Phaeophyceae


* The members of phaeophyceae or brown algae are found primarily in marine habitats. They show great variation in size and form. They range from simple branched, filamentous forms

(Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres. They possess chlorophyll a, c, carotenoids and xanthophylls.

* They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them. Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol. The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus. The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ -the frond.

* Vegetative reproduction takes place by fragmentation.

* Asexual reproduction in most brown algae is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella.

* Sexual reproduction may be isogamous, anisogamous or oogamous. Union of gametes may take place in water or within the oogonium (oogarnous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella.

* The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Rhodophyceae:

* Rhodophyta are commonly called red algae because of the predominance of the red pigment, r-­phycoerythrin in their body. Majority of the red algae are marine with greater concentrations found in the warmer areas.

They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.

* The red thalli of most of the red algae are multicellular. Some of them have complex body organisation. The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.

* The red algae usually reproduce vegetatively by fragmentation.

* They reproduce asexually by non-motile spores and sexually by non-motile gametes.

* Sexual reproduction is oogamous and accompanied by complex post fertilisation developments.

* The common members are: Polysiphonia, Porphyra Gracilaria and Gelidium.

Bryophytes

* Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills.

* Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction. They usually occur in damp, humid and shaded localities.

They play an important role in plant succession on bare rocks/ soil.

* The plant body of bryophytes is more differentiated than that of algae. It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures.

* The main plant body of the bryophyte is haploid.

It produces gametes, hence is called a gametophyte.

* The sex organs in bryophytes are multicellular. The male sex organ is called antheridium. They produce biflagellate antherozoids. The female sex organ called archegonium is flask-shaped and produces a single egg. The antherozoids are released into water where they come in contact with archegonium. An antherozoid fuses with the egg to produce the 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. Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate to produce gametophyte.

* Bryophytes in general are of little economic importance but some mosses provide food for herbaceous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat that have long been used as fuel, and because of their capacity to hold water as packing material for trans­shipment of living material. Mosses along wi6 lichens are the first organisms to colonise rocks and hence, are of great ecological importance. They decompose rocks making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

* 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.

* 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.

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

* 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 anthertdta to archegonia, opening of archegonial neck, and the movement of antherozoids into the archegonial neck.

* Campbell (1940), Smith (1955), Takhtajan (1953) divided bryophyte, 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. The latin word Hepatica means liver. Thus the members of hepticopsida are popularly known as liverworts. The members of bryopsida are commonly known as mosses.

Liverworts:

* It The liverworts grow usually in moist, shady habitats such as banks of streams, marshy 'ground, damp soil, bark of trees and deep in the woods.

* The plant body of a liverwort is thalloid, e.g., Marchantia. The thallus is dorsiventral and closely appressed to the substrate. The leafy members have tiny leaf-like appendages in two rows on the stem-like structures.

* Asexual reproduction in liverworts takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (sing. gemma). Gemmae are green, multicellular, asexual buds, which develop in small receptacles called gemma cups located on the thalli. The gemmae become detached from the parent body and germinate to form new individuals.

* During sexual reproduction, male and female sex organs are produced either on the same or on different thalli. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are' produced within the capsule. These spores germinate to form free-living gametophytes.

Mosses:

* The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages. The first stage is the protonema stage, which develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage. The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axis' bearing spirally arranged leaves. They are attached to the soil through multicellular and branched rhizoids.

This stage bears the sex organs

* Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema.

* In sexual reproduction, the sex organsantheridia and archegonia are produced at the apex of the leafy shoots. After fertilisation, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. The sporophyte in mosses is more elaborate than that in liverworts. The capsule contains spores. Spores are formed after meiosis. The mosses have an elaborate mechanism of spore dispersal.

* Common examples of mosses are Funaria, Polytrichum and Sphagnum.

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