Class 12th Biology 2012 Set2 Delhi Board Paper Solution

1. In bacteria, fungi and lower plants, various kinds of thick-walled spores(cysts) are formed which help them to survive in unfavourable conditions - these germinate on availability of suitable environment.

2. In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal - they germinate to form new plants under favourable moisture and temperature conditions. They do so by reducing their metabolic activity and going into a date of ‘dormancy’.

3. In animals, the organism, if unable to migrate, might avoid the stress by escaping in time. The familiar case of bears going into hibernation during winter is an example of escape in time. Some snails and fish go into aestivation to avoid summer-related problems (heat and desiccation).

4. Under unfavourable conditions, many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.