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  Life cycle and classification of bryophytes, Biology tutorial

Introduction:

Bryophytes are a group of simple, small, green land dwelling plants of which some are aquatic including of Hornworts (that is, Anthocerotopsida), Liverworts (that is, Hepaticopsida) and Mosses (that is, Bryopsida).

These are the biggest group of land plants, which number around 25,000 different species found all through the world. Most of them are found in regions which are damp and humid. They can survive being frozen in snow devoid of damage.

Since Bryophytes are the simple plants, most have no internal modes for transporting water or nutrients. They are frequently stated to encompass leaves however these are not equivalent to the leaves of vascular plants. Bryophytes are generally one cell thick, however at times the midrib and stalk are quite a few cells thick however do not have any water conducting tissue. They do not have any roots however do encompass filamentous rhizoids that are at times multicellular which do little more than fasten them down.

Such plants don't produce flowers and thus never generate seeds. Plants which do not have flower are termed as cryptogam and reproduce through spore production, by using other cryptogams being fungi, slime moulds and ferns. The procedure by which they produce spores is known as alternation of the generations.

Features of Bryophytes:

a) Bryophytes grow up in damp and shady places.

b) The thalloid gametophyte distinguished in to rhizoids, axis (that is, stem) and leaves.

c) Vascular tissues (that is, xylem and phloem) not present.

d) The gametophyte bears multi-cellular and jacketed sex organs (that is, antheridia and archegonia).

e) Sexual reproduction is of oogamous kind.

f) Multicellular embryo builds up within archegonium.

General life cycle:

1) Archegonia: Archegonia are multicellular, stalked, flask-shaped female sex organs.

2) Archegonia comprise of an elongated upper part termed as neck and lower swollen part venter. The neck comprises of an axial row of cells termed as neck canal cells bounded through a sterile jacket. The venter as well build up of a 1-2 layer-thick wall of sterile cells that surrounds a bigger egg cell or the ovum and the smaller ventral canal cell merely above the egg.

3) At maturity, the tip of archegonium opens and the neck canal cells and also the ventral canal cells disintegrate, opening the neck for the entry of the antherozoids.

4) Antheridia comprise of rounded structure comprising of a single layered jacket surrounding a central mass of cells, that is, androcytes.

5) Each and every change into slender biflagellate antherozoids.

6) The antherozoids are discharged if the antheridium ruptures, therefore permitting them to swim freely in a water film. The antherozoids enter via the open necks and fuses by egg to form the diploid zygote.

7) After, divisions of zygote, a multicellular embryo are made that is nourished by the gametophyte.

8) The embryo grows and makes a mature sporophyte, in which sporogenous tissue will make spore tetrads, which in turn are discharged as the spores, forming either the gametophyte or the protonema that in turn makes the typical gametophyte.

Morphology of Bryophytes:

Division Bryophyla is classified into three classes:

1) Hepaticopsida (liverworts)

2) Anthocerotopsida (hornworts)

3) Bryopsida (mosses).

Hepaticopsida:

The gametophyte in the liverworts is either a thalloid ribbon-like body or a foliose structure.

The thalloid liverworts are generally prostrate, dorsiventral, dichotomously branched, having a dissimilar midrib and many unicellular rhizoids on the ventral surface. Internal differentiation is changeable, from extremely simple and comprising of some layers of cells to somewhat complicated, having an epidermal layer, air chambers and pores, a well developed photosynthetic tissue and some layers of storage cells. Beneath the light microscope, the epidermis or the leaf cells often represent certain organelles termed oil bodies which have terpenoids and differ in color, number and distribution. The sex organs are made up of either on the dorsal surface of the thallus in a median longitudinal furrow or on the specialized branches which elevate them into the air.

The foliose liverworts encompass stems having unicellular rhizoids and three rows of leaves: two dorsal rows of big, bibbed leaves inserted diagonally on the stem; and the ventral row made by smaller leaves which might become reduced or wanting and are inserted crossway on the stem. There are, though, some isophyllous taxa where all three rows of leaves are present and crosswise inserted on the stem. Both the ventral and dorsal leaves are variously dissected, generally comprising of a unistratose lamina and at times a band of some long thick-walled cells at mid leaf, the vitta. The stem exhibits little internal differentiation by an outer cortical layer which might be made up of smaller cells than those of the inner medullar area.

In the liverworts, the sporophyte consists of a small seta which extends if the spores are mature and the capsule wall comprises of one to numerous layers of cells without stomata. The capsule wall opens by means of four valves which expose the mature capsules and elaters to the wind. The sporogenous tissue gives rise to spores and elaters. The latter encompass hygroscopic movements, due to the joint action of one or some spiral wall thickenings and modifies in water tension which help in the dispersal of spores.

Anthocerotopsida:

The gametophyte in the hornworts is a thalloid, multi-layered lobed body which might be smooth on the dorsal surface.

Each and every gametophytic cell includes, as a rule, a single ventricular chloroplast, however there might be up to 12 chloroplasts in the Megaceros. The mid portion of the chloroplast comprises a single pyrenoid. Growth takes place by an apical cell situated in the notch of the thallus. On the ventral surface there are many smooth-walled rhizoids and pores that communicate by mucilage chambers often filled with Nostoc, that is, a nitrogen-fixing alga. The pores are at times interpreted as stomata which are remnants of an ancestor having radial symmetry. The sex organs build up from surface cells; the antheridia are found in the dorsal chambers with openings toward the dorsal surface whereas the archegonia encompass the neck canal cells and the oosphere surrounded through undifferentiated gametophytic cells.

Whereas the gametophyte might be thought of as primitive and quite simple, the sporophyte consists of a fairly complex structure. The capsule wall is a multistratose jacket surrounding the sporogenous tissue which gives mount to spores and pseudoelaters and the columella. The outer layer of the jacket includes stomata. Dehiscence takes place by one, two and four longitudinal lines and maturation carries on from the tip downwards. The uncertain growth of the sporophyte is due to an intercalary meristem positioned among the foot and the capsule; because of this meristem, the sporophyte might generate spores for an indefinite period of time. Dispersal is helped by the drying generally multi-cellular pseudoelaters and capsule walls which coil and twist and discharge the spores.

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