Discuss briefly the general characteristic features and classification of Rhodophyceae.

Ans. Occurrence: It includes about 400 genera and 2,500 species. Most of the members of this class are marine. There is an insignificant percentage of forms which inhabit fresh water pools, ponds, banks of lakes and slowly running streams. The common freshwater members are Lemanea, Batrachospermum, Thorea, and Hildenbrandia. The fresh water forms need more aeration and, therefore, they occur in running water though it is not essential for all such species. Asterocytis and Compsopogon are the genera which are found in quiet water or rather stagnant pools while the former and Porphyridium may be terrestrial also. Catenella and Bostrychia occur in brackish water. Erythrocladia is a prostate epiphyte or endophyte. Colaconema is an endophyte within the membranes of various algae. Goniotrichum is also an epiphyte.

Rhodophyceae also includes parasitic species. Polysiphonia fastigiata is a semi-parasite on Ascophyllum nodosum, a brown alga while Chorecolax is a parasite on Polysiphonia. Corallina shows a marked calcification. In short, the members are world-wide in distribution being found in tropics and temperate regions although most of the Florideae inhabit warmer seas. Some filamentous types like Acrochaetium are epiphytes and endophytes and their many species are found on Hydroids, Bryozoa, Sponges and on the shells of Molluscs.

Size and form : The members of this class differ very much in their shape, size and form though the diversity is not so great as in Chlorophyceae and Phaeophyceae. The most simple type is Porphyridium, which is unicellular and terrestrial and forms deep red masses on damp soils. Asterocytis and Goniotrichum are filamentous types while there are a number of genera which are microscopic. Generally, the plant body is attached to a rock or any other substratum with the help of rhizoids. Plants are branched and ribbon-shaped with apical growth. They may be monosiphonous or polysiphonous.

Important features : The members of Rhodophyceae are characterized by the complete absence of motile reproductive units. They exhibit a variety of colours; from bright red and purple to dark brown, brownish green, blue green or even black. These colours are due to the presence of various pigments located in the chromatophores. Besides the normal green pigments, carotin and xanthophyll, there is the characteristic pigment, the water soluble phycoerythrin in association with blue pigment, the phycocyanin. The chlorophyll content is decidedly low. Batrachospermum, perhaps, lacks phycoerythrin and contains only chlorophyll mixed with phycocyanin.

The method of reproduction is altogether different and is very much complicated. The terminology of sex organs is also quite different. The male, no doubt, is called anteridium but it gives rise to non-motile spermatia while the female is procarp being differentiated into a lower broad region the carpogonium and a long neck called trichogyne. The post-fertilization stages are also very much complicated. The assimilatory product, floridean starch, is also present in the form of reserve food. The pyrenoids are present and sometimes the granules of starch may accumulate round them.

Structure of a cell: A cell is made up of two clear layers, an inner layer of cellulose and an outer layer of pectic substances. There is calcification in Corallinaceae. The deposit is first visible on the outer layer and then it deepens but it never touches the protoplast lamella. Magnesium occurs in deep water forms while a few forms have a deposit of iron, e.g., Batrachospermum.

Cell contains cytoplasm and vacuoles. Pigments are located in chromatophore or chromatophores within the cell. Cells in the earlier stages are uninucleate although in the later stages the number of nuclei may increase. Clear nucleolus and chromatin are present in the nucleus. The shape of the nuclei may be irregular as in some Ceramiales.

Reproduction: The reproduction is vegetative, sexual and asexual. Most of the members multiply vegetatively by fragmentation. When the plant gets broken into many pieces, each piece gives rise to a new individual.

The sexual reproduction is brought about with the help of male and female organs of reproduction and it is always oogamous. As already mentioned, the male reproductive organs give rise to non-motile spermatia.

The female organ procarp has a carpogonium in which the egg nucleus is located. Trichogyne or neck of the procarp serves as a receptive spot. The male nucleus unites with the female nucleus resulting in the formation of zygote. The zygote nucleus passes into the auxiliary cells, where it gives rise to many carpospores directly (Bangioideae) or indirectly (Floridean). The carpospores may contain X number of chromosomes (if the first nuclear division is meiotic) or may contain 2X number of chromosomes (if the first nuclear division is mitotic). Accordingly, they are called Haplobiontic or Diplobiontic. When haploid in nature, the carpospores form the new plants directly as in Batrachospermum.

When they are diploid they form tetrasporic plants (Asexual plants) which bear tetrasporangia. These sporangia develop plants as in Polysiphonia. Asexual reproduction is also affected by the neutral spores. Monospores found in Chantransia stages of Thorea and Batrachospermum also bring about asexual reproduction

Classification : Rhodophyceae has been divided into sub-classes and seven orders in following ways:

Sub-Class I. Bangioideae: It possesses unicellular or colonial or filamentous forms with a single axle or stellate chromatophore.

It has only one order.

1. Bangiales. The members of this order are haplobiontic in character. Sub-Class II. Florideae : Branched or unbranched filamentous forms with sex organs highly developed. Reproduction, fertilization and postfertilization stages are complicated. It has six orders.

1. Nemalionales

2. Gelidiales

3. Cryptonemiales

4. Gigartinales

5. Rhodymeniales

6. Ceramiales

Nemalionales: The Order includes a number of fresh water forms which are quite simple. It is characterized by the complete absence of tetrasporic plants or tetraspores. After fertilization, the zygote divides and forms carpospores.

The first nuclear division is meiotic, i.e. carpospores are formed direct from the zygote and are haploid in nature. It, therefore, includes haplobiontic species.

The order has been divided into the following families :

1. Acrochaetiaceae

2. Batrachospermaceae

3. Lemnaceae

4. Naccariaceae

5. Bonnemaisoniaceae

6. Thoreaceae

7. Helminthocladiaceae

8. Chaetangiaceae

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