Which part of cactus plant is modified into spines?

Cacti are adapted to the harsh desert environment, where water is scarce. To conserve water, cacti have evolved to reduce transpiration, the loss of water through the leaves, by modifying them into spines. 

Spines are much smaller than leaves and have a much smaller surface area, so they lose less water through transpiration. Additionally, the spines also provide shade for the stem, which helps to reduce water loss from the stem. By reducing transpiration, cacti are able to survive in the desert, where water is limited.