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  Bryophyta and Pteridophyta, Biology tutorial

Bryophyta and Pteridophyta (Filicinophyta)

Introduction:

The bryophytes are simplest of land plants. It is probable that they developed from green algae. They have 2 classes, Liverworts (Hepaticae) and Mosses (Musci). They are not well adapted to life on land. So they live in damp shady places. They are small, plain. Their growth and conducting tissues are either not present or badly developed. Vascular tissues are xylem and phloem tissues. They don't contain true roots, stems and leaves. This is due to these in the true forms have vascular tissues that are not present in bryophytes. They are attached to by thin filaments known as rhizoids which produce from stem. So rhizoids serve generally as means of anchorage. Water and mineral salts are absorbed from entire surface of plant. Rhizoid does not function like plant roots either. Plant surface is delicate and hasn't cuticle. For this reason they are in danger of losing water. That is the reason they can live only in dark shady places. Although few have been discovered to survive long (up to a year) at up to 20oc by means not yet well understood.

Life Cycles:

In bryophytes, 2 generations alternate in life cycles. There is haploid gametophyte generation and diploid sporophyte generation. You will see that two generations are always alternating. Gametophyte is always haploid and produces its gametes by mitosis. Sporophyte is always diploid and produces its spores by meiosis so that there is the return to haploid condition. Haploid spores provide rise to gametophyte generation that is more conspicuous and lasts longer. It is thus dominant generation. In bryophyte gametophyte is dominant. In all other plants sporophyte (i.e.) part that produces spores. You also must appreciate that when you draw the life cycle, it is common in biology to put dominant generation on top half of diagram. You must further note that diagram can also signify summary of life cycles in every plants, flowering plants included. Notice that to make gametes mitosis takes place and meiosis manufactures spores. In animals, though it is meiosis which produces gametes.

External characteristics of Liverworts:

Liverworts e.g. Pellia

The liverworts are simpler than mosses and stay more limited to damp shady places, such as river banks, damp rocks and wet vegetation. Body is composed of the dull green flat thallus. It has midrib that is more obvious on lower side. It has unicellular rhizoids also occurring from lower side. Frequently branches dichotomously at tip. When it has absorbed enough nutrients, the vertical seta starts to grow from top and this seta later carries black capsules containing haploid spores made by meiosis. Capsule later split into 4 to release spores that germinate to form thallus. Similar to mosses, several species of liverworts reproduce by creating gemmae. Gemmae are tiny round or spherical reproductive structures that are borne inside gemmae cups. Gemmae cups form on top of thallus. Gemmae formation is the significant shape of asexual reproduction in several species of liverworts and mosses.

The Moses:

Like liverworts, they also live in damp shady places. Though they are a bit more differentiated because they have multicellular roots, stem, and spirally arranged leaves. Like liverworts, mosses require water for fertilization. When plants' surface is wet, mature antheridia soak up water and burst, releasing male gametes (sperm) onto surface. Each sperm contain 2 flagella. With assistance of these, they swim towards archegonia, each of which has one female gamete or ovum. Fertilization occurs in archegonium. The diploid zygote is thus formed. This grows out of archegonium to become the new sporophyte.

Pteridophytes - Ferns (Filicinophyta):

General Characteristics:

Ferns are generally limited to damp shady places. They are common in tropical rain forests. Only few of them can produce in full sunlight.

Structure:

Structurally ferns are more grown than mosses. Due to this they have vascular tissue composed of xylem and phloem. Vascular tissues transport water and nutrient round plant body. Xylem carries water and mineral solutions such as sugar. Emergence of the vascular tissue is the significant evolutionary step above bryophytes and algae. This vascular tissue is discovered in sporophyte generation. When water and mineral resources and organic matter such as sugar can be carried into plants, it signifies that they have adequate supply of these materials. They can afford to produce large and complicated without being cut-off supply as there is transport system. Secondly, the tissues provide stiffness as they have lignified cells that have great strength and rigidity. Other lignified tissues, Schlerenchyma also grow in vascular plants and further add to mechanical role of xylem.

The sporophyte generation has true leaves, stems and roots. This generation of fern thus has benefit of support; food, water and minerals supply; skill to produce big and tough and even to struggle for light. Though, fern is still handicapped as gametophyte generation that is still tiny and vulnerable to drying out than bryophyte gametophyte. Gametophyte generation of fern is known as prothcillus. It makes sperm that should swim to reach female gametes.

External Features of Tree Fern:

The frond (leaves) of sporophyte can be up to a meter tall. It contains thick horizontal stem or rhizome which bears adventitious roots. It can reproduce vegetative by breaking of branches from core stem. These provide rise to separate plants. Bases of fronds are covered with scales known as ramenta that project younger leaves from frost or drought. Young leaves are typically rolled up. Main axis of leaf is known as rachis and leaflets on either side are called pinnae and small rounded subdivisions of pinnae are known as pinnules.

Reproduction:

When spores are made, they are initially green but turn out to be brown when mature. Arrangement of spores on frond varies from fern to fern. Sporangia grow in clusters known as son that are covered by indusium. Inside every sporangium is diploid mother cell which divides by meiosis to make haploid spores. When mature, indusium shrive and drops off to expose the sporangia walls that start to dry out, they ultimately break to release spores. These spores germinate to form gametophyte generation.

It is of green color and photosynthetic. It is attached by unicellular rhizoids to soil. Delicate prothallus has no cuticle and is prone to drying out. It can just survive in damp places. It makes simple antheridia and archegonia on the lower surface. These are sex organs that protect gametes within them. Gametes are made by gamete mother cell.

Archegonium create ovum whereas antheridium sperm. Al does so by mitosis as in bryophytes. Sperm has flagella with which it swims to fertilize ovum. Diploid zygote so created develops in, sporophyte generation. The fertilization is reliant upon presence of water. Young zygote carries on to get food from gametophyte until its own root and leaves can take over role of nutrition. Then gametophyte withers and dies.

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