Biology Class 12 NCERT Solutions: Chapter 13 Organisms and Populations Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q. 7 Name important defense mechanisms in plants against herbivory.


  • Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivory.
  • Morphological defence mechanisms:
  • Cactus leaves (Opuntia) are modified into sharp spines (thorns) to deter herbivores from feeding on them.
  • Sharp thorns along with leaves are present in Acacia to deter herbivores.
  • In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them.
  • Chemical defence mechanisms:
  • All parts of Calotropis weeds contain toxic cardiac glycosides, which can prove to be fatal if ingested by herbivores.
  • Chemical substances such as nicotine, caffeine, quinine, and opium are produced in plants as a part of self-defense.

Q. 8 An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?


Orchid and Mango Tree
  • An orchid growing on the branch of a mango tree is an epiphyte. Epiphytes are plants growing on other plants which however, do not derive nutrition from them.
  • Therefore, the relationship between a mango tree and an orchid is an example of commensalisms, where one species gets benefited while the other remains unaffected. In the above interaction, the orchid is benefited as it gets support while the mango tree remains unaffected.

Q. 9 What is the ecological principle behind the biological control method of managing with pest insects?


  • The basis of various biological control methods is on the concept of predation.
  • Predation is a biological interaction between the predator and the prey, whereby the predator feeds on the prey.
  • Hence, the predators regulate the population of preys in a habitat, thereby helping in the management of pest insects.
Use of Predators

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