Life Sciences Term 3

• Chemical defences:  plants produce chemicals called phytoecdysteroids to defend against insects. The chemicals cause insects to moult prematurely, lose weight and if enough is ingested, metabolic damage and death.  Cultivated tobacco plants produce nicotine.  The leaves are eaten by insects and kills them. The leaves of mopane trees contain high levels of tannins making the leaves distasteful to herbivores

 Cultivated tobacco plants

   Leaves of the mopane tree

Thorns: this is a common term for a sharp structure found on plants for protection against herbivores. There are various types of sharp structures:

• Prickles are modified extensions of the cortex and epidermis of a plant that shape into a sharp, needle-like structure, for example rose bushes
  
• Thorns are modified branches or stems that form hard, pointed and sharp ends that can pierce the skin of herbivores. Examples are acacia trees, kei apples and lemon trees
  
• Spines are modified leaves that have a cylindrically shaped hard and sharp point, for example aloes and cacti.  Spines also reduce water loss by the plant

 Prickles of a rose bush

 Thorrns on an acacia tree

 Spines on a cacti