Name the phase all organisms have to pass through before they can reproduce sexually. (SHORT ANSWER QUESTIONS) (1MARK) (CBSE 2011)

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Answer :- Juvenile phase. 

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Mutations

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Charles Darwin (1859) in his book ' Origin of Species ' postulated that evolution or formation of newer species of organisms from the pre-existing ones occurs through accumulation of variations in the organisms. Without variations all the individuals of a species shall be alike. None of them would have better characteristics for being favoured in the struggle for existence and natural selection. A change in the environment could kill all of them. Variations increase the adaptability of the individuals and make them better fitted in the struggle for existence and hence natural selection. There are two sources of variations in the populations - recombinations and mutations . (i) Recombinations are also called continuous variations . They are not new traits but are formed by new combination of genes or traits already existing in the population. Recombinations are produced due to three reasons:  (a) Crossing over of nonsister chromatid segments during meiosis.  (b) Random segrega
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CLASSIFICATION Division : Bryophyta Class     : Hepaticopsida Order     : Marchantiales Family   : Marchantiaceae Genus   : Marchantia MORPHOLOGICAL FEATURES OF THE THALLUS- 1. The main plant body is gametophytic thallus. Gametophyte is small, well developed and  autotrophic. 2. The plant body is dorsiventral, the dorsal surfaces is dark green having a distinct midrib and  dichotomously branched. 3. The ventral or the lower surface bear numerous rhizoids and scales. 4. The rhizoids are unicellular and branched structures meant for anchorage and absorption. They  may be smooth walled or tuberculated (bearing peg-like projections). 5. The scales are purplish multicellular structures which protect the thallus from the dessication  by secreting the mucilage. 6. The dorsal surface also bears small gemma cups on the mid ribs. Inside the gemma cups many  erect stalked gemmae are present which helps in vegetative reproduction of Marchantia. 7. Sexual reproduction is oogamous. The male reproduc
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POLYMORPHISM Definition One of the important characters of Eumetazoa is the division of labour. In the vast majority, i.e., from flat worms to chordates, the division of labour involves the assignment of different functions to different parts or organs of the individual. In coelenterates, low organization and lack of organs do not permit any great degree of specialization for different functions within the limits of an individual. Therefore, in these animals the different vital functions are assigned to different individuals. This form of the division of labour is known as polymorphism. Polymorphism is, thus, the phenomenon of occurrence of an animal in more than one morphological and functional form. The individuals of polymorphic animals are known as the zooids or persons. 2. Grades Some coelenterates possess only two types of zooids, others have three types, and still others several types. These forms are respectively described as dimorphic, trimorphic and polymorphic. There
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