How does a spine on a cactus help it to survive?

Cactus needles have a number of functions.

Firstly, because cactus plants live in deserts and semi-arid areas, the needles reduce the loss of moisture that normally occurs through evaporation. As the cactus is a "flowering plant", like all plants in this family it has leaves, but in this case, the leaves are the needles. there is very little surface area on the needles, so less moisture is lost through evaporation.

Because of the shape of the needles, they also help capture whatever moisture is available. Dew and other forms of precipitation will tend to drip from the needles to the soil directly around the roots of a cactus, enabling the roots to utilise as much of the moisture as possible.

Cactus needles also deter plant-eating animals.