Write short notes on the following:

Xylem and phloem are vascular tissues which are a complex conducting tissue formed of more than one type of cell. Xylem transports water and minerals while phloem conducts organic food. Bryophytes do not have to conduct tissues. They are non-vascular terrestrial plants of moist habitats in which a multicellular diploid sporophyte lives as a parasite on an independent multicellular haploid gametophyte.

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generations in mosses. Water conducting cells with walls perforated by plasmodesma-derived pores occur in the calobryales and Pallaviciniaceae among liverworts and in Takakia among mosses. Imperforate water conducting cells are present in bryoid mosses. A polarized cytoplasmic organization and a distinctive axial system of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses and in less specialized parenchyma cells of the leafy stem and seta in other mosses including sphagnum. A similar organization suggested to reflect specialization in long distance symplasmic transport of nutrients, also occurs in other parts of the plant in mosses, including rhizoids and caulonemata, and may be observed in thallus parenchyma cells of liverworts. Perforate water conducting cells in the calobryales, Metzgeriales and Takakia, and hydroids in bryoid mosses, probably evolved independently. Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise putative food-conducting of Bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.

The bryophytes show an alternation of generations between the independent gametophyte generation, which produces the sex organs and sperm and eggs, and the dependent sporophyte generation, which produces the spores. The sporophyte releases spores, from which the gametophytes ultimately develop.

The life-cycle of a bryophyte shows regular alternation of gametophytic and sporophytic generations. This process of alternation of generations was demonstrated for the first time in 1851 by Hofmeister. Thereafter in 1894 Strasburger could actually show the periodic doubling and halving of the number of chromosomes during the life-cycle.

The haploid phase (n) is the gametophyte or sexual generation. It bears the sexual reproductive organs which produce gametes, i.e. antherozoids and eggs. With the result of gametic union a zygote is formed which develops into a sporophyte. This is the diploid phase (2n). The sporophyte produces spores which always germinate to form gametophytes.

During the formation of spores, the spore mother cells divide meiotically and haploid spores are produced. The production of the spores is the beginning of the gametophytic or haploid phase. The spores germinate and produce gametophytic or haploid phase. The spores germinate and produce gametophytes which bear sex organs.

Ultimately the gametic union takes place and zygote is resulted. It is diploid (2n). This is the beginning of the sporophytic or diploid phase. This way, the sporophyte generation intervenes between fertilization and meiosis; and gametophyte generation intervenes between meiosis and fertilization.

Peristome is an anatomical feature that surrounds an opening to an organ or structure.

In Bryophytes, the peristome is a specialized structure in the sporangium that allows for gradual spore discharge, instead of releasing them all at once. Most Bryophytes produce a capsule with a lid which falls off when the spores inside are mature and thus ready to be dispersed. The opening that revealed is called the stoma and is surrounded by one or two peristomes. Each peristome is a ring of triangular teeth formed from the remnants of dead cells with thickened cell walls. There are usually 16 such teeth in a single peristome, separate from each other and able to both fold in to cover the stoma as well as fold back to open the stoma. This articulation of the teeth is termed arthrodontous and is found in the moss subclass Bryopsida. In other groups of mosses, the capsule is either nematodontous with an attached operculum, or else splits open without operculum or teeth. There are two subtypes of arthrodontous peristome.

(a) The first is termed haplolepidous and consists of a single circle of 16 peristome teeth.

(b) The second type is the diplolepidous peristome found in subclass Bryidae. In this type, there are two rings of peristome teeth-an inner endostome and an exostome. The endostome is a more delicate membrane, and its teeth are aligned between the teeth of the exostome. There are a few mosses in the Bryopsida that have no peristome in their capsules. These mosses still undergo the same cell division patterns in capsule development, but the teeth do not fully develop.

The plant body of Marchantia is gametophytic or haploid (x) in nature. The plant body is thallus like prostrate dorsoventrally differentiated, dichotomously branched, and possess a prominent midrib. The thallus is dark green in colour. The thallus may attain a length of 2 cm. to 10cm. Each lobe of thallus possess a distinct midrib and an apical notch, its margin is weavy. The dorsal surface of thallus has small rhomboidal to polygonal areas which marks the position of stoma-like opening into the air chamber. On the ventral surface thallus on either side of the midrib present two or more rows of scales which protect the growing point. On the lower side of central surface of thallus are present the rhizoids which are of two type smooth walled and tuberculated and do the function of fixation and absorption.

A vertical cross section of the Marchantia thallus (gametophyte) shows a high degree of internal differentiation. It is clearly differentiated into upper photosynthetic and lower storage regions. Starting from upper side, it consist of an upper epidermis which is usually colourless or pale green. The epidermis has pores. The pores are barrel shaped, consisting of four or five superimposed layers of cells and having bother an upper and lower opening. The pores thus communicate with air chamber below. Below upper epidermis is a loose green dorsal region with a single horizontal layer of air chamber. Each chamber opens outside by means of a pore. From the floor of chamber simple or branched photosynthetic filament arise.

Just below the air chambers (Photosynthetic region) there is a compact, colourless ventral region known as storage region, and its cells consist of starch grains. Some cell may have a single oil body or be filled with mucilage. From the lower epidermis arises two types of smooth and tuberculated rhizoids and four or more rows of scales.

Polytrichum, a large genus of about 100 species is widely distributed throughout the world, chiefly in temperate and tropical countries. Only four species have been reported from India (Bruhl, 1935) namely P. densifolium, P. xanthopilum, P. juniprinum and P. alpinum. These are commonly found on ranges. P. commune is cosmopolitan in distribution.

It is mainly found in cool temperate and tropical regions. Like other mosses it is also found in cool and shady places. They may be found growing in bogs and marshes, on soil of firm or loose texture, on rocks and cliffs and as epiphytes on trunks of trees.

The Gametophyte is the plant body which differentiated into underground rhizome and the upright erect leafy shoot.

(A) Rhizome: It is the underground part of the gametophyte which grows horizontally. It bears numerous rhizoids. The rhizoids are long, thread like with thick walls and obligate septa. They grow downward in the substratum. The rhizoids coil, twist and interwined like strings in a rope. The rhizome besides the rhizoids, bears small, brown or colourless numerous leaves in three rows.

(B) Leafy Shoot: It is erect leafy axis arising from the rhizome. It may be 25-30 cm tall. It is differentiated externally into a stem like central axis which bears large dark green leaves.

The leaves are arranged spirally in three rows. The leaves are borne on the stem in different manners. The leaves borne on rhizome and transition zone are small, brown or colourless and triangular in outline. The leaves borne on the erect stem are large and green and consist of proximal sheathing base and distal limb.

(i) They have star shaped pointy leaves. ;

(ii) Each leave is arranged at ninety degrees at a strong stem.

(iii) It does not grow tall.

(iv) They are dark green in colour.

(v) They don’t have a woody tissue.

(vi) It has the usual lifespan of 3-5 years however the maximum recorded lifespan is 10 years.

In general the economic importance of bryophytes is very little. On the whole none of them is directly important to man except a number of species of sphagnum. The dried plants of sphagnum are used as an absorbent on surgical dressing as a backing material for transhipment of living plants as a material to increase the water retaining properties of certain poor types of soil and as a means of retaining high soil acidity required by certain decorative and economically important plants.

The mosses, ecologically are also of much importance. They are helpful in soil formation, in soil production, and in soil conservation. They provide the essential steps in the development of soils, because they can grow even in dry rocks where growth of higher plant is impossible. In due course of time their plant body dies and decay forming humus. The humus increases the fertility of soil.

Bryophytes are very important in initiating soil formation on barren terrain, in maintaining soil moisture, and in recycling nutrients in forest vegetation. Indeed, discerning the presence of particular bryophytes is useful in assessing the productivity and nutrient status of forest types. Further, through the study of bryophytes, various biological phenomena have been discovered that have had a profound influence on the development of research in such areas as genetics and cytology.

External structure: The leaves are formed on the main stem as well as on the branches. The leaves occurring on branches are overlapping and on stem axis they are little apart. The leaves are small thin and scale like without mid rib, sessile and arranged spirally on the stem and branches.

Internal structure; The leaf is single layered in thickness. It is composed of two types of cells (i) Capillary cells, the ordinary type of cells and green (ii) Photosynthetic or assimilatory cells. The two kinds of cells are alternated with each other to form a network. The elongated small assimilatory cells are alternated by wide hyaline capillary cells. The assimilatory cells contain chloroplast and photosynthetic in nature. The capillary cells are colourless (hyaline) without protoplasmic contents and hence they are dead and filled with water.

A vertical cross section of the Marchantia thallus (gametophyte) shows a high degree of internal differentiation. It is clearly differentiated into upper photosynthetic and lower storage regions. Starting from upper side, it consists of an upper epidermis which is usually colourless or pale green. The epidermis has pores. The pores are barrel shaped, consisting of four or five superimposed layers of cells and having bother an upper and lower opening. The pores thus communicate with air chamber below. Below upper epidermis is a loose green dorsal region with a single horizontal layer of air chamber. Each chamber opens outside by means of a pore. From the floor of chamber simple or branched photosynthetic filament arise.

Just below the air chambers (Photosynthetic region) there is a compact, colourless ventral region known as storage region, and its cells consists of starch grains. Some cell may have a single oil body or be filled with mucilage. From the lower epidermis arises two types of smooth and tuberculated rhizoids and four or more rows of scales.

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