Why desert plants have spine-like structures?

In desert plants, the leaves are modified into spines-like structures to prevent water loss through transpiration.

[Extra information for reference: 

Desert plant-like Cactus adapted to survive in a desert with its specific features such as:

a) The long roots that can go deep in the soil for the absorption of water.

b) The leaves are modified into spines to prevent water loss through transpiration.

c) The stem is covered with a thick layer of wax to retain water.

The desert plants make their food by photosynthesis. The stomata of desert plants are closed during day time to prevent transpiration and conserve water. The stomata conduct a gaseous exchange at night. The carbon dioxide entered into the leaf during the daytime.

The photosynthesis takes place during day time. The intermediate product is stored into vacuole which is sent to the stroma of chloroplast during day time. It gets broken down into carbon dioxide and pyruvate. This carbon dioxide enters into the Calvin cycle.]