The cactus adapted to its harsh environment by developing thick skin to slow the evaporation of water. The leaves changed in order to reduce the surface area that water could evaporate from and over time became defensive spines to protect itself from animals trying to get inside its thick skin for the moisture.
Leaves
Quil is to porcupine as spine is to cactus
protects it from predators.
A spine.
'Spine' is another word for 'thorn'.
Spine
Cactus, like other plants, uses photosynthesis to get energy.
stem. if i am wrong please correct me
Thorns are leaf modifications in opuntia (a type of cactus) and a stem modification in bougainvillea. So it depends on the type of plant.
Cactus
Cactus plants use their stems for storage. So that's where starches are stored. In fact, stems are where a cactus plant stores everything. Other, non cactus plants may use their roots for storage. But cactus roots are much too fibrous and shallow.
The cactus wren
A cactus competes with other plants for water.