated antheridia and archegonia at the apex of same plant,
i. e. monoecious.
- The main axis acts as male shoot (antheridiophore) which bear many antheridia. The lateral branch acts as female shoot (archegoniophore) which bear many archegonia.
- Each antheridium consists of stalk and club-shaped body surrounded by jacket. Numerous elongated, spirally coiled, biflagellate antherozoids are formed inside it.
- Each archegonium is flask-shaped structure consisting of short multicellular stalk, lower swollen portion (venter) and upper tube-like neck. Venter has two-layered wall and contain a large ovum or egg cell. Above the egg lies a Venter Canal Cell (VCC). The jacket of neck consists of single layer of cells arranged in six vertical rows. Neck contains 6-9 Neck Canal Cells (NCC).
- Multicellular, uniseriate sterile structure called paraphysis are present in between the antheridia and archegonia. The antherozoids released by bursting of antheridium swim in water and reach up to archegonium being attracted by sugar (chemotaxis). The zygote formed by fusion of antherozoid and egg develops a thick wall and is known as oospore.
- The diploid oospore divides and develops into a sporophyte, which obtain nourishment from the parent gametophyte. Sporophyte consists of foot, seta and capsule.
- The capsule is differentiated into operculum (a circular cup-shaped 2-3 layered lid), annulus (ring like layer of thickened cells at the base of operculum), peristome (32 tooth-like projections arranged in two rings of 16 each), collumella (solid sterile column containing water and food), spore sac and apophysis (sterile solid basal portion having chloroplasts in many cells and stomata in epidermis). The stoma is surrounded by single ring-like guard cell. The diploid spore mother cells undergo meiosis to form thousands of haploid single celled spores.
- Under dried condition the capsule dehisces and the spores on getting favourable conditions germinate into a green, much branched, filament-the protonema (juvenile stage). Protonema consists of rhizoids and a number of aerial green, prostrate chloronemal branches.
It is the juvenile stage of moss. It results from the germinating meiospore. When fully grown, it consists of a under green, branching system of filaments called the protonema. The filamentous protonema branches freely. It forms filament green and felt-like coating on the damp soil resembling the algal growth. Many of the branches grow and spread in the moist soil. These are the green chloronemal branches. The other branches penetrate the soil. They are colourless or brown. The cells of these rhizoidal branches lack chloroplasts. They have oblique septa between them. The protonema stage in Funaria is only vegetative and transitory.