Q.25. What do you understand by alternation of generation? explain it in the life cycle of Ectocarpus? (2009)
Ans. Alternation of generation: -
Generally, the life cycle of a typical plant consists of two stages :
(i) Gametophytic phase: - In this phase, the plant body is made up of haploid cells.
(ii) Sporophytic phase: - The plant body is composed of diploid cells.
Alteration of generation is a sequence in which both gametophytic phase and sporophytic phase come one after another.
Alternation of generation in the life cycle of Ectocarpus: -
In isomorphyic life cycle of Ectocarpus, a distinct isomorphic alternation of generation is seen. In this species, both haploid and diploid plants are found in nature. The sporophyte reduplicates by neutral spores and gametophyte by apogamous gametes.
On the sporophyte, unilocular and plurilocular sporangia are formed. Unilocular sporangia produce haploid zoospore, each on germination gives rise to a gametophytic plant while in plurilocular sporangia diploid zoospores or neutral spores are produced, each germinates to form a sporophytic plant.
The gametophytic plant, formed as a result of germination of haploid zoospores, bears plurilocular gametangia which produce gametes. These gametes fuse and as a result a diploid zygote is formed. In majority of the cases, fusion occurs between the two identical gametes of different plants showing heterothallism. However, anisogamy is also known.
The zygote germinates and divides repeatedly by mitotic divisions and as a result a diploid or sporophytic plant is formed. This sporophytic plant again forms unilocular or plurilocular sporangia.
Thus, a regular alternation of generation is found in Ectocarpus, i.e. there is a regular alternation of haploid and diploid plants. Because the plants of both the generations are morphologically similar, the life cycle is isomorphic type.
Q.26. Explain effect of Environment on Ectocarpus. (2011)
Ans. Effect of environment on Ectocarpus: -
The life cycle of Ectocarpus is affected by environment speciflcally. Mostly only one type of Ectocarpus plants found at one place or environment which shows specific features of this algae. Those oceanic region where temperature is low, all the plants are sporophytic and do not show alternation of generation. In these plants diplontic life cycle is found.
Those plant which are found near the sea sore of Meditarian sea are gametophytic. These plant shows haplontic life cycle and it is not well expressed. Those plants which are found at American Sea sore exhibit isomorphic alternation of generation. This type of physical distribution of Ectocarpus shows that the life cycle of this algae is affected by temperature, light duration and tides.
Sargassum
Q.27. Describe the life-cycle of Sargassum. Explain the alternation of generation unit. (2013)
Related Questions -
Q. Draw a labellad diagram of v.s. of Sargassum female conceptacle. (2006)
Q. Give systematic position with identification charaters of Sargassum. (2005)
Q. Draw T.S. of male and female conceptacle of sargassum. (2003)
Q. Describe the life cycle of sargassum with the help of suitable diagrams. (1994, 97, 99, 02)
Ans. Systematic position:-
Division - Algae
Class - Phaeophyceae
Order - Fucales
Family - Sargassaceae
Genus - Sargassum
Occurrence: - It is a marine algae of tropical sea. It is also known as gulf weed. Some species attached to rocks some are free floating. The plants are also occur in Indian coast.
Structure: - It is macroscopic plant look like on angiospermic plant. The plant body of sargassum is more or less hushy in habit and is radially symmetrical. The plant body divide into a holdfast, main axis and primary and secondary laterals, leaves, air bladders and cylindrical and flattened like structures known as receptacle. The main branch is erect and has nodes and internodes. The leaf like structure are sterile and bears numerous dot like structures, cryptostoma, where also present some air bladders which are the reservoir of air and as such help in floating.
Reproduction: -They reproduces both by vegetative and sexual method.
Vegetative reproduction: - Sargassum reproduces vegetatively by fragmentation or multiplication. The separation commonly occurs by accidentally or death and decay of older parts. Each fragment grows into a new plant.
2. Sexual reproduction: - This a is oogamous type and takes place by means of definite male (antheridia) and female (oogonia) reproductive organs called respectively which are borne in the flask-shaped conceptacles found on conceptacles receptacle. Some cryptostomata are also present on the receptacle.
Structure of Mature Conceptacle: - The plant of sargassum consists of male and female conceptacle.
It is a flask shaped structure embedded in the receptacle tissue and it opens out through an ostiole which is found over the surface of receptacle. Each cell of wall contains chromatophores. The wall cells give rise to branched hair called paraphyses. Sex organs are borne in wall of conceptacles. They are unisexual bearing either antheridia or oogonia.
Development of Antheridia: - It develop in lower portions of branched paraphyses and has avoid shape. It occur in large number and so crowded in a single conceptacle.
It contains 64 biflagellate male cells. They are detached at this stage with the help of water and comes out through ostiole which are float on the surface of water the wall of it gelatinous and male cells are liberated. They are pearshaped in structure with laterally inserted two unequal flagella.
Development of Oogonium: - It develop as rounded sessile and extent embeded in the tissue of conceptacle. At maturity it contains eight nuclei from the diploid nucleus the cytoplasm undergoes no cleavage. It round off to form a single egg which is eight nucleate out of these eight nuclei one enlarges. The other seven are finally degenerate during fertilization which are liberated through the ostiole in sea water and soon attached on the wall of conceptacle.
Fertilization: - The liberated antherozoids swims towards the oogonia conceptacles and each oogonium surrounded by antherozoidss. The sperm attaches to oogonial wall by their anterior flagella while the posterior one keep lashing in water. One of their penetrates the wall of the oogonium and enters the ovum.
Male nucleus entered and lies close by the side of the female nucleus which are ultimately fused to form zygote.
Germination of Zygote: - Zygote begins germination without undergoing any resting period. It divides by a transverse wall into an upper and a lower cell. The lower cell elongates and forms rhizoidal processes and upper cell forms to diploid erect plant.
Alternation of Generation and Nuclear Behaviour in Sargassum: -
The plant body of Sargassum is diploid. It bears sex organs in diploid conceptacles which develop on receptacles. Diploid antheridia and oogonia are formed by the cells of the wall of conceptacles. The diploid nucleus of each antheridium by meiosis division forms four haploid nuclei which form 64 haploid anthérozoids by mitotic division. Similarly, the diploid nucleus of the oogoniuin forms four haploid nuclei by meiotic division which form 8 haploid nuclei by mitotic division of these 8 haploid nuclei. 7 degenerate and the remaining one acts as the functional nucleus from which develop an haploid egg. During fertilization single haploid nucleus or antherozoid fuses with the haploid egg nucleus to form diploid zygote. The nucleus of diploid zygote does not undergo meiosis. It divides mitotically to form two cells. From these cells develop young diploid sporophytic sargassum plant. In Sargassum plant Sporophytic generation is predominant in comparative to gametophytic i.e. gametophytic stage is less developed. This type of alternation of generation is called as cyclosporic alternation of generation.